Glossary of astronomy

Thisglossary of astronomyis a list of definitions of terms and concepts relevant toastronomyandcosmology,their sub-disciplines, and related fields. Astronomy is concerned with the study ofcelestial objectsand phenomena that originate outside theatmosphere of Earth.The field of astronomy features an extensive vocabulary and a significant amount of jargon.

A[edit]

A-type star: In theHarvard spectral classificationsystem, a class ofmain-sequencestar having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 timesthe mass of the Sun,and have surface temperatures of 7,600–10,000 kelvin.
absolute magnitude: A measure of astar's absolute brightness. It is defined as theapparent magnitudethe star would show if it were located at a distance of 10parsecs,or 32.6light-years.
accretion disk: A roughly circular mass of diffuse material inorbitaround a central object, such as astarorblack hole.The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.
active galactic nucleus(AGN): A compact region in the center of agalaxydisplaying a much higher than normalluminosityover some part of theelectromagnetic spectrumwith characteristics indicating that the luminosity is not produced bystars.A galaxy hosting an AGN is called anactive galaxy.
albedo: A measure of the proportion of the totalsolar radiationreceived by anastronomical body,such as aplanet,that isdiffusely reflectedaway from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by ablack body) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g.cloud coverand geological or environmental surface features).
albedo feature: A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness (albedo) compared to adjacent areas.
Am star: Achemically peculiar starbelonging to the more general class ofA-type stars.The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. Seemetallicity.
aphelion: The point at which a body orbiting the Earth'sSunis furthest from the Sun. Contrastperihelion.
apoapsis: The point at which an orbiting body is furthest from itsprimary.Contrastperiapsis.
apogee: The point at which a body orbiting the Earth (such as theMoonor anartificial satellite) is furthest from the Earth. Contrastperigee.
apparent magnitude: A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of theatmosphere.The brighter the object appears, the lower its magnitude.
appulse: The closest approach of one celestial object to another, as viewed from a third body.
apsis: In theorbitof aplanetary body,one of the two extreme points of distance between the body and itsprimary– either the point of minimal distance, called theperiapsis,or the point of maximal distance, called theapoapsis.The term may also be used to refer to the value of the distance rather than the point itself. Allelliptical orbitshave exactly two apsides.
argument of periapsis: The angle from anorbitingbody'sascending nodeto itsperiapsis,measured in the direction of motion. It is one of six canonicalorbital elementsused to characterize an orbit.
artificial satellite: An object that has been intentionally placed intoorbitby humans, often around the Earth but also around other bodies within theSolar System.Contrastnatural satellite.
ascending node: Theorbital nodeat which an orbiting object moves north through theplane of reference(in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of theSolar System). The position of the ascending node with respect to a reference direction, called thelongitude of the ascending node,is used along with otherparametersto describe an orbit. Contrastdescending node.
aspect: The position of aplanetor Earth'sMoonwith respect to theSun,as viewed from Earth.^[1]
asterism: Any pattern ofstarsrecognizable in Earth'snight sky.An asterism may form part of an officialconstellationor it may be composed of stars from more than one constellation.
asteroid: Aminor planetof the innerSolar System,i.e. one that orbits theSunat a distance no greater than the orbit ofJupiter.Asteroids are somewhat arbitrarily distinguished from many different types of similar objects:small Solar System bodiesprimarily composed of dust and ice instead of mineral and rock are known ascomets;bodies less than one meter in diameter are known asmeteoroids;very large asteroids are sometimes calledplanetoidsorplanetesimals;and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known asdistant minor planets.
asteroid belt: Thecircumstellar discin theSolar Systemlocated roughly between the orbits ofMarsandJupiterthat is occupied by numerous irregularly shapedsmall Solar System bodiesranging in size from dust particles toasteroidsandminor planets.The asteroid belt is often called themain asteroid beltormain beltto distinguish it from other asteroid populations in other parts of the Solar System.
astrobiology: An interdisciplinary field that studies the origins, evolution, distribution, and future of living systems in the universe, encompassing research on organic compounds inspace,abiogenesisandextreme-environment adaptationon Earth, thehabitabilityofextrasolar planets,the possible existence ofextraterrestrial life,and how humans might be able to detect extraterrestrialbiosignatures,among other topics.
astrodynamics: Seeorbital mechanics.
astrogeology: A field that studies the geology of solidified bodies such as the planets and their moons, asteroids, comets, and meteorites. Investigations are centered around the composition, structure, processes, and history of these objects.
astrometric binary: A type ofbinary systemwhere evidence for an unseen orbiting companion is revealed by its periodicgravitational perturbationof the visible component. See alsospectroscopic binary.
astrometry: The branch of astronomy that involves precise measurements of the positions and movements ofstarsand othercelestial bodies.
astronomical body: A type of naturally occurring physical entity, association, or structure within theobservable universethat is a single, tightly bound, contiguous structure, such as astar,planet,moon,orasteroid.Though the terms astronomical "body" andastronomical "object"are often used interchangeably, there are technical distinctions.
astronomical catalogue: A list of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery.
astronomical object: A type of naturally occurring physical entity, association, or structure that exists within theobservable universebut is a more complex, less cohesively bound structure than anastronomical body,consisting perhaps of multiple bodies or even other objects with substructures, such as aplanetary system,star cluster,nebula,orgalaxy.Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.
astronomical symbol: Any abstract pictorial symbol used to represent one or moreastronomical objects,events, or theoretical constructs, e.g. those of theplanetsof theSolar System,thephases of the Moon,thezodiacal constellations,and thesolsticesandequinoxes.Many of these symbols were commonly used historically, though in the modern era they are usually limited toalmanacsandastrology,and their appearance in scientific literature has become increasingly infrequent. Exceptions include the symbols for the Sun (☉), the Earth (🜨), and the Moon (☾), which are sometimes used forastronomical constantsand in other forms of shorthand.
astronomical unit(AU): A unit of length used primarily for measuring distances within theSolar Systemor secondarily between the Earth and distant stars. Originally conceived as thesemimajor axisof theEarth's orbitaround the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10⁻⁶parsecs; 1.5813×10⁻⁵light-years).
astronomy: The scientific study ofcelestial objectsandphenomena,the origins of those objects and phenomena, and their evolution.
astrophotography
astrophysics: The branch of astronomy that employs principles ofphysicsandchemistryto determine the nature ofastronomical objectsand phenomena, examining properties such asluminosity,density,temperature,andchemical composition(rather than the positions or motions of objects in space, which is more specifically the emphasis ofcelestial mechanics).
atmosphere: A gaseous envelope held in place by the gravity of a planet. This shell of gas has no clearly defined exterior boundary, but instead grows increasingly tenuous with altitude. The term can also be applied to a stellar atmosphere, referring to the visible outer layers of a star.
axial precession: A slow, continuous, gravity-induced change (aprecession) in the orientation of anastronomical body'saxis of rotation.The term often refers in particular to the gradual shift in the orientation ofEarth's rotational axiswith respect toits orbital planeover a cycle of approximately 25,772 years, which is caused predominantly by the gravitational influence of theMoonand theSunon the Earth'sequatorial bulge.The phenomenon is similar to but much larger in magnitude than other changes in the alignment of Earth's axis such asnutationandpolar motion,and is the cause of the apparentprecession of the equinoxesin thenight sky.
axial tilt: The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between itsequatorialplane andorbital plane.Axial tilt usually does not change considerably during a singleorbital period;Earth's axial tilt is the cause of theseasons.Axial tilt is distinct fromorbital inclination.
axis of rotation
azimuth: An angular measurement of an object's orientation along the horizon of the observer, relative to the direction oftrue north.When combined with thealtitudeabove the horizon, it defines an object's current position in thespherical coordinate system.

B[edit]

barycenter: The commoncenter of massabout which any two or more bodies of a gravitationally bound systemorbit.The barycenter is one of thefociof theelliptical orbitof each body participating in the system; its location is strongly influenced by themassof each body and the distances between them. For example, in aplanetary systemwhere the mass of the centralstaris significantly larger than the mass of an orbitingplanet,the barycenter may actually be located within the radius of the star, such that the planet appears to orbit the star itself, though both bodies actually orbit the shared barycenter.
baryogenesis: The process by which the class of subatomic particles known asbaryonswere generated in the early Universe, including the means by which baryons outnumber antibaryons.
Big Bang: The prevailingcosmological modelfor the origin of theobservable universe.It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
binary star: Astar systemconsisting of exactly twostarsorbiting around their commonbarycenter.The term is often used interchangeably withdouble star,though the latter can also refer to anoptical double star,a type of optical illusion which is entirely distinct from true binary star systems.
black hole: A concentration of mass so compact that it creates a region of space from which not even light can escape. The outer boundary of this region is called theevent horizon.
blazar
break-up velocity: The surface velocity at which thecentrifugal forcegenerated by a rapidly spinning star matches the force ofNewtonian gravity.At rotational velocities beyond this point, the star begins to eject matter from its surface.^[2]
brown dwarf: Asubstellar objectthat is too low in mass to sustain thenuclear fusionofhydrogen-1in its core, with the latter being a characteristic of stars on themain sequence.Brown dwarfs can still generate energy fromgravitational contractionand by the fusion ofdeuterium.
bulge

C[edit]

Calibrator star: A star used for calibration of high-powered teleescopes.
coudé spectrograph: This is aspectrographplaced at theCoudé focusof areflecting telescope.The focus remains stationary as the telescope is re-oriented, which is advantageous for the stable mounting of heavy spectroscopic instruments.^[3]
celestial equator: The imaginarygreat circleof a body'scelestial spherethat is coplanar with the body's terrestrialequator.On Earth, the plane of the celestial equator is the basis of theequatorial coordinate system.Due to Earth'saxial tilt,this plane is currently inclined at an angle of 23.44 degrees with respect to theecliptic.
celestial mechanics: The branch ofastronomythat studies themotionsof all types ofastronomical objects,includingstars,planets,andnaturalandartificial satellites,among others.
celestial meridian: Seemeridian.
celestial pole: One of two coordinates in the Earth'sskyat which a hypothetical indefinite extension of the Earth'saxis of rotation"intersects" thecelestial sphere,i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which allfixed starsappear to revolve during the course of a day. The celestial poles form the north and south poles of theequatorial coordinate system.
celestial sphere: An imaginary sphere that encompasses the Earth's entireskyand is stationary with respect to thebackground stars.It is the basis forspherical astronomy.
centaur: Asmall Solar System bodywith either aperihelionor asemi-major axisbetween those of theouter planets,i.e. generally inward of theKuiper beltbut beyond theJupiter trojans.Centaurs arecis-Neptunian objectsthat typically exhibit characteristics of bothasteroidsandcomets,and generally also have unstable orbits because they cross the orbits of one or more of thegiant planets.
central massive object(CMO): Any very large concentration of mass at the center of agalaxy,typically either asupermassive black holeor acompact stellar nucleus,but sometimes both.
chromosphere
chromospheric activity index: A parameter indicating themagnetic activityin a star'schromosphere.One measure of this activity is $log R' HK$ ,where $R' HK$ is the ratio of theequivalent widthof a star'ssingly ionizedcalciumH and K lines,after correction forphotosphericlight, to thebolometric flux.^[4]Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active ( $log R' HK$ above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1).^[5]
circumstellar disc
cis-Neptunian object (CNO)
clearing the neighbourhood
color index: A numeric value that is used to compare the brightness of a star measured from differentfrequencybands of theelectromagnetic spectrum.Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.
comet: A relatively small, icy body that displays extended features when it approaches theSun.The energy from the Sun vaporizesvolatileson a comet's surface, producing a visiblecomaaround the cometary body. Sometimes a comet can produce a longtailradiating away from the Sun.
commensurability: A property of two objects orbiting the same body whoseorbital periodsare in arational proportion.For example, the orbital period ofSaturnaround the Sun is very nearly 5/2 the orbital period ofJupiter.
common proper motion: A term used to indicate that two or more stars share the same motion through space, within themargin of observational error.That is, either they have nearly the sameproper motionandradial velocityparameters, which may suggest that they aregravitationally boundor share a common origin,^[6]or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time).
compact star: Anyastronomical bodywith a very high mass relative to its radius, compared to most ordinary atomic matter. The term typically refers to very high-density objects such aswhite dwarfs,neutron stars,andblack holes,or tostellar remnantswith very small radii.
compact stellar nucleus: Seenuclear star cluster.
conjunction: A phenomenon during which twoastronomical objectsorspacecrafthave either the sameright ascensionor the sameecliptic longitudeas observed from a third body (usually the Earth), such that, from the observer's perspective, the objects appear to closely approach each other in the sky.
constellation: A region on thecelestial spheresurrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based inmythology,while the modern demarcation of their borders was established by theInternational Astronomical Unionin 1930. Compareasterism.
corona: An aura ofplasmathat surrounds cooler stars such as theSun.It can be observed during asolar eclipseas a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
coronal loop
coronal mass ejection(CME): A significant release ofplasmaand the accompanying magnetic field from theSun'scorona,often following asolar flareor present during asolar prominenceeruption.
cosmic distance ladder
cosmic dust: Dustwhich exists inouter spaceor has fallen on Earth, generally composed of fine particles of solid matter far smaller than those found in terrestrial dust.
cosmic microwave background(CMB)
cosmic ray: A type ofradiationconsisting of high-energyprotonsand atomic nuclei which move through space at nearly the speed of light, and which may originate from theSunor from outside theSolar System.Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
cosmogony: Any model concerning the origin of either theuniverseor thecosmos.
cosmology: The scientific study of theorigin, evolution, and eventual fateof theUniverse.
critical rotation
critical velocity: The surface velocity at the equator of a rotating body where thecentrifugal forcebalances theNewtonian gravity.At this rotation rate, mass can be readily lost from the equator, forming acircumstellar disc.See alsobreak-up velocity.^[7]
culmination: The apparent movement of anastronomical object(e.g. theSun,theMoon,aplanet,astar,aconstellation,etc.) across the observer's localmeridian.During each day, the Earth's rotation causes every astronomical object toappear to movealong a circular path on thecelestial sphere,creating two points at which it crosses the meridian: anupper culmination,at which the object reaches its highest point above thehorizon,and alower culmination,at which it reaches its lowest point, nearly 12 hours later. When not otherwise qualified, thetime of culminationtypically refers to the time at which the upper culmination occurs.^[8]

