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Spiral galaxy

(Redirected fromSpiral nebula)
This article is about the class of galaxy. For the type of logic puzzle, seeSpiral Galaxies (puzzle).

Spiral galaxiesform aclass of galaxyoriginally described byEdwin Hubblein his 1936 workThe Realm of the Nebulae[1]and, as such, form part of theHubble sequence.Most spiral galaxies consist of a flat, rotatingdiskcontainingstars,gas and dust,and a central concentration of stars known as thebulge.These are often surrounded by a much fainterhaloof stars, many of which reside inglobular clusters.

An example of a spiral galaxy, theMessier 77(also known as NGC 1068)

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hotOB starsthat inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,[2]extending from the central bulge, at the ends of which the spiral arms begin. The proportion ofbarred spiralsrelative tobarless spiralshas likely changed over the history of theuniverse,with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of thegalaxiesin the visible universe (Hubble volume) have bars.[3]

TheMilky Wayis a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.[4]The most convincing evidence for the stars forming a bar in theGalactic Centercomes from several recent surveys, including theSpitzer Space Telescope.[5]

Together withirregular galaxies,spiral galaxies make up approximately 60% of galaxies in today's universe.[6]They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[7]

Structure

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Tuning-fork-style diagram of the Hubble sequence

Spiral galaxies may consist of several distinct components:

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms

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Main article:Spiral arm
Barred spiral galaxyUGC 12158

Spiral armsare regions of stars that extend from the center of barred andunbarredspiralgalaxies.These long, thin regions resemble aspiraland thus give spiral galaxies their name. Naturally, differentclassifications of spiral galaxieshave distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

Bulge

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Main article:Galactic bulge

Abulgeis a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.

NGC 1300ininfraredlight

Using the Hubble classification, the bulge of Sa galaxies is usually composed ofPopulation II stars,which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller[9]and are composed of young, bluePopulation I stars.Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host asupermassive black holeat their centers. In our own galaxy, for instance, the object calledSagittarius A*is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence ofactive nucleiin some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such asMessier 106.

Bar

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Spiral galaxy NGC 2008

Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.[10][11]Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures[12][13]which typically have a maximum visibility at half the length of the in-plane bar.

Spheroid

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19 face-on spiral galaxies from theJames Webb Space Telescopein near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that have not yet fully formed and are encased in gas and dust appear bright red.[14]

The bulk of the stars in a spiral galaxy are located either close to a single plane (thegalactic plane) in more or less conventional circularorbitsaround the center of the galaxy (theGalactic Center), or in aspheroidalgalactic bulge around the galactic core.

However, some stars inhabit aspheroidal haloorgalactic spheroid,a type ofgalactic halo.The orbital behaviour of these stars is disputed, but they may exhibitretrogradeand/or highlyinclinedorbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into andmergewith the spiral galaxy—for example, theSagittarius Dwarf Spheroidal Galaxyis in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free ofdust,and in further contrast, stars in the galactic halo are ofPopulation II,much older and with much lowermetallicitythan theirPopulation Icousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of smallred dwarfsclose to theSunare thought to belong to the galactic halo, for exampleKapteyn's StarandGroombridge 1830.Due to their irregular movement around the center of the galaxy, these stars often display unusually highproper motion.

Oldest spiral galaxies

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BRI 1335-0417is the oldest and most distant known spiral galaxy, as of 2024.The galaxy has aredshiftof 4.4, meaning its light took 12.4 billion years to reach Earth.[15]

The oldest grand design spiral galaxy on file isBX442.At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companiondwarf galaxy.Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.[16][17]

A1689B11is an extremely old spiral galaxy located in theAbell 1689galaxy cluster in the Virgo constellation.[18]A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang.[19][20]

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In June 2019,citizen scientiststhroughGalaxy Zooreported that theusual Hubble classification,particularly concerningspiral galaxies,may not be supported, and may need updating.[21][22]

Origin of the spiral structure

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Spiral galaxyNGC 6384taken byHubble Space Telescope
The spiral galaxyNGC 1084,home of fivesupernovae[23]

History

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The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms wasBertil Lindbladin 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called thewinding problem.Measurements in the late 1960s showed that theorbital velocity of stars in spiral galaxieswith respect to their distance from the galactic center is indeed higher than expected fromNewtonian dynamicsbut still cannot explain the stability of the spiral structure.

Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:

  • star formation caused bydensity wavesin thegalactic diskof the galaxy.
  • the stochastic self-propagating star formation model (SSPSF model) – star formation caused by shock waves in the interstellar medium. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.

These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.

Density wave model

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Main article:Density wave theory
Animation of orbits as predicted by the density wave theory, which explains the existence of stable spiral arms. Stars move in and out of the spiral arms as they orbit the galaxy.