D[edit]

debris disk: A ring-shapedcircumstellar discof dust and debris orbiting its host star. It is created by collisions betweenplanetesimals.A debris disk can be discerned from aninfrared excessbeing emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.
declination: In theequatorial coordinate system,the celestial equivalent of terrestriallatitude.Coordinates north of thecelestial equatorare measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See alsoright ascension.
decretion disk: Acircumstellar discformed from gas ejected from a central star that now follows a nearlyKeplerian orbitaround it. This type of disk can be found around manyBe stars.^[9]
deep-sky object(DSO): Anyastronomical objectthat is not an individualstaror an object within the Earth'sSolar System.The classification is used mostly in amateurobservational astronomyto distinguish faint objects in thenight skysuch asstar clusters,nebulae,andgalaxies.
degenerate star: A star composed ofdegenerate matter,e.g. awhite dwarfor aneutron star.These stars are in an advanced state ofevolutionand have suffered extremegravitational collapse,such that normal atoms cannot exist in them.^[1]
descending node: Theorbital nodeat which an orbiting object moves south through theplane of reference(in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of theSolar System). Contrastascending node.
detached object: Adynamicalclass ofminor planetin the outer reaches of theSolar Systemwhosepoint of closest approach to the Sunis so distant that the object is only moderately or weakly affected by the gravitational influence ofNeptuneand the other known planets, such that it appears to be "detached" from the rest of the Solar System. Detached objects are thus distinct from other populations oftrans-Neptunian objects,such ascubewanosandscattered discobjects.
direct motion: Seeprograde motion.
diurnal motion: The apparent motion of anastronomical object(e.g. theSun,aplanet,or a distantstar) around the twocelestial polesin the Earth'snight skyover the course of one day. Diurnal motion is caused by Earth's rotation about its own axis, such that every object appears to follow a circular path called thediurnal circle.
double star: Any pair ofstarswhich appear near each other on the celestial sphere, either because the two stars coincidentally lie along nearly the same line of sight from the Earth, though they are in fact physically distant from each other, or because the two stars are actually located in physical proximity to each other, by which they may form a co-moving pair or abinary starsystem.
dwarf planet
dwarf star: Anystarbelonging to a category of ordinarymain-sequencestars like theSun,in contrast toevolved giant starslikeBetelgeuseandAntares.Confusingly, the term has also come to includestellar remnantsknown aswhite dwarfsas well as low-masssubstellar objectsknown asbrown dwarfs.

E[edit]

early-type star: A hotter and more massive star, in contrast tolate-type starsthat are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged. It may be used to refer to the higher-temperature members of any particular population or category of stars, rather than of all stars in general.
eccentricity: Seeorbital eccentricity.
ecliptic: Theplanedefined by theEarth's orbitaround theSun.Hence, the position of the Sun as viewed from the Earth defines the intersection of this plane with thecelestial sphere.The ecliptic is widely used as areference planefor describing the position of otherSolar Systembodies within variouscelestial coordinate systems.It differs from thecelestial equatorbecause of theaxial tiltof the Earth.
ecliptic coordinate system: Anastronomical coordinate systemcommonly used to specify the apparent positions, orbits, and axial orientations of objects within theSolar System,with an origin at the geometric center of either theSunor the Earth, afundamental planedefined by the plane of Earth's orbit around the Sun (i.e. theplane of the ecliptic), aprimary directiontowards thevernal equinox,and a right-handed convention. This system is convenient because most of theplanetsand manysmall Solar System bodiesorbit the Sun with only slightinclinationsto the ecliptic. It may be implemented in either spherical or rectangular coordinates.
effective temperature: (of a star or planet) The temperature of an idealblack bodythat would emit the same total amount ofelectromagnetic radiation.
elliptical galaxy: A type ofgalaxywith an approximatelyellipsoidalshape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along withspiralandlenticular galaxies.
elliptical orbit: A type ofKepler orbitwith anorbital eccentricityof less than 1 (often inclusive ofcircular orbits,which have eccentricity equal to 0), or one with negativeenergy.Elliptical orbits take the shape of anellipse,and are very common in two-body astronomical systems.
elongation: Theangular separationbetween theSunand an orbiting body, such as aplanet,as it appears from Earth.
ephemeris: A list or table of the expected positions ofastronomical objectsorartificial satellitesin theskyat various dates and times. Modern ephemerides are often provided by computer software.
epoch: A moment in time used as a reference point for some time-varying astronomical quantity, such as thecelestial coordinatesororbital elementsof anastronomical object,because such quantities are subject toperturbationsand change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin.
equator: The imaginary line on a gravitationally rounded spheroid such as aplanetthat represents the intersection of the spheroid's surface with aplaneperpendicular to itsaxis of rotationand equidistant from itsgeographical poles.The plane of the Earth's terrestrial equator is the basis for thecelestial equator.
equatorial coordinate system: Anastronomical coordinate systemdefined by an origin at the geometric center of the Earth, afundamental planecreated by projecting the Earth's terrestrialequatoronto thecelestial sphere(forming thecelestial equator), aprimary directiontowards thevernal equinox,and a right-handed convention. This system is widely used to specify the positions of celestial objects as viewed from Earth. It may be implemented in either spherical or rectangular coordinates.
equinoctial: Of, relating to, or occurring at anequinox.
equinox: Either of the two precise times of year when the imaginary plane of the Earth'sequator,extended indefinitely in all directions,passes through the center of theSun(i.e. the two points at which this plane intersects theplane of the ecliptic); or, equivalently, when the Sun's apparent geocentriclongitudeis either 0 degrees or 180 degrees.^[10]The two equinoxes, known as thevernal equinoxand theautumnal equinox,occur on or nearMarch 20andSeptember 22each year. On the day of an equinox, the center of the visible Sun appears to bedirectly abovethe equator, and the durations of day and night are approximately equal all over the planet. Comparesolstice.
escape velocity: The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object'skinetic energyandgravitational potential energyis equal to zero. It is a function of the mass of the body and of the distance between the object and the body'scenter of mass.An object which has achieved escape velocity is neither on the surface nor in a closedorbitof any radius.
evolutionary track: A curve on theHertzsprung–Russell diagramthat a solitarystarof a particular mass and composition is expected to follow during the course of itsevolution.This curve predicts the combination of temperature andluminositythat a star will have during part or all of its lifetime.^[11]
extinction: Theabsorptionandscatteringofelectromagnetic radiationby matter (dust and gas) between an emittingastronomical objectand theobserver.Atmospheric extinction varies by the wavelength of the radiation, with theattenuationbeing greater for blue light than for red.
extragalactic astronomy: The branch of astronomy that studies objects and phenomena outside of theMilky Way galaxy,i.e. all objects not covered bygalactic astronomy.
extrasolar object: Anyastronomical objectthat exists outside theSolar System.The term is generally not applied tostarsor any objects larger than a star or the Solar System itself, such asgalaxies.
extrasolar planet: Anyplanetoutside the Earth'sSolar System.
exobiology: Seeastrobiology.

F[edit]

facula: A bright spot on astar'sphotosphereformed by concentrations ofmagnetic field lines.For theSunin particular, faculae are most readily observed near thesolar limb.An increase in faculae as a result of a stellar cycle increases the star's total irradiance.
field galaxy: Anygalaxythat does not belong to a larger cluster of galaxies and is gravitationally isolated.
field star: A randomly situatedstarthat lies along theline of sightto a group of physically associated stars under study, such as astar cluster.These field stars are important to identify in order to prevent them from contaminating the results of a study.^[12]
first light: The first use of a newly constructedtelescopeor other instrument to take anastronomical image.
first magnitude star: A term used to classify the brightest stars in the night sky, withapparent magnitudeslower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars.
First Point of Aries(♈︎): The location of theMarch equinoxupon thecelestial sphere,used as a reference point incelestial coordinate systems.Located in theconstellation Pisces,the First Point of Aries defines theecliptic coordinateof (0°, 0°) and represents the point at which theSunmeets thecelestial equatorwhile traveling from south to north each year. It is directly opposite theFirst Point of Libra.
First Point of Libra: The location of theSeptember equinoxupon thecelestial sphere,used as a reference point incelestial coordinate systems.Located in theconstellation Virgo,the First Point of Libra represents the point at which theSunmeets thecelestial equatorwhile traveling from north to south each year. It is directly opposite theFirst Point of Aries.
fixed stars: The "background" ofastronomical objectsin thenight skywhich are so distant from observers on Earth that they do not appear to move relative to each other, as opposed to the "foreground" of objects within theSolar Systemwhich do. The fixed stars are typically taken to include allstarsother than theSun,as well as all otherextrasolaranddeep-sky objects.
flare star: A class ofvariable starthat undergoes sudden, dramatic increases in brightness due tomagnetic activityon its surface. This change in brightness occurs across theelectromagnetic spectrumfromradiowaves toX-rays.Most flare stars are faintred dwarfs.
Fulton gap: The apparent uncommonness ofplanetshaving a size between 1.5 and 2 times that of the Earth.

G[edit]

galactic astronomy: The branch of astronomy that studies objects and phenomena within theMilky Way galaxy,as opposed to everything outside of the Milky Way, which is the domain ofextragalactic astronomy.
galactic anticenter: The direction in space that is directly opposite thecenterof theMilky Way Galaxy,as viewed from Earth; considered as a point on thecelestial sphere,the Milky Way's anticenter is in theconstellation Auriga.
Galactic Center: The rotationalcenterof theMilky Way galaxy,consisting of asupermassive black holeof 4.100 ± 0.034 millionsolar masses.It is approximately 8,200 parsecs (27,000 ly) away from Earth in the direction of theconstellations Sagittarius,Ophiuchus,andScorpius,where the Milky Way appears brightest.
galactic coordinate system
galactic corona
galactic nucleus: The region at the center of agalaxy,usually home to a very dense concentration of stars and gas. It almost always includes asupermassive black holewhich, when active, can generate a much higherluminosityin a compact region than its surroundings. This excess luminosity is known as anactive galactic nucleus,and the brightest such active galaxies are known asquasars.
galactic period: The time a givenastronomical objectwithin agalaxytakes to complete oneorbitaround thegalactic center.Estimates of the duration of one revolution of theSolar Systemabout the center of theMilky Wayrange from 225 to 250 million terrestrial years.
galactic tide: Thetidal forceexperienced by objects subject to thegravitational fieldof agalaxysuch as theMilky Way.
galactocentric distance: A star or cluster's distance from thegravitational centerof a particulargalaxy.For example, theSunis about 27,000light-years(approximately 8kiloparsecs) away from theGalactic Centerof theMilky Way.^[13]Galactocentric distance may also refer to a galaxy's distance from another galaxy.
galaxy: A large, gravitationally bound system ofstars,stellar remnants,interstellar gas,dust,anddark matter,each of which orbits acenter of mass.Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology aselliptical,spiral,orirregular.Most of the galaxies in theobservable universeare between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter though some, including theMilky Way,are much larger.
galaxy cluster: A large-scale structure consisting of hundreds or thousands ofgalaxiesbound together by gravity. Galaxy clusters are distinct from similarly namedgalactic clustersand other types ofstar clustersand from smaller aggregates of galaxies known asgalaxy groups.Galaxy groups and galaxy clusters can themselves cluster together to formsuperclusters.
galaxy group: A gravitationally bound aggregation of up to 50galaxies,each at least as luminous as theMilky Way Galaxy.Larger aggregations may be calledgalaxy clusters,and galaxy groups and clusters can themselves cluster together to formsuperclusters.
Galilean moons: A collective name for the fourmoonsofJupiterdiscovered byGalileo Galileiin 1610:Io,Europa,Ganymede,andCallisto.
gamma-ray astronomy: The subfield ofastronomythat studiesastronomical objectsdetectable atgamma-raywavelengths.
gamma-ray burst(GRB): A cataclysmic event that generates a brief but intense outburst ofgamma rayradiation which can be detected from billions oflight-yearsaway. The source of most GRBs is theorized to besupernovaorhypernovaexplosions of high-mass stars. Short GRBs may also result from the collision ofneutron stars.
gas giant: Agiant planetcomposed mainly ofhydrogenandheliumgases rather than heavier elements, e.g.JupiterandSaturnin theSolar System.
geocenter: Thegeometric centerof the Earth, i.e. the arithmetic mean position of all points within theoblate spheroidthat is the precise shape of the Earth.
geocentric: With reference to, or pertaining to, thegeometric centerof theEarth;^[14]centered upon the Earth, e.g. a geocentric orbit.
geocentric zenith: The point projected upon thecelestial sphereby a straight line that passes through thegeocenterand an observer; i.e. the observer'szenithas defined with respect to the center of the Earth.^[14]
geometric albedo: The ratio of the brightness of an astronomical body at aphase angleof zero to an idealized flat, fully reflecting,diffusively scattering(Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.
geometric position: The position of an object (celestialor otherwise) with respect to thecenter of the Earthor to the position of an observer, i.e. as defined by a straight line between the center of the Earth (or the observer) and the object at a given time, without any corrections forlight-time,aberration,etc.^[14]
geostationary orbit: Acircular geosynchronous orbit,which maintains a constant altitude of 35,786 kilometres (22,236 mi) directly above Earth'sequatorin thesame directionasEarth's rotationsuch that, to an observer on Earth's surface, the orbiting object appears motionless, in a fixed position in the sky.Artificial satellitesare often placed in geostationary orbit so that antennas on Earth do not have to rotate to track them.
geosynchronous orbit(GSO): Asynchronous orbitabout the Earth, i.e. with anorbital periodequal to Earth'srotational period,such that the orbiting object appears to return to exactly the same position in the sky after a period of onesidereal day.All geosynchronous orbits have asemi-major axisequal to 35,786 kilometres (22,236 mi);geostationary orbitsare a special case of geosynchronous orbits.
giant planet: Any very large or massiveplanet,includinggas giantsandice giants.
globular cluster: A tight, spherical conglomeration of many thousands ofstarswhich are gravitationally bound to each other and whichorbitagalactic coreas asatellite.They differ fromopen clustersin having a much higher combined mass, with a typical lifespan extending for billions of years.
gravitational collapse
gravitational lens: Any very large distribution of mass, such as agalactic cluster,which can bend passing light from a distant source by a noticeable degree. The effect, known asgravitational lensing,can make background objects appear to an observer to take on a ring or arc shape.
gravitational-wave astronomy: A branch ofobservational astronomywhich analyzes minute distortions in the curvature ofspacetimeknown asgravitational wavesto collect observational data about astronomical objects and events such asneutron stars,black holes,supernovae,and theBig Bang.

H[edit]

H II region: An ionizednebulapowered by young, massiveO-type stars.Ultraviolet photonsfrom these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly inspectral lines of hydrogenand other elements. Because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms ofspiral galaxiesand in star-formingirregular galaxies.
heliocenter: The precisegeometric centerof the Earth'sSun,i.e. the arithmetic mean position of all points within the approximatespheroidthat is the shape of the Sun.
heliocentric: With reference to, or pertaining to, thegeometric centerof the Earth'sSun;^[14]centered upon the Sun, e.g. a heliocentric orbit.
heliopause
heliosphere: The vast, bubble-like cavity in theinterstellar mediumwhich surrounds and is created by theplasmaemanating from the Earth'sSun.The heliosphere encompasses the entirety of theSolar Systemand a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.
Hertzsprung–Russell diagram: A plot ofluminosityversuseffective temperaturefor a population ofstars;depending on the usage, the star'sabsolute magnitudemay be substituted for luminosity, and itscolor indexorspectral typefor temperature. Single stars of known mass and composition follow predictabletracksacross this chart over the course of theirevolution.Hence, knowing a star's mass andmetallicityallows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, includingmain-sequence,red giant,andwhite dwarfstars.
Hill sphere: The approximate region around anastronomical objectwithin which its gravitational attraction dominates the motions ofsatellites.It is computed with respect to the next most gravitationally attractive object, such as the nearest star or thegalactic core.Satellites moving outside this radius tend to be perturbed away from the main body.^[15]
horizon: The apparent boundary between the surface of a celestial body and its sky when viewed from the perspective of an observer on or near that body's surface; more specifically, theplaneperpendicular to a line from an observer to thezeniththat passes through the point of observation.^[14]
hour angle: For a givencelestial object,the angular distance on thecelestial spheremeasured westward along thecelestial equatorfrom the observer's localmeridianto thehour circlethat passes through the celestial object;^[14]or, equivalently, the angle between theplanecontaining Earth'srotational axisand thezenith,and the plane containing Earth's rotational axis and the object of interest. Analogous toright ascension,the hour angle is one of many ways commonly used to specify the longitudinal position of an object upon the celestial sphere.
hour circle: Any imaginarygreat circledrawn upon the celestial sphere that passes through both of thecelestial polesand is therefore perpendicular to thecelestial equator.^[14]Similar to ameridianbut additionally taking into account the terrain and the depth to thegeocenterat a ground observer's particular location, the concept of the hour circle is employed to describe the longitudinal position of a celestial object relative to the observer's local meridian.
hybrid pulsator: This is a class ofpulsating starsthat display pulsation frequencies of two different classes of variables. An example are variables displaying characteristic frequencies of bothDelta ScutiandGamma Doradus variables.On theHertzsprung–Russell diagram,these stars are positioned where theinstability stripsof both variable classes overlap.^[16]
hydrogen burning limit: A critical mass below which an astronomical object cannot sustain its surfaceluminositythrough nuclear fusion. This mass limit, equal to about 7% of themass of the Sun,forms the dividing line betweenbrown dwarfsand hydrogen-fusing stars.^[17]
hypergalaxy: A system consisting of a largegalaxyaccompanied by multiple smallersatellite galaxies(oftenelliptical) as well as itsgalactic corona.TheMilky WayandAndromedasystems are examples of hypergalaxies.^[18]

I[edit]

ice giant: Agiant planetcomposed mainly of elements heavier thanhydrogenorhelium(such asoxygen,carbon,nitrogen,andsulfur), especially chemicalvolatileswith freezing points above 100 K (−173 °C), e.g.UranusandNeptunein theSolar System.
inclination: Seeorbital inclination.
inferior planet: An archaic term that is sometimes used to refer to the planetsMercuryandVenus.The name originated from the fact that these planets orbit closer to theSunthan the Earth and hence, in thegeocentric cosmologyofPtolemy,both appear to travel with the Sun across the sky. This is in contrast to the so-calledsuperior planets,such asMars,which appear to move independently of the Sun.
infrared astronomy: The subfield ofastronomythat studiesastronomical objectsdetectable atinfraredwavelengths.
International Astronomical Union(IAU)
interstellar medium(ISM): Thematterthat exists in the space between thestarsin agalaxy.This medium mainly consists ofhydrogenandhelium,but is enhanced by traces of other elements contributed by matter expelled from stars.
interstellar reddening: An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known asextinction.This effect causes more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon ofredshift.
invariable plane: The imaginary plane passing through thebarycenterof aplanetary systemand perpendicular to itsangular momentumvector, and which may be regarded as the weighted average of all planetaryorbitaland rotational planes comprising the system.
ionosphere
irregular galaxy
irregular moon: Anatural satellitefollowing a distant,inclined,and ofteneccentricandretrogradeorbit about itsprimary.Irregular moons are thought to be captured from other orbits, as opposed toregular moons,which are thought to formin situ.
isochrone: A curve on theHertzsprung–Russell diagramthat represents theevolutionarypositions of stars having the same age but differing masses. This is in contrast to anevolutionary track,which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.

J[edit]

Jeans instability: A physical state in which aninterstellar cloudof gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.
Julian year(a): A unit of time defined as exactly 365.25 days of 86,400SIseconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as thetropical year.It is therefore widely used as the basis for defining the standard astronomicalepochand thelight-year.

K[edit]

Kelvin–Helmholtz mechanism
Kepler orbit: The motion of oneorbitingbody relative to another, as anellipse,parabola,orhyperbola,which forms a two-dimensionalorbital plane(or sometimes a straight line) in three-dimensional space. Kepler orbits are idealized mathematical constructions which consider only the point-like gravitational attraction of two bodies, neglecting more complex orbitalperturbationsthat may exist in reality.
Kuiper belt: Acircumstellar discofsmall Solar System bodiessuch asasteroids,trojans,andcentaursin the outerSolar System,extending between 30 and 50AUfrom theSun.It is similar to theasteroid beltbut far larger, and is home to severaldwarf planets,includingPluto.

L[edit]

Lagrangian point: Any of a set of points near two largebodiesinorbitat which a smaller object will maintain a constant position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points thegravitational forcesof the large bodies, thecentripetal forceof orbital motion, and (in certain scenarios) theCoriolis accelerationall align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labelsL₁toL₅.The phenomenon is the basis for the stable orbits oftrojan satellitesand is commonly exploited byman-made satellites.
Laniakea Supercluster
late-type star
libration: A slight oscillating motion of theMoonas seen from the Earth, a result of theMoon's elliptical orbit.It can allow normally hidden parts of the Moon'sfar sideto be visible along thelimbsof the lunar disk.
light-year(ly): A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at thespeed of lightin vacuum would travel in oneJulian year:approximately 9.46 trillion kilometres (9.46×10¹²km) or 5.88 trillion miles (5.88×10¹²mi). Though the light-year is often used to measuregalactic-scale distances in non-specialist publications, the unit of length most commonly used in professionalastrometryis theparsec.
limb darkening: An optical effect seen in stars (including theSun), where the center part of the disk appears brighter than the edge orlimbof the image.
line of apsides: The imaginary line connecting the twoapsides(theperiapsisand theapoapsis) of anelliptical orbit,and which therefore represents the distance of the orbit's longest axis.
Local Group
longitude of the ascending node(☊ or Ω): The angle between a specified reference direction, called theorigin of longitude,and the direction of anorbit'sascending node,as measured on a specifiedplane of reference.The angle is typically measured eastwards from the reference direction to the ascending node (i.e. counterclockwise as seen from the north). It is one of six canonicalorbital elementsused to characterize an orbit.
luminosity: The total amount of energy emitted per unit time by astar,galaxy,or otherastronomical object.InSIunits, luminosity is measured injoulesper second orwatts,and is often given in terms ofastronomical magnitude.Luminosity is related to but distinct fromvisual brightness.
lunar: Of or relating to the Earth'sMoon.
lunar phase: The shape of the portion of theMoonthat is illuminated by direct sunlight as viewed from Earth. This shape is referred to as a phase because it gradually changes in a regular cycle over the course of asynodic month:as the orbital positions of the Moon around Earth and Earth around theSunchange, the visibility of the side of the Moon thatconstantly facesEarth alternates between completely illuminated (known as afull moon) and completely darkened by the Moon's own shadow (known as anew moon). There are also intermediate phases, during which the visible side may be only partially sunlit, e.g. when the Moon appears as a crescent. During the part of the lunar cycle in which the illuminated portion is growing larger, the Moon is said to bewa xing;when the illuminated portion is becoming smaller, it is said to bewaning.The phase of the Moon at any particular time appears the same from every point on Earth.