Bertil Lindbladproposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.[24]

Historical theory of Lin and Shu

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Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of slightly elliptical orbits

The first acceptable theory for the spiral structure was devised byC. C. LinandFrank Shuin 1964,[25]attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.[26]

Star formation caused by density waves

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The following hypotheses exist for star formation caused by density waves:

  • As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (theJeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
  • As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
  • As clouds get swept up by the spiral arms, they collide with one another and driveshock wavesthrough the gas, which in turn causes the gas to collapse and form stars.

More young stars in spiral arms

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Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,[27]their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.[24]

Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.[24]As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.[26]

Gravitationally aligned orbits

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Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate themolecular cloudsin whichnew starsform, and evolution towards grand-design bisymmetric spirals is explained.[28]

Distribution of stars in spirals

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The similar distribution of stars in spirals

The stars in spirals are distributed in thin disks radial with intensity profiles such that[29][30][31]

withbeing the disk scale-length;is the central value; it is useful to define:as the size of the stellar disk, whose luminosity is

.

The spiral galaxies light profiles, in terms of the coordinate,do not depend on galaxy luminosity.

Spiral nebula

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Drawing of theWhirlpool Galaxyby Rosse in 1845

Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to asspiral nebulae,due toLord Rosse,whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the "Whirlpool Galaxy",and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern inMessier 99andMessier 33respectively. In 1850 he made the first drawing ofAndromeda Galaxy's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula.[32]

The question of whether such objects were separate galaxies independent of the Milky Way, or a type ofnebulaexisting within our own galaxy, was the subject of theGreat Debateof 1920, betweenHeber CurtisofLick ObservatoryandHarlow ShapleyofMount Wilson Observatory.Beginning in 1923,Edwin Hubble[33][34]observedCepheid variablesin several spiral nebulae, including the so-called"Andromeda Nebula",proving that they are, in fact, entire galaxies outside our own. The termspiral nebulahas since fallen out of use.

Milky Way

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Milky Way Galaxy's spiral arms and barred core – based onWISEdata

TheMilky Waywas once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.[35][36]Their suspicions were confirmed bySpitzer Space Telescopeobservations in 2005,[37]which showed that the Milky Way's central bar is larger than what was previously suspected.

Famous examples

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Further information:List of spiral galaxies