M[edit]

Massive compact halo object(MACHO): A kind ofastronomical bodythat might explain the apparent presence ofdark matteringalaxy halos.A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with anyplanetary system.Examples of MACHOs includeblack holesorneutron starsas well asbrown dwarfsandrogue planets.
magnetosphere: A mostly convex region formed when a plasma, such as thesolar wind,interacts with the magnetic field of a body, such as aplanetorstar.
magnitude: A numericallogarithmicscale indicating the brightness of anastronomical object,where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with thenaked eye,although this can vary depending on sky conditions and eyesight.
main sequence: A category ofstarswhich form a continuous and distinctive band on plots of stellar temperature versusluminosity,in particular theHertzsprung–Russell diagram.These stars are characterized by being inhydrostatic equilibriumand undergoingnuclear fusionofhydrogen-1in their core region. TheSunis a main-sequence star.
major axis: Seesemi-major axis.
March equinox: The precise time of year on Earth when the Sun appears to cross thecelestial equator,while generally trending northward at eachzenithpassage. It represents the moment at which theNorth Poleof the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is thevernal equinoxin the Northern Hemisphere and theautumnal equinoxin the Southern Hemisphere. ContrastSeptember equinox.
mean anomaly( $M$ ): The fraction of anelliptical orbit'speriodthat has elapsed since the orbiting body passedperiapsis,expressed as the angular distance from thepericenterwhich a fictitious body would have if it moved in a perfectlycircular orbitin the sameorbital periodas the actual body in its elliptical orbit. Unlike thetrue anomaly,the mean anomaly does not correspond to a real geometric angle but is instead a contrived parameter used to make calculating the position of the orbiting body in thetwo-body problemmathematically convenient.
mean-motion resonance (MMR): Seeorbital resonance.
meridian: A line running north–south across theskyand passing through the point directly overhead known as thezenith.
meridian astronomy: The measurement of positions of celestial objects based on observation of the times of theirtransitacross themeridianand of theirzenithdistance at those times, with the intention of obtaining accuratestarpositions which are self-consistent over large areas of sky.^[19]
Messier object: One of a set of 110 "nebulous"astronomical objects,103 of which were catalogued as non-cometsby French comet hunterCharles Messierbetween 1771 and 1781. The Messier catalogue includes most of thedeep-sky objectseasily visible from the Northern Hemisphere.
meteor: The visible passage of a glowingmeteoroid,micrometeoroid,comet,orasteroidthrough the Earth'satmosphere,usually as a long streak of light produced when such an object is heated toincandescenceby collisions with air molecules in the upper atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also shedding material in its wake.
meteorite: A solid piece of debris from ameteorthat originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.
meteoroid: A small rock or boulder that has entered a planetaryatmosphere.If it survives to reach the surface, it is then termed ameteorite.
meteor shower: A series ofmeteorsthat seemingly radiate from a single area in thenight sky.These are produced by debris left over from a larger body, such as acomet,and hence they follow roughly the sameorbit.This makes many meteor showers predictable events, as they recur every year.
metallicity: A measure of the abundance of elements other thanhydrogenandheliumwithin an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.
micrometeorite: A very smallmeteoritethat has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50μmto 2mm.Micrometeorites are a major component ofcosmic dust.
micrometeoroid: A very smallmeteoroid,usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed amicrometeorite.
microvariable: A stellar object such as avariable starthat undergoes very small variations inluminosity,in which the amplitude of the fluctuations amounts to just a few thousandths of amagnitude.Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.^[20]
Milky Way: 1. Thebarred spiral galaxythat includes the Earth'sSolar System.The name describes the galaxy's appearance from the Earth: a hazy band of light visible in thenight sky,formed from billions ofstarsthat cannot be individually distinguished by the naked eye. The Milky Way Galaxy has a diameter of 100,000–200,000light-yearsand is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located on the inner edge of one of the Milky Way's spiral arms, about 27,000 light-years from theGalactic Center,which theSunorbits with a period of 240 million years.; 2. The hazy band of light itself, which from Earth appears as a band because the galaxy's disk-shaped structure is viewed side-on from within.
minor axis: Seesemi-minor axis.
minor planet: An object in directorbitaround theSunthat is neither a dominantplanetnor originally classified as acomet.Amoonis not a minor planet because it orbits another body instead of the Sun.
minor-planet moon: Anatural satellitethat orbits aminor planet.See alsomoonletandsubsatellite.
molecular cloud: Aninterstellar cloudin which the prevailing physical conditions allow molecules to form, includingmolecular hydrogen.
moment of inertia factor: A dimensionless quantity that characterizes the radial distribution of mass inside a planet or moon.
moon: Seenatural satellite.
Moon: The solid, rocky body that orbits the Earth as its onlynatural satellite,completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible fortideson Earth; because oftidal locking,only one side of the Moon is ever visible from the Earth. Sunlight reflected from its surface makes the Moon appear very bright in thenight sky,though its orbital position with respect to the Earth and theSuncauses its visibility to change in a regular cycle ofphaseswhen viewed from the Earth. The adjectivallunaris often used specifically to describe the orbit, gravity, and other properties of the Earth's Moon.
moonlet: An especially smallnatural satelliteorbiting aplanet,dwarf planet,or otherminor planet.See alsominor-planet moonandsubsatellite.
moonmoon: Seesubsatellite.
Morgan–Keenan stellar classification system
morning width: The horizontal angular distance between the riseazimuthof acelestial bodyand the east direction.^[21]^[22]^[23]
moving group: A loose grouping ofstarswhich travel together through space. Although the members were formed together in the samemolecular cloud,they have since moved too far apart to be gravitationally bound as acluster.
multi-messenger astronomy: A type ofastronomybased on the acquisition of information aboutastronomical objectsthrough the coordinated observation and interpretation of four disparate classes of "messenger" signals withextrasolarorigins:electromagnetic radiation,gravitational waves,neutrinos,andcosmic rays.Because these four extrasolar messengers are created by different astrophysical processes, their presence or absence during a celestial event can reveal useful information about their sources.
multiverse

N[edit]

N galaxy: An early classification foractive galaxiesthat had the visual appearance of a galaxy with a particularly bright, star-like nucleus. As a group, they are intermediate betweenSeyfert galaxiesandQuasar.Most aregiant ellipticalsthat are radio sources and display prominentemission lines.^[24]
naked eye: The human eye as used without any magnifying or light-collecting optical aid, such as a telescope, nor any eye protection. Many astronomical objects emit or reflect visible light that is sufficiently bright to fall within the limits of normal human visual perception, allowing observers to see them from the Earth's surface without any special equipment. Vision corrected to normal acuity using eyeglasses or contact lenses is still considered unaided.
natural satellite: Anyastronomical bodythatorbitsaplanet,minor planet,or sometimes anothersmall Solar System body.
near-Earth object(NEO): Anysmall Solar System body,such as anasteroidorcomet,whose orbit brings it into proximity withEarth,generally by being less than 1.3AUfrom theSunat itsclosest approach.
nebula: Anyastronomical objectof indistinct nebulosity. In modern usage, the term typically refers to aninterstellar cloudofdust,hydrogen,helium,and otherionized gases.Historically, it was also used to refer to extended sources ofluminositythat could not be resolved into their individual components, such asstar clustersandgalaxies.
neutrino: A type of elementary particle, electrically neutral and with an extremely small rest mass, that interacts with other particles only via the weak interaction and the gravitational interaction. Neutrinos therefore typically pass through normal matter unimpeded and undetected.
neutron star: A type ofcompact starthat is composed almost entirely ofneutrons,which are a type ofsubatomic particlewith no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of theSun,but with a radius of only 12 km (7.5 mi), making them among the densest known objects in the universe.
New General Catalogue(NGC)
night sky: The appearance of the Earth'sskyatnighttime,when theSunis below thehorizon,and more specifically when clear weather and low levels of ambient light permit visibility ofcelestial objectssuch asstars,planets,and theMoon.The night sky remains a fundamental setting for both amateur and professionalobservational astronomy.
non-inclined orbit: Anyorbitthat iscoplanarwith a specifiedplane of reference,such that theorbital inclinationis 0 degrees forprogradeorbits and 180 degrees forretrogradeones.
nuclear star cluster(NSC): A compact and dense concentration of stars located at the center of a galaxy.
number density: The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically in cm³or m³.With stars, cubic parsecs (pc³) are often used.
nutation: A continuous, gravity-induced change in the orientation of anastronomical body'saxis of rotationwhich results from the combined effects of small, short-term variations. Nutation is distinguished fromprecession,which is a similar but longer-term change in axial orientation.

O[edit]

O–C diagram: A diagram ofobservedminuscalculatedvalues over time, showing how observed data differ from theoretical values which have been calculated according to a particularscientific model.It is often used as a diagnostic tool to determine the accuracy of the model. With avariable star,it is typically used to compare phase differences over time.^[25]
OB association: A group of massivestarswhich are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars ofstellar classificationsO and B.
obliquity: Seeaxial tilt.
observation arc: The duration of time between the earliest and latestobservationsmade by astronomers of an object within theSolar System,which defines the length of the path traced by the object between these same observations. The term is primarily used in the discovery and tracking ofasteroidsandcomets,which can be difficult to continuously track because of their size and great distance from Earth. Very short observation arcs, e.g. where the time between the initial observation and the most recent observation is less than 30 days, are of limited descriptive power because they represent only a very small fraction of the total path traced by the object in itsorbitaround the Sun (or otherprimary), and therefore result in a high degree ofuncertaintywhen estimating the shape and characteristics of the object's orbit.
observable universe
observational astronomy: The practice and study of directly observingastronomical objectswith the use oftelescopesand other astronomical instruments. It is concerned with recording data about theobservable universe,as opposed totheoretical astronomy,which is concerned with calculating the measurable implications of astronomical models.
occultation: A celestial event that occurs when a distantastronomical bodyorobjectis hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view.Solarandlunareclipses are specific types of occultations.
Oort cloud: A vast theoretical cloud of predominantly icyplanetesimalshypothesized to surround theSunat distances ranging from 2,000 to 200,000AU.It is thought to be divided into two regions: a disc-shapedinner Oort cloudand a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of theSolar System.
opacity: A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whetherconvectionoccurs.
open cluster: A gravitationally bound group of up to one thousand stars that formed together in the samemolecular cloud.
opposition: The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.
orbit: Thegravitationallycurved trajectory of anobject,such as the trajectory of aplanetaround astaror anatural satellitearound a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximatelyelliptical orbitsaround a commoncenter of masspositioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.
orbit plot: A schematic diagram of a completeorbit.For a binary system, it is typically presented from theprimary'sframe of reference.^[26]
orbital eccentricity: A parameter that determines how much anorbitdeviates from a perfect circle. For anelliptical orbit,the eccentricity ranges from greater than zero to less than one.
orbital elements: The set of parameters that uniquely define anorbit.
orbital inclination: The tilt of an object'sorbitaround an astronomical body, expressed as the angle between theorbital planeor axis of direction of the orbiting object and aplane of reference.
orbital mechanics
orbital node: One of two points at which theplaneof anorbitintersects a specifiedplane of referenceto which it isinclined;in some contexts, the two nodes may be distinguished as theascending nodeand thedescending node.Anon-inclined orbit,which is coplanar with the reference plane, has no nodes.
orbital period: The time a givenastronomical objecttakes to complete oneorbitaround another object. For objects in theSolar System,the orbital period is often referred to as thesidereal period.
orbital plane: The imaginarygeometric planedefined by theorbitof anastronomical bodyaround itsprimary.TheEarth's orbital plane,which defines theecliptic,is commonly used as aplane of referencefor the orbits of other objects in theSolar System.
orbital resonance: The situation that occurs when two or moreorbitingbodies exert regular, periodic gravitational influences on each other such that one or more of theirorbital parameters(e.g.eccentricity,semi-major axis,inclination,etc., or any combination thereof) exist in some definite mathematical relationship with each other. Most commonly, the term refers tomean-motion orbital resonance,in which the bodies'orbital periodsare related by a ratio of small integers. For example, thedwarf planet Plutoexists in a stable 2:3 resonance withNeptune,such that Pluto completes two orbits around theSunin the same time it takes Neptune to complete three. Resonance may act on any time scale, from short-term tosecular,and often leads to either long-term stabilization of the orbits or their eventual destabilization.
orbital speed: Thespeedat which an astronomical body or objectorbitsaround abarycenter,or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entireorbital period,or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs atperiapsis.
origin of longitude
osculating orbit: The hypothetical, idealizedKepler orbitthat an orbiting object would follow around itsprimaryif allperturbationswere absent, i.e. the orbit that coincides with the instantaneousorbital state vectorsat a given moment in time.^[27]
outer space: The vast,nearly emptyexpanse that exists beyond the Earth and between allcelestial bodies,characterized generally by extremely low densities of particles, extremely low temperatures, andminimal gravity.Most of the volume of theUniverseis intergalactic space, and even galaxies and star systems consist almost entirely of empty space.

P[edit]

parsec(pc): A unit of length defined as the distance at which a star would show aparallaxshift of exactly onearcsecondas observed from Earth's orbit. It is equal to 3.2616light-yearsor 206,265astronomical units.The word "parsec" is aportmanteauof the wordsparallaxandsecond.
partial solar eclipse
peak magnitude: For a variable star, this is the highest amplitude achieved during a rise in luminosity, followed by a decline. This data point can provide useful distance information for acataclysmic variable.It can be determined from alight curveof the stellar variability.
periapsis: The point at which an orbiting body is closest to itsprimary.Contrastapoapsis.
perigee: The point at which a body orbiting the Earth (such as theMoonor anartificial satellite) is closest to the Earth. Contrastapogee.
perihelion: The point at which a body orbiting the Earth'sSunis closest to the Sun. Contrastaphelion.
perturbation: The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of itsprimaryalone, or any force which complicates theorbitalcharacteristics of the body such that the idealizedKepler orbitof thetwo-body problemis not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/oratmospheric resistance.
phase angle: Theelongationor angle between an orbiting body and the Sun as viewed from a particular perspective, such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow.Inferior planetssuch asVenusgenerally have low phase angles as seen from Earth, so they often appear as a slim crescent;superior planetssuch asMarsandJupiterusually have high phase angles, so that little of the shadowed side is visible.
photometric system
photosphere
plane of reference: An arbitrarily chosen, imaginaryplanefrom which to measure and defineorbital elementssuch asinclinationandlongitude of the ascending node.Theecliptic plane,invariable plane,andequatorial planeare all commonly used as reference planes in various contexts.
plane of the sky: An imaginary plane that is perpendicular to the line of sight. Typically this is used as a reference plane for theinclinationof an orbital plane of a distant star system.^[28]
planet: A type ofastronomical body orbitingthe Sun, which is massive enough to beroundedby its owngravity(but not massive enough to achievethermonuclear fusion) and hascleared its neighbouring regionof allplanetesimals.The termexoplanetis used in reference to a planet-like object that is not orbiting the Sun.
planetary: Of or relating to aplanetor planets.
planetary body: Anysecondary bodythat is geologically differentiated or inhydrostatic equilibriumand therefore has aplanet-like geology, such as aplanet,dwarf planet,or otherplanetary-mass object,but excluding smaller objects such asplanetesimals.
planetary differentiation: The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).
planetary nebula: A type ofemission nebulaformed from a glowing shell of expanding plasma that has been ejected from ared giantstar late in its life. The name derives from their resemblance to aplanet.An example is theRing Nebula.
planetary science: The scientific study ofplanets,moons,andplanetary systems,with the aim of understanding their formation, composition, topography, dynamics, and interactions with other bodies.
planetary system: Any set ofgravitationallybound non-stellarobjects in or out oforbitaround astarorstar system.In general, planetary systems include one or moreplanets,though such systems may also consist ofdwarf planets,moons,asteroids,meteoroids,planetesimals,anddebris discs,among other objects.
planetary-mass object(PMO)
planetesimal: Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of aplanetwhose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gasdrag.The term is most commonly applied to small bodies thought to exist inprotoplanetary disksanddebris disksduring the process of planet formation, but is also sometimes used to refer to various types ofsmall Solar System bodieswhich are left over from the formation process. There is no precise distinction between a planetesimal and aprotoplanet.
planetoid: Another name for aminor planetordwarf planet.
polar orbit: Anorbitin which the orbiting object passes directly over or nearly over bothpolesof the body being orbited during each revolution. It therefore has aninclinationequal or nearly equal to 90 degrees to the body'sequator.
precession: Any slow change in the orientation of an object'saxis of rotation.For the Earth in particular, this phenomenon is referred to as theprecession of the equinoxes.Apsidal precessionrefers to a steady change in the orientation of an orbit, such as the precession in the orbit ofMercurythat was explained by the theory ofgeneral relativity.
precession of the equinoxes
primary: The main physical body of a gravitationally bound, multi-object system. The primary constitutes most of the system's mass and is generally located near the system'sbarycenter.
prograde motion: Orbital or rotational motion of an object in the same direction as the rotation of the object'sprimary.The direction of rotation is determined by aninertial frame of referencesuch as thefixed stars.Contrastretrograde motion.
projected separation: The observed physical separation between two astronomical objects, as determined from theirangular separationand estimated distance.^[29]For planets anddouble stars,this distance is usually given inastronomical units.The actual separation of the two objects depends on the angle of the line between the two objects to theline-of-sightof the observer.
proper motion: The rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year.
protoplanet: A largeplanetaryembryo that originated within aprotoplanetary diskand has since undergone internal melting to produce an interior of non-uniform composition. Protoplanets represent an intermediate step in the formation of a full-sizedplanet;they are thought to form out of smallerplanetesimalsas they collide with each other and gradually coalesce into larger bodies.
protoplanetary disk
protostar: A concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes apre-main-sequence star.
pseudo-synchronous rotation: For aneccentric orbit,this is a nearsynchronizationof revolution and rotation atperiastron.^[30]
pulsar: A highly magnetized rotatingneutron starorwhite dwarfthat emits a beam of electromagnetic radiation. This beam is observed only when it is pointing toward Earth, making the object appear to pulse.

Q[edit]

quadratic field strength: A method of computing the mean strength of a varyingstellar magnetic field.It is determined by calculating theroot mean squareof a series oflongitudinal magnetic field strengthmeasurements taken at different times.^[31]
quadrature: A configuration in which two celestial bodies have apparentecliptic longitudesthat differ by 90 degrees as viewed from a third body, e.g. when a planet'selongationis perpendicular to the direction of theSunas viewed from the Earth. The term is used especially to describe the position of asuperior planetor theMoonat its first and last quarter phases.
quasar: A distant, point-like energy source originating from a powerfulactive galactic nucleus.Itsluminosityis generated by theaccretionof gas onto asupermassive black hole.Quasars emit radiation across theelectromagnetic spectrumfromradio wavestoX-rays,and theirultravioletand optical spectra are characterized by strong, broademission lines.

R[edit]

radial velocity: The velocity of an object along the line of sight to the observer, which in astronomy is usually determined viaDoppler spectroscopy.Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of thegravitational perturbationof another body, or because of radial pulsations of its surface. The latter, for example, occurs with aBeta Cephei variablestar.
radio astronomy: The subfield ofastronomythat studiesastronomical objectsatradio frequencies,using largeradio antennasknown asradio telescopes.
radio source: Anyastronomical objectthat emits strongradio wavesinto space. These objects are the basis forradio astronomy.
red-giant branch: A conspicuous trail of enlarged red stars found on theHertzsprung–Russell diagramfor a typicalglobular cluster.It begins at themain-sequence turnoffpoint and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip. This branch consists of older stars that have evolved away from the main sequence but have not yet initiatedhelium fusionin theircore region.
redshift
regular moon: Anatural satellitefollowing a relatively close andprogradeorbit with little or noorbital inclinationororbital eccentricity.Regular moons are thought to formin situabout theirprimary,as opposed toirregular moons,which are thought to be captured.
relativistic jet
réseau: A grid of fine lines or crosshatches engraved upon a transparent glass plate, which when placed in front of film during a photographic exposure produces a corresponding grid in the resulting photograph by creating permanent shadows on the film negative. These grids are used in some photographic telescopes to produce reference markers in photographs of distant stars, allowing precise and convenient measurement of astrometric positions.
retrograde motion: Orbital or rotational motion of an object in the direction opposite the rotation of the object'sprimary.The direction of rotation is determined by aninertial frame of referencesuch as thefixed stars.Contrastprograde motion.
right ascension: In theequatorial coordinate system,the celestial equivalent of terrestriallongitude.It divides thecelestial equatorinto 24 hours, each of 60 minutes.
ring system: A disk- or ring-shaped accumulation of various solid material such asdustandmoonletsthat orbits anastronomical bodysuch as aplanet.Ring systems are common components ofsatellite systemsaroundgiant planets,as with theRings of Saturn.See alsocircumplanetary disk.
Roche limit: The distance from an astronomical object at which thetidal forcematches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form aring.Outside this limit, loose material will tend tocoalesce.
rogue planet: Anyplanetary-mass objectthat orbits agalactic centerdirectly rather than astarorsubstellar object.Such objects have often been ejected from theplanetary systemin which they formed, or otherwise have never been gravitationally bound to any star system.
Rosseland optical depth: Anextinction coefficientof anatmosphere,which describes the netopacityto radiation at a given depth. Seeoptical depth.^[32]
rotation period: The time that an object takes to complete a single revolution about its ownaxis of rotationrelative to thebackground stars.It is not necessarily the same as the object'ssynodic dayorsidereal day.
rotational modulation: A phenomenon which causes theluminosityof a star to vary as rotation carriesstar spotsor other localized activity across the line of sight. Examples includeRS CVnandBY Dra variables.^[33]

S[edit]