See also

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Classification

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Other

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References

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  1. ^Hubble, E.P.(1936).The realm of the nebulae.Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven:Yale University Press.ISBN9780300025002.OCLC611263346.Alt URL(pp. 124–151)
  2. ^D. Mihalas (1968).Galactic Astronomy.W. H. Freeman.ISBN978-0-7167-0326-6.
  3. ^"Hubble and Galaxy Zoo Find Bars and Baby Galaxies Don't Mix".Science Daily.16 January 2014.
  4. ^"Ripples in a Galactic Pond".Scientific American.October 2005. Archived fromthe originalon 6 September 2013.
  5. ^R. A. Benjamin; E. Churchwell; B. L. Babler; R. Indebetouw; M. R. Meade; B. A. Whitney; C. Watson; M. G. Wolfire; M. J. Wolff; R. Ignace; T. M. Bania; S. Bracker; D. P. Clemens; L. Chomiuk; M. Cohen; J. M. Dickey; J. M. Jackson; H. A. Kobulnicky; E. P. Mercer; J. S. Mathis; S. R. Stolovy; B. Uzpen (September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy".The Astrophysical Journal Letters.630(2): L149–L152.arXiv:astro-ph/0508325.Bibcode:2005ApJ...630L.149B.doi:10.1086/491785.S2CID14782284.
  6. ^Loveday, J. (February 1996)."The APM Bright Galaxy Catalogue".Monthly Notices of the Royal Astronomical Society.278(4): 1025–1048.arXiv:astro-ph/9603040.Bibcode:1996MNRAS.278.1025L.doi:10.1093/mnras/278.4.1025.
  7. ^Dressler, A. (March 1980)."Galaxy morphology in rich clusters — Implications for the formation and evolution of galaxies".The Astrophysical Journal.236:351–365.Bibcode:1980ApJ...236..351D.doi:10.1086/157753.
  8. ^Salucci, P. (2019). "The distribution of dark matter in galaxies".The Astronomy and Astrophysics Review.27(1): 2.arXiv:1811.08843.Bibcode:2019A&ARv..27....2S.doi:10.1007/s00159-018-0113-1.
  9. ^Alister W. Graham and C. Clare Worley (2008),Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations
  10. ^de Vaucouleurs, G.; de Vaucouleurs, A.; Corwin, H. G., Jr.; Buta, R. J.; Paturel, G.; Fouqué, P. (2016),Third Reference Catalogue of Bright Galaxies
  11. ^B.D. Simmons et al. (2014),Galaxy Zoo: CANDELS barred discs and bar fractions
  12. ^Astronomy Now (8 May 2016), Astronomers detect double ‘peanut shell’ galaxies
  13. ^Bogdan C. Ciambur and Alister W. Graham (2016),Quantifying the (X/peanut)-shaped structure in edge-on disc galaxies: length, strength, and nested peanuts
  14. ^"Webb reveals structure in 19 spiral galaxies".esa.int.Retrieved30 January2024.
  15. ^"ALMA Spots Candidate for Most Distant Known Spiral Galaxy".sci-news. 20 May 2021.Retrieved20 May2021.
  16. ^Oldest spiral galaxy is a freak of cosmoshttp:// zmescience /space/oldest-spiral-galaxy-31321/
  17. ^Gonzalez, Robert T. (19 July 2012)."Hubble Has Spotted an Ancient Galaxy That Shouldn't Exist".io9.Retrieved10 September2012.
  18. ^"[BBC2005] Source 11 -- Galaxy".24 June 2018.
  19. ^"The most ancient spiral galaxy confirmed".PhysOrg. 3 November 2017.
  20. ^Tiantian Yuan; Johan Richard; Anshu Gupta; Christoph Federrath; Soniya Sharma; Brent A. Groves; Lisa J. Kewley; Renyue Cen; Yuval Birnboim; David B. Fisher (30 October 2017)."The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field".The Astrophysical Journal.850(1): 61.arXiv:1710.11130.Bibcode:2017ApJ...850...61Y.doi:10.3847/1538-4357/aa951d.S2CID119267114.
  21. ^Royal Astronomical Society(11 June 2019)."Citizen scientists re-tune Hubble's galaxy classification".EurekAlert!.Retrieved11 June2019.
  22. ^Masters, Karen L.; et al. (30 April 2019)."Galaxy Zoo: unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding".Monthly Notices of the Royal Astronomical Society.487(2): 1808–1820.arXiv:1904.11436.Bibcode:2019MNRAS.487.1808M.doi:10.1093/mnras/stz1153.
  23. ^"A spiral home to exploding stars".ESA / Hubble.Retrieved2 April2014.
  24. ^abcBelkora, L. (2003).Minding the Heavens: the Story of our Discovery of the Milky Way.CRC Press.p. 355.ISBN978-0-7503-0730-7.
  25. ^Lin, C. C.; Shu, F. H. (August 1964). "On the spiral structure of disk galaxies".The Astrophysical Journal.140:646–655.Bibcode:1964ApJ...140..646L.doi:10.1086/147955.
  26. ^abHenbest, Nigel (1994),The Guide to the Galaxy,Cambridge University Press,p. 74,ISBN9780521458825,Lin and Shu showed that this spiral pattern would persist more or less for ever, even though individual stars and gas clouds are always drifting into the arms and out again.
  27. ^"Main Sequence Lifetime".Swinburne Astronomy Online.Swinburne University of Technology.Retrieved8 June2019.
  28. ^Francis, C.; Anderson, E. (2009). "Galactic spiral structure".Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.465(2111): 3425–3446.arXiv:0901.3503.Bibcode:2009RSPSA.465.3425F.doi:10.1098/rspa.2009.0036.S2CID12461073.
  29. ^F. Shirley Patterson (1940),The Luminosity Gradient of Messier 33
  30. ^Gerard de Vaucouleurs (1957),Studies of the Magellanic Clouds. III. Surface brightness, colors and integrated magnitudes of the Clouds.
  31. ^Freeman, K. C. (1970)."On the Disks of Spiral and so Galaxies".Astrophysical Journal.160:811.Bibcode:1970ApJ...160..811F.doi:10.1086/150474.
  32. ^Alexander, S. On the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulae. Astronomical Journal, vol. 2, iss. 37, p. 97-103 (1852)
  33. ^"NASA - Hubble Views the Star That Changed the Universe".
  34. ^Hubble, E. P.(May 1926). "A spiral nebula as a stellar system: Messier 33".The Astrophysical Journal.63:236–274.Bibcode:1926ApJ....63..236H.doi:10.1086/142976.
  35. ^Gerard de Vaucouleurs (1964),Interpretation of velocity distribution of the inner regions of the Galaxy
  36. ^Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL26Al map features ".Space Science Reviews.120:315–316.Bibcode:1996A&AS..120C.315C.
  37. ^McKee, Maggie (16 August 2005)."Bar at Milky Way's heart revealed".New Scientist.Retrieved17 June2009.
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