Saber's beads: A broken arc of illuminations seen at the limb of very young or very old lunar crescents. The visual similarity to the moments before and after atotal solar eclipsewas first noted by American astronomer Stephen Saber.
satellite galaxy: A smaller companiongalaxythat orbits within thegravitational potentialof a more massive and luminous host galaxy; e.g. theLarge Magellanic Cloudis a satellite galaxy of theMilky Way.
scattered disc
scintillation: Rapid variations in the apparentbrightness,color, or position of astar(or any other distant luminous object) as viewed through a medium, such as the Earth's atmosphere, caused by the passing of light through layers ofturbulencein the medium. Most terrestrial scintillation effects are the result ofatmospheric refractioncaused by small-scale fluctuations in air density, and are much more pronounced near thehorizon,since light rays near the horizon must travel longer paths through the atmosphere before reaching the observer.
secular: Continuing, or changing in a non-periodic way, over a long period of time.^[1]
secular motion: Any change in movement that happens over a very long time period.^[34]Examples include theperihelion precession of Mercury,thetidal accelerationof the Earth–Moon system, andprecession of the Earth's axis.
selenocentric: With reference to, or pertaining to, the geometric center of the Earth'sMoon.^[14]
semi-diameter: The angle at the position of an observer subtended by the equatorial radius of the Sun, the Moon, or a planet.^[14]
semi-major axis: One half of the longest diameter (the major axis) of anellipse.It is expressed in units of length and often used to give a physical dimension to a two-bodyelliptical Kepler orbit,such as for abinary starsystem or star–planet system. When the distance between the orbiting bodies is unknown, the semi-major axis may be given as an angle.
September equinox: The precise time of year on Earth when the Sun appears to cross thecelestial equator,while generally trending southward at eachzenithpassage. It represents the moment at which theNorth Poleof the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is theautumnal equinoxin the Northern Hemisphere and thevernal equinoxin the Southern Hemisphere. ContrastMarch equinox.
sidereal day: Therotation periodof an object (e.g. the Earth) with respect to the distantfixed starsof its owncelestial sphere(rather than to its primary star, e.g. theSun), measured as the time it takes for the fixed stars, as viewed from a particular point on the object's surface, to return to the same position in theskyon consecutive nights. The Earth's sidereal day is equal to approximately 86,164.09 seconds (23 hours, 56 minutes, 4.09 seconds), about four minutes shorter than thesolar day,which instead reckons time based on the Sun's position in the sky.
sidereal period: Theorbital periodof an object within theSolar System,e.g. the Earth's orbital period around the Sun. The name "sidereal" implies that the object returns to the same position relative to thefixed starsof thecelestial sphereas observed from the Earth.
sidereal time: The calculation of the passage of time based on thediurnal motionof thefixed starsin the Earth'ssky.^[14]The fundamental unit of sidereal time is thesidereal day,i.e. the time interval between two successive returns of the fixed stars to the localmeridian,as viewed from a given location on the Earth's surface.
sidereal year
sky: Everything that lies above the surface of theEarth,including theatmosphereandouter space.In the context ofastronomy,the term "sky" is also used as another name for thecelestial sphere.See alsonight sky.
small Solar System body(SSSB)
solar day: Asynodic dayon Earth, i.e. therotation periodof the Earth with respect to theSun,measured as the time it takes for the Sun, as viewed from a particular point on the Earth's surface, to return to the same position in thesky(e.g. to cross the samemeridian) on consecutive days. Because the Earth's orbit around the Sun affects the angle at which the Sun is seen from the Earth, the Sun appears to take slightly longer to return to the same position than do thefixed stars,which results in the solar day being on average about four minutes longer than thesidereal day.The length of the solar day is also not constant, but rather changes over the course of the year because the Earth's orbit isnot perfectly circularand because its rotational axis isnot perpendicularto its orbital plane. Onemeansolar day(averaged over the Earth's orbital period) is currently equal to 86,400 seconds, or exactly 24 hours.
solar eclipse: Anoccultationof theSunby the Earth'sMoon,in which a portion of the Earth passes through the shadow cast by the Moon, temporarily blocking sunlight, fully or partially, from reaching that portion of the Earth's surface. A solar eclipse occurs when the Moon isprecisely alignedbetween the Sun and the Earth. Because all three bodies are continuously moving, the shadow of the Moon traces out a narrow path across the Earth's surface, and from any given location within or very close to this path, the eclipse is visible only for a short duration. Depending on the observer's location and on the apparent sizes of the solar and lunar disks in the sky, an eclipse may appear to betotal,partial,orannular.^[14]
solar flare
solar mass(M_☉): A standard unit of mass equal to the mass of the Earth'sSun,or approximately1.98847×10³⁰kg.It is commonly used to express the masses of otherstarsand astronomical objects relative to the Sun.
solar prominence: A large, bright, transient feature, often in the shape of aloop,consisting of plasma extending outward from theSun'sphotosphereinto thecorona.Prominences may be hundreds of thousands of kilometers long.
solar radius(R_☉): A standard unit of distance equal to the radius of the Earth'sSun(typically measured from the Sun's center to the layer in thephotosphereat which theoptical depthequals 2/3), or approximately 695,700 kilometres (432,300 mi). It is commonly used to express the radii of otherstarsand astronomical objects relative to the Sun.
solar storm: Seegeomagnetic storm.
Solar System: The gravitationally boundplanetary systemof the Earth'sSunand all of the objects that orbit it, either directly or indirectly, including the eight trueplanets,fivedwarf planets,and numeroussmall Solar System bodiessuch asasteroids,comets,andnatural satellites.
solar time: The calculation of the passage of time based on thediurnal motionof theSunin the Earth'ssky.^[14]The fundamental unit of solar time is thesolar day,i.e. the time interval between two successive returns of the Sun to the localmeridian,as viewed from a given location on the Earth's surface. Because the duration of this interval changes during the Earth's orbit around the Sun,apparent solar timeis distinguished frommean solar time.Solar time andsidereal timewere employed by astronomers as time reckoning systems before the introduction ofephemeris time.
solar wind: A stream of charged particles, primarilyprotons,electrons,andAlpha particles,released from theSun'scoronaand flowing outwards at up to 900 kilometres per second (2,000,000 mph) into interplanetary space.^[1]Phenomena influenced by the solar wind includeaurora,geomagnetic storms,and the plasma tails ofcomets.
solstice: Either of the two precise times of year when theSunreaches its most northerly or most southerly point in theskyas seen from Earth; or, equivalently, when the Sun's apparent geocentriclongitudeis either 90 degrees or 270 degrees. The solstices occur on or near June 20 and December 21 each year. TheJune solstice,called thesummer solsticein the Northern Hemisphere, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime for any given point in the Northern Hemisphere; the reverse is true in the Southern Hemisphere, where the June date is thewinter solstice.
spectral classification: Seestellar classification.
spectroscopic binary: A type ofbinary starsystem where the individual components have not been resolved with atelescope.Instead, the evidence for the binarity comes from shifts observed in thespectrum.This is caused by theDoppler effectas theradial velocityof the components change over the course of each orbit.
spectroscopy
speed of light
spherical astronomy: A branch ofobservational astronomywhich is used to locate the positions of astronomical objects on thecelestial sphereas they would appear from a particular date, time, and location on Earth. It relies on the mathematical methods ofspherical geometryand the measurements ofastrometry.
spiral galaxy
standard gravity( $ɡ 0$ or $ɡ n$ ): The nominalgravitational accelerationof an object in a vacuum near the surface of the Earth, as a result ofEarth's gravityand, less importantly, thecentrifugal forcegenerated by its rotation. It is by definition equal to9.80665m/s²(approximately32.17405ft/s²).
star: A massive, luminousspheroidofplasmaheld together by its owngravitywhich, for at least a portion of its life, radiates energy intoouter spacedue to thethermonuclear fusionofhydrogenintoheliumwithin its core. Astronomers can determine the mass, age, platemperature, chemical composition, and many other properties of a star by observing its motion through space, itsluminosity,and itsemission spectrum.
star catalogue
star cluster
star system: Any small number ofstarsthat orbit each other, bound by gravitational attraction, such as abinary starsystem. In the broadest sense, very large groups of stars bound by gravitation such asstar clustersandgalaxiesare also star systems. Star systems are distinct fromplanetary systems,which includeplanetsand other bodies such ascomets.
starburst galaxy: Anygalaxythat has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as amerger.
starfield: Any set ofstarsvisible in an arbitrarily sized field of view of atelescope,usually in the context of some region of interest within thecelestial sphere.^[35]^[36]For example, the starfield surrounding the starsBetelgeuseandRigelcould be defined as encompassing some or all of theOrion constellation.
stellar: Of or relating to astarorstar system.
stellar atmosphere: The outermost region of astar,located above the stellar core, radiation zone, and convection zone. Although it constitutes only a small portion of the star's mass, for some evolved stars the stellar envelope can encompass a significant fraction of the radius.
stellar classification: The categorization ofstarsbased upon theirspectra.The modernMorgan–Keenan spectral classificationscheme is a two-dimensional classification based on temperature andluminosity.
stellar designation
stellar dynamics
stellar envelope: 1. The region within the volume of a star that transports energy from the stellar core to thestellar atmosphere;or another name for the stellar atmosphere itself.; 2. Thecommon envelopeof gases encompassing abinary starsystem.
stellar evolution
stellar evolution model: Anastrophysicalmodel of a star'sstellar evolutionover time based upon its mass and chemical composition.^[37]
stellar magnetic field: Amagnetic fieldgenerated by the convective motion ofplasmainside astar,responsible for phenomena such asstarspotsandcoronal loops.
stellar parallax
stellar remnant
submillimetre astronomy: The subfield ofastronomythat studiesastronomical objectsdetectable at submillimetre wavelengths (i.e.terahertz radiation).
subsatellite: Anynaturalorartificial satellitethat orbits another natural satellite, i.e. "a moon of a moon".
substellar object: Anastronomical objectwhose mass is smaller than the smallest mass at which thefusionof hydrogen nuclei can be sustained (equivalent to approximately 0.08solar masses), includingbrown dwarfsand somestellar remnants,as well as certainplanetary-mass objects.
Sun
supercluster
superior planet: An archaic term that is sometimes used to refer toplanetsthat orbit further from the Sun than the Earth, such asSaturn.The name originated from thegeocentric cosmologyofPtolemy.Contrastinferior planet.
supermassive black hole(SMBH): One of a class of very largeblack holeswhich possess masses ranging from hundreds of thousands to many billions of times themass of the Sun.These are typically found at agalactic core,where they can have a profound effect upon the evolution of the surroundinggalaxy.
supernova: An extremelyluminous,transientstellar explosion occurring during a massivestar's finalevolutionarystages or when awhite dwarfis triggered into runaway nuclear fusion.
surface gravity(g): Thegravitational accelerationexperienced at the equatorial surface of anastronomical bodyor otherobject,including that produced by the effects of rotation. It is typically expressed in units of acceleration such asmeters per second squared(m/s²) or as a multiple of the Earth'sstandard gravity,which is equal to9.80665m/s².
synchronous orbit: Anyorbitin which an object orbits itsprimarywith anorbital periodequal to the averagerotational periodof the primary and in thesame directionas the primary's rotation.
synodic day: The time it takes for an object to rotate once about its ownaxis(i.e. itsrotation period) relative to theprimaryit is orbiting (rather than to the much more distantfixed stars). The synodic day may be described as the time between two consecutivesunrises(in the case where the primary is astar), which is not necessarily the same as thesidereal day.As it does on Earth, an object's synodic day may change slightly in duration over the course of theorbital perioddue toeccentricityandaxial tilt;Earth's synodic day is often called asolar day.
synodic period: The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to thebackground starsvisible in the second body'scelestial sphere.Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in thesame locationin thenight skyas observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from theorbital period,a result of the fact that both the body being studied (e.g.Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (theSun).
synodic time: The calculation of the passage of time based on successiveconjunctionsof an astronomical object, such as aplanet(i.e. successive returns of the object to the sameaspectin the Earth'ssky).^[14]
syzygy: The straight-line configuration of three celestial bodies in a gravitational system.

T[edit]

tangential velocity: The component of thevelocityof a star or otherastronomical bodythat is perpendicular to the line of sight of theobserver(i.e. in the tangent plane). This component can be computed from the body's observedproper motionand its measured distance from the observer.^[19]
telescope
telluric star: Astarwith nearly featureless continuum spectra that can be used to correct for the effect oftelluric contaminationof theEarth's atmosphereon the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800Å.These features need to be corrected for in order to more accurately measure the spectrum.^[38]
termination shock: The boundary within theheliosphere,approximately 75 to 90AUfrom theSun,beyond which thesolar windslows to subsonic speeds (relative to the Sun) as a result of interactions with the localinterstellar medium.
terminator: The line that divides the illuminated side of amoonorplanetfrom its dark side. The line moves as the object rotates with respect to its parentstar.
theoretical astronomy: A branch of astronomy that uses analytical and computational models based on principles from physics and chemistry to describe, explain, and model the properties of astronomical objects and phenomena, with the ultimate goal of accurately predicting the observable or testable consequences of those models.
thick disk population
thin disk population: The layer of theMilky Waygalaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and centered on thegalactic plane.Stars belonging to this population generally follow orbits that lie close to this plane.^[39]This is in contrast to members of thethick disk populationandhalo stars.
tidal braking: The transfer of momentum between an astronomical body and an orbiting satellite as the result oftidal forces.This can cause changes in therotation periodsfor both bodies as well as modification of their mutual orbit. A satellite in aprogradeorbit will gradually recede from its primary while slowing the rotation rate of both bodies.
tidal force
tidal locking: The net result of continuedtidal brakingsuch that, over the course of an orbit, there is no net transfer ofangular momentumbetween an astronomical body and its gravitational partner. When theorbital eccentricityis low, the result is that thesatelliteorbits with the same face always pointed toward itsprimary.^[40]An example is theMoon,which is tidally locked with the Earth.
tidal stream: A stream ofstarsand gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of agalaxysuch as theMilky Way.^[41]
tilt erosion: The gradual reduction of theobliquityof an orbitingsatellitedue to tidal interactions.^[42]
Tisserand's parameter( $T$ ): A measure of the orbital motion of a relatively small body (e.g. anasteroidorcomet) with respect to a larger, perturbing body (e.g. aplanet), used forrestricted three-body problemsin which the three bodies all differ greatly in mass. The parameter is calculated from theorbital elementsof each body, including the small body'ssemimajor axis,eccentricity,andinclination,and is useful in specifically identifying small bodies observed before and after planetary encounters, as its numerical value remains largely constant throughout the body's lifetime. It is also used to distinguish between different kinds of orbits which are characteristic of different classes of bodies.^[19]
topocentric: With reference to, or pertaining to, a point on the surface of the Earth.^[14]
total solar eclipse
trans-Neptunian object(TNO)
transit: 1. The passage of a particularcelestial objectacross a particularmeridian.; 2. An astronomical event during which a celestial body or object passes visibly across the face of a much larger body. An example is thetransit of Venusacross the face of theSun,which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the netluminosityfrom the two objects, thetransit methodcan be used to detectextrasolar planetsas they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called anoccultationoreclipse.
trojan
tropical year
true anomaly( $ν$ , $θ$ ,or $f$ ): The angle between the direction ofperiapsisand the current position of an orbiting body as it moves along anelliptical orbit,as measured from the nearestfocusof the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being theeccentric anomalyand themean anomaly,and also one of six canonicalorbital elementsused to characterize an orbit.
Tully–Fisher relation: Anempirical relationshipbetween the mass or intrinsicluminosityof aspiral galaxyand itsangular velocityoremission linewidth. It can be used to estimate the distance of the galaxy, and hence forms a rung on thecosmic distance ladder.
twilight: The time period immediately before sunrise and after sunset during which, despite the Sun being completely below the horizon, the scattering of sunlight by the Earth's atmosphere supplies significant illumination to the ambient environment. Several definitions of twilight are commonly distinguished, includingastronomical,civil,andnautical twilight.^[14]
two-body problem

U[edit]

UBV photometric system
universe: 1. The entirety ofspaceandtimeand their contents, includinggalaxies,stars,planets,all other forms ofmatterandenergy,and thephysical lawsandconstantsthat describe them. When not otherwise qualified, "the Universe" usually refers to theentireUniverse, whose spatial extent is unknown because it is not directly measurable; this is distinguished from theobservable universe,whose size it is possible to measure.; 2. One of many hypothetical parallel universes which exist ascausallydisconnected constituent parts of a largermultiverse,which itself comprises all of space and time and their contents.

V[edit]

variable star: Anystarthat is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star'sradial velocityor its profile ofspectral lines.
velocity dispersion: Thestatistical dispersionof velocities about the mean velocity for a group of objects, such as stars in aglobular clusteror galaxies in agalactic cluster.This value can be used to derive the combined mass of the group by using thevirial theorem.
Virgo Supercluster(Virgo SC)

W[edit]

weak-line star: A reference to the faintness of thespectral linesfor a star compared to standard stars with the samestellar classification.Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals" —these are sometimes called metal weak stars.^[43]
white dwarf: A type ofstellar remnantcomposed mostly ofelectron-degenerate matter.A white dwarf lacks the mass needed to continue thenuclear fusionprocess with its constituent atoms, so the object's energy output normally comes from radiative cooling. SeenovaandType Ia supernova.
Wilson–Bappu effect: A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933Åand theabsolute visual magnitudeof the emittinglate-type stars.This linear relation makes it useful for determining the distances of G, K, and M-type stars.^[44]

X[edit]

XBONG: A seemingly normalgalaxythat does not appear to have anactive galactic nucleus,yet displays an anomalous level of excessX-ray emission.^[45]
X-ray source: A source ofX-rays.They are usually produced when a high-mass object, usually aneutron starorblack holeand a companion star are in abinary system.

Z[edit]

zenith: The point in theskythat is directly overhead from the perspective of a particular location on the Earth.
zero-age main sequence(ZAMS): The sequence of positions along theHertzsprung–Russell diagramachieved by newly formed, chemically homogeneousstarswhich have finished contracting and have reachedhydrostatic equilibrium,with energy being derived solely fromnuclear fusion.^[46]
zodiac: The area of theskythat extends approximately 8 degrees north or south (incelestial latitude) of theecliptic,the apparent path of theSunacross thecelestial sphereover the course of the year as observed from Earth. The Sun,Moon,and visible planets appear to travel across a band of twelveZodiac constellationswithin this belt as the Earth orbits the Sun.
zodiacal light: A band of light in the night sky, thought to be sunlight reflected from cometary dust concentrated in the plane of the zodiac, or ecliptic.

References[edit]

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^Daintith, John; Gould, William (2009).The Facts on File Dictionary of Astronomy.Infobase Publishing.ISBN 978-1438109329.
^Silaj, J.; Jones, C. E.; Tycner, C.; Sigut, T. A. A.; Smith, A. D. (March 2010), "A Systematic Study of Hα Profiles of Be Stars",The Astrophysical Journal Supplement,187(1): 228–250,Bibcode:2010ApJS..187..228S,doi:10.1088/0067-0049/187/1/228,S2CID 122679154.
^Astronomical Almanac.United States Naval Observatory.2008. Glossary.
^Salaris, Maurizio; Cassisi, Santi (2005),Evolution of stars and stellar populations,John Wiley and Sons,p. 110,ISBN 0-470-09220-3,retrieved2012-02-29.
^Ridpath, Ian (2012),A Dictionary of Astronomy,OUP Oxford,p. 163,ISBN 978-0199609055,retrieved2016-10-15.
^"Distance & Speed Of Sun's Orbit Around Galactic Centre Measured".Universe Today.2017-02-14.Retrieved2019-03-09.
^^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o"Section M: Glossary".The Astronomical Almanac Online.U.S. Naval Observatory and H.M. Nautical Almanac Office. Archived fromthe originalon 2022-02-23.Retrieved2021-11-22.
^Chebotarev, G. A. (April 1964), "Gravitational Spheres of the Major Planets, Moon and Sun",Soviet Astronomy,7:618,Bibcode:1964SvA.....7..618C.
^Sánchez Arias, J. P.; et al. (January 2017), "Asteroseismology of hybrid δ Scuti-γ Doradus pulsating stars",Astronomy & Astrophysics,597,id. A29,arXiv:1609.02864,Bibcode:2017A&A...597A..29S,doi:10.1051/0004-6361/201629126.
^Forbes, John C.; Loeb, Abraham (February 2019), "On the Existence of Brown Dwarfs More Massive than the Hydrogen Burning Limit",The Astrophysical Journal,871(2): 11,arXiv:1805.12143,Bibcode:2019ApJ...871..227F,doi:10.3847/1538-4357/aafac8,S2CID 119059288,227
^Einasto, J. (1978), "Hypergalaxies",The large scale structure of the universe; Proceedings of the Symposium, Tallinn, Estonian SSR, September 12-16, 1977. (A79-13511 03-90),vol. 79, Dordrecht, D. Reidel Publishing Co., pp. 51–60,Bibcode:1978IAUS...79...51E.
^^a ^b ^cRidpath, Ian (2007).Oxford Dictionary of Astronomy(2nd ed.). New York: Oxford University Press.ISBN 9780199214938.OL 18293428M.
^Rufener, F.; Bartholdi, P. (June 1982), "List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry",Astronomy and Astrophysics Supplement Series,48:503–511,Bibcode:1982A&AS...48..503R.
^Westlake, Hannah (2015).Astronomy for beginners: all you need to know to get started in astronomy(Third ed.). Imagine Publishing Limited. p. 151.ISBN 978-1785461477.
^Esken, Thomas."Gcal 4.1: Appendix N Glossary".gnu.org.GNU.Retrieved19 January2020.
^Umar, Khalid Bin (18 August 2019)."Al Khwarizmi: A Medieval Polymath".TAWARIKHKHWANI.tawarikhkhwani.Retrieved19 January2020.
^Miller, J. S. (December 1981), "The galaxy components of BL Lacertae objects, N systems, and quasi-stellar objects.",Publications of the Astronomical Society of the Pacific,93:681–694,Bibcode:1981PASP...93..681M,doi:10.1086/130908,S2CID 119578058.
^Sterken, C. (July 2005), Sterken, C. (ed.), "The O-C Diagram: Basic Procedures",The Light-Time Effect in Astrophysics, Proceedings of ASP Conference Series, held in Brussels 19-22 July 2004.,vol. 335, San Francisco: Astronomical Society of the Pacific, p. 3,Bibcode:2005ASPC..335....3S.
^Aitken, Robert G. (1935),The Binary Stars,New York: McGraw-Hill Book Company, pp. 70–124.
^Moulton, Forest R. (1970) [1902].Introduction to Celestial Mechanics(2nd revised ed.).Mineola, New York:Dover. pp. 322–23.ISBN 0486646874.
^Kutner, Marc Leslie (2003),Astronomy: A Physical Perspective,Cambridge University Press, p. 87,ISBN 9780521529273.
^MacEvoy, Bruce; Tirion, Wil (2015),The Cambridge Double Star Atlas,Cambridge University Press, p. 4,ISBN 978-1107534209.
^Hut, P. (June 1981), "Tidal evolution in close binary systems.",Astronomy and Astrophysics,99:126–140,Bibcode:1981A&A....99..126H.
^Bychkov, V. D.; et al. (April 2009), "Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars",Monthly Notices of the Royal Astronomical Society,394(3): 1338–1350,Bibcode:2009MNRAS.394.1338B,doi:10.1111/j.1365-2966.2008.14227.x,S2CID 120268049.
^Schrijver, C. J.; Zwaan, C. (2008),Solar and Stellar Magnetic Activity,Cambridge Astrophysics, vol. 34, Cambridge University Press, p. 25,ISBN 978-1139425421.
^Rodono, M.; et al. (September 1986), "Rotational modulation and flares on RS CVn and BY Dra-type stars. I - Photometry and SPOT models for BY Dra, AU Mic, AR Lac, II Peg and V 711 Tau (= HR 1099)",Astronomy and Astrophysics,165(1–2): 135–156,Bibcode:1986A&A...165..135R.
^Ridpath, Ian, ed. (2018),A Dictionary of Astronomy,Oxford Quick Reference Online (3 ed.), Oxford University Press,ISBN 978-0192542618.
^HubbleSOURCE: Constellation Mythological Forms,NASA,retrieved2011-10-20.
^Kepler Mission Star Field,NASA,retrieved2011-10-20.
^Andersen, J. (2000), Livio, Mario (ed.),Unsolved Problems in Stellar Evolution,Space Telescope Science Institute Symposium, vol. 12, Cambridge University Press, p. 97,ISBN 0521780918.
^Husser, Tim-Oliver (2012),3D-Spectroscopy of Dense Stellar Populations,Universitätsverlag Göttingen, pp. 54–59,ISBN 978-3863950927
^"Components of the Milky Way",Galaxies,Center for Computational Physics in Hawaii, archived fromthe originalon 2011-10-24,retrieved2012-02-27
^Barnes, Rory, ed. (2010),Formation and Evolution of Exoplanets,John Wiley & Sons,p. 248,ISBN 978-3527408962.
^Sanders, Jason (2015),Dynamics of the Milky Way: Tidal Streams and Extended Distribution Functions for the Galactic Disc,Springer Theses, Springer, p. 9,ISBN 978-3319187723.
^Heller, R.; et al. (April 2011), "Tidal obliquity evolution of potentially habitable planets",Astronomy & Astrophysics,528:16,arXiv:1101.2156,Bibcode:2011A&A...528A..27H,doi:10.1051/0004-6361/201015809,S2CID 118784209,A27.
^Jaschek, Carlos; Jaschek, Mercedes (1990),The Classification of Stars,Cambridge University Press,p. 257,ISBN 0-521-38996-8.
^Pace, G.; et al. (April 2003), "The Wilson-Bappu effect: A tool to determine stellar distances",Astronomy and Astrophysics,401(3): 997–1007,arXiv:astro-ph/0301637,Bibcode:2003A&A...401..997P,doi:10.1051/0004-6361:20030163,S2CID 17029463.
^Yuan, Feng; Narayan, Ramesh (September 2004), "On the Nature of X-Ray-Bright, Optically Normal Galaxies",The Astrophysical Journal,612(2): 724–728,arXiv:astro-ph/0401117,Bibcode:2004ApJ...612..724Y,doi:10.1086/422802,S2CID 15648424.
^Hansen, Carl J.; Kawaler, Steven D. (1999),Stellar Interiors: Physical Principles, Structure, and Evolution,Astronomy and Astrophysics Library, Springer Science & Business Media, p. 39,ISBN 038794138X.

External links[edit]

"Astronomical Glossary",A Knowledgebase for Extragalactic Astronomy and Cosmology,NASA/IPAC, January 10, 2006,retrieved2012-02-19
Astronomy Terms,Sky & Telescope Media,retrieved2018-03-09
"ESO Astronomical Glossary",Public Outreach,European Southern Observatory,retrieved2018-03-09
"Glossary",HubbleSite – Reference Desk,Space Telescope Science Institute (STScI),retrieved2018-03-09
"Glossary of (comet and) astronomical terms",International Comet Quarterly,Harvard University,retrieved2012-02-20

[Mitton-1] Mitton, Jacqueline (2007).Cambridge Illustrated Dictionary of Astronomy.Cambridge, UK: Cambridge University Press.ISBN 978-0-521-82364-7.

[Andre2008-2] Maeder, Andre (2008),Physics, Formation and Evolution of Rotating Stars,Astronomy and Astrophysics Library,Springer Science & Business Media,ISBN 978-3540769491.

[Walker1987-3] Walker, Gordon (1987),Astronomical Observations, An Optical Perspective,Cambridge University Press, p. 109,ISBN 9780521339070.

[apj_pt1_279_763-4] Noyes, R. W.; et al. (April 15, 1984), "Rotation, convection, and magnetic activity in lower main-sequence stars",Astrophysical Journal, Part 1,279:763–777,Bibcode:1984ApJ...279..763N,doi:10.1086/161945.

[Schröder2009-5] Schröder, C.; Reiners, Ansgar; Schmitt, Jürgen H. M. M. (January 2009),"Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo"(PDF),Astronomy and Astrophysics,493(3): 1099–1107,Bibcode:2009A&A...493.1099S,doi:10.1051/0004-6361:200810377.^{[permanent dead link]}

[perryman2009-6] Perryman, Michael (2009),Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data,Cambridge University Press, p. 80,ISBN 978-0521514897.

[Townsend2004-7] Townsend, R. H. D.; et al. (May 2004), "Be-star rotation: how close to critical?",Monthly Notices of the Royal Astronomical Society,350(1): 189–195,arXiv:astro-ph/0312113,Bibcode:2004MNRAS.350..189T,doi:10.1111/j.1365-2966.2004.07627.x,S2CID 14732824.

[FoF-8] Daintith, John; Gould, William (2009).The Facts on File Dictionary of Astronomy.Infobase Publishing.ISBN 978-1438109329.

[apjs187_1_228-9] Silaj, J.; Jones, C. E.; Tycner, C.; Sigut, T. A. A.; Smith, A. D. (March 2010), "A Systematic Study of Hα Profiles of Be Stars",The Astrophysical Journal Supplement,187(1): 228–250,Bibcode:2010ApJS..187..228S,doi:10.1088/0067-0049/187/1/228,S2CID 122679154.

[10] Astronomical Almanac.United States Naval Observatory.2008. Glossary.

[maurizio_santi2005-11] Salaris, Maurizio; Cassisi, Santi (2005),Evolution of stars and stellar populations,John Wiley and Sons,p. 110,ISBN 0-470-09220-3,retrieved2012-02-29.

[Ridpath2012-12] Ridpath, Ian (2012),A Dictionary of Astronomy,OUP Oxford,p. 163,ISBN 978-0199609055,retrieved2016-10-15.

[UTGD-13] "Distance & Speed Of Sun's Orbit Around Galactic Centre Measured".Universe Today.2017-02-14.Retrieved2019-03-09.

[TAAO-14] ^^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o"Section M: Glossary".The Astronomical Almanac Online.U.S. Naval Observatory and H.M. Nautical Almanac Office. Archived fromthe originalon 2022-02-23.Retrieved2021-11-22.

[Chebotarev1964-15] Chebotarev, G. A. (April 1964), "Gravitational Spheres of the Major Planets, Moon and Sun",Soviet Astronomy,7:618,Bibcode:1964SvA.....7..618C.

[Sánchez_Arias_et_al_2017-16] Sánchez Arias, J. P.; et al. (January 2017), "Asteroseismology of hybrid δ Scuti-γ Doradus pulsating stars",Astronomy & Astrophysics,597,id. A29,arXiv:1609.02864,Bibcode:2017A&A...597A..29S,doi:10.1051/0004-6361/201629126.

[Forbes_Loeb_2019-17] Forbes, John C.; Loeb, Abraham (February 2019), "On the Existence of Brown Dwarfs More Massive than the Hydrogen Burning Limit",The Astrophysical Journal,871(2): 11,arXiv:1805.12143,Bibcode:2019ApJ...871..227F,doi:10.3847/1538-4357/aafac8,S2CID 119059288,227

[Einasto1977-18] Einasto, J. (1978), "Hypergalaxies",The large scale structure of the universe; Proceedings of the Symposium, Tallinn, Estonian SSR, September 12-16, 1977. (A79-13511 03-90),vol. 79, Dordrecht, D. Reidel Publishing Co., pp. 51–60,Bibcode:1978IAUS...79...51E.

[Ridpath-19] Ridpath, Ian (2007).Oxford Dictionary of Astronomy(2nd ed.). New York: Oxford University Press.ISBN 9780199214938.OL 18293428M.

[Rufener1982-20] Rufener, F.; Bartholdi, P. (June 1982), "List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry",Astronomy and Astrophysics Supplement Series,48:503–511,Bibcode:1982A&AS...48..503R.

[AFB2015-21] Westlake, Hannah (2015).Astronomy for beginners: all you need to know to get started in astronomy(Third ed.). Imagine Publishing Limited. p. 151.ISBN 978-1785461477.

[Esken2000-22] Esken, Thomas."Gcal 4.1: Appendix N Glossary".gnu.org.GNU.Retrieved19 January2020.

[Umar2019-23] Umar, Khalid Bin (18 August 2019)."Al Khwarizmi: A Medieval Polymath".TAWARIKHKHWANI.tawarikhkhwani.Retrieved19 January2020.

[Miller1981-24] Miller, J. S. (December 1981), "The galaxy components of BL Lacertae objects, N systems, and quasi-stellar objects.",Publications of the Astronomical Society of the Pacific,93:681–694,Bibcode:1981PASP...93..681M,doi:10.1086/130908,S2CID 119578058.

[Sterken_2005-25] Sterken, C. (July 2005), Sterken, C. (ed.), "The O-C Diagram: Basic Procedures",The Light-Time Effect in Astrophysics, Proceedings of ASP Conference Series, held in Brussels 19-22 July 2004.,vol. 335, San Francisco: Astronomical Society of the Pacific, p. 3,Bibcode:2005ASPC..335....3S.

[Aitken1935-26] Aitken, Robert G. (1935),The Binary Stars,New York: McGraw-Hill Book Company, pp. 70–124.

[27] Moulton, Forest R. (1970) [1902].Introduction to Celestial Mechanics(2nd revised ed.).Mineola, New York:Dover. pp. 322–23.ISBN 0486646874.

[Kutner_2003-28] Kutner, Marc Leslie (2003),Astronomy: A Physical Perspective,Cambridge University Press, p. 87,ISBN 9780521529273.

[McEvol_Tirion2015-29] MacEvoy, Bruce; Tirion, Wil (2015),The Cambridge Double Star Atlas,Cambridge University Press, p. 4,ISBN 978-1107534209.

[30] Hut, P. (June 1981), "Tidal evolution in close binary systems.",Astronomy and Astrophysics,99:126–140,Bibcode:1981A&A....99..126H.

[Bychkova2009-31] Bychkov, V. D.; et al. (April 2009), "Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars",Monthly Notices of the Royal Astronomical Society,394(3): 1338–1350,Bibcode:2009MNRAS.394.1338B,doi:10.1111/j.1365-2966.2008.14227.x,S2CID 120268049.

[Schrijver_Zwaan2008-32] Schrijver, C. J.; Zwaan, C. (2008),Solar and Stellar Magnetic Activity,Cambridge Astrophysics, vol. 34, Cambridge University Press, p. 25,ISBN 978-1139425421.

[Rodono1986-33] Rodono, M.; et al. (September 1986), "Rotational modulation and flares on RS CVn and BY Dra-type stars. I - Photometry and SPOT models for BY Dra, AU Mic, AR Lac, II Peg and V 711 Tau (= HR 1099)",Astronomy and Astrophysics,165(1–2): 135–156,Bibcode:1986A&A...165..135R.

[Ridpath2018-34] Ridpath, Ian, ed. (2018),A Dictionary of Astronomy,Oxford Quick Reference Online (3 ed.), Oxford University Press,ISBN 978-0192542618.

[CMF-35] HubbleSOURCE: Constellation Mythological Forms,NASA,retrieved2011-10-20.

[KMSF-36] Kepler Mission Star Field,NASA,retrieved2011-10-20.

[Andersen2000-37] Andersen, J. (2000), Livio, Mario (ed.),Unsolved Problems in Stellar Evolution,Space Telescope Science Institute Symposium, vol. 12, Cambridge University Press, p. 97,ISBN 0521780918.

[Husser2012-38] Husser, Tim-Oliver (2012),3D-Spectroscopy of Dense Stellar Populations,Universitätsverlag Göttingen, pp. 54–59,ISBN 978-3863950927

[c2h2-39] "Components of the Milky Way",Galaxies,Center for Computational Physics in Hawaii, archived fromthe originalon 2011-10-24,retrieved2012-02-27

[Heller2011a-40] Barnes, Rory, ed. (2010),Formation and Evolution of Exoplanets,John Wiley & Sons,p. 248,ISBN 978-3527408962.

[sanders2015-41] Sanders, Jason (2015),Dynamics of the Milky Way: Tidal Streams and Extended Distribution Functions for the Galactic Disc,Springer Theses, Springer, p. 9,ISBN 978-3319187723.

[Heller2011b-42] Heller, R.; et al. (April 2011), "Tidal obliquity evolution of potentially habitable planets",Astronomy & Astrophysics,528:16,arXiv:1101.2156,Bibcode:2011A&A...528A..27H,doi:10.1051/0004-6361/201015809,S2CID 118784209,A27.

[jaschek1990-43] Jaschek, Carlos; Jaschek, Mercedes (1990),The Classification of Stars,Cambridge University Press,p. 257,ISBN 0-521-38996-8.

[Pace2003-44] Pace, G.; et al. (April 2003), "The Wilson-Bappu effect: A tool to determine stellar distances",Astronomy and Astrophysics,401(3): 997–1007,arXiv:astro-ph/0301637,Bibcode:2003A&A...401..997P,doi:10.1051/0004-6361:20030163,S2CID 17029463.

[Yuan2004-45] Yuan, Feng; Narayan, Ramesh (September 2004), "On the Nature of X-Ray-Bright, Optically Normal Galaxies",The Astrophysical Journal,612(2): 724–728,arXiv:astro-ph/0401117,Bibcode:2004ApJ...612..724Y,doi:10.1086/422802,S2CID 15648424.

[Hansen1999-46] Hansen, Carl J.; Kawaler, Steven D. (1999),Stellar Interiors: Physical Principles, Structure, and Evolution,Astronomy and Astrophysics Library, Springer Science & Business Media, p. 39,ISBN 038794138X.

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