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This article is about the unit of length. For other uses, seeLight year (disambiguation).

Light-year
Map showing stars and star systems lying within 12.5 light-years of theSun[1]
General information
Unit systemastronomy units
Unit oflength
Symbolly[2]
Conversions
1 ly[2]in...... is equal to...
metric(SI) units
  • 9.4607×1015m
  • 9.46073Pm
imperialandUSunits
  • 5.8786×1012mi
astronomical units

Alight-year,alternatively spelledlight year(ly), is aunit of lengthused to expressastronomical distancesand is equal to exactly 9,460,730,472,580.8 km (Scientific notation:9.4607304725808 × 1012km), which is approximately 5.88 trillion mi. As defined by theInternational Astronomical Union(IAU), a light-year is the distance thatlight travels in a vacuumin oneJulian year(365.25 days).[2]Because it includes the word "year", the term is sometimes misinterpreted as aunit of time.[3]

Thelight-yearis most often used when expressing distances to stars and other distances on agalacticscale, especially innon-specialistcontexts andpopular sciencepublications.[3]The unit most commonly used in professionalastronomyis theparsec(symbol: pc, about 3.26 light-years).[2]

Definitions[edit]

As defined by theInternational Astronomical Union(IAU), the light-year is the product of theJulian year[note 1](365.25 days, as opposed to the 365.2425-dayGregorian yearor the 365.24219-dayTropical yearthat both approximate) and thespeed of light(299792458m/s).[note 2]Both of these values are included in theIAU (1976) System of Astronomical Constants,used since 1984.[5]From this, the following conversions can be derived. The IAU-recognized abbreviation for light-year is "ly",[2]although other standards likeISO 80000:2006(now superseded) have used "l.y."[6][7]and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (fromannée-lumière,año luzandanno luce,respectively), "Lj" in German (fromLichtjahr), etc.

1 light-year =9460730472580800metres (exactly)
≈9.461petametres
≈9.461trillion kilometres (5.879trillion miles)
≈63241.077astronomical units
≈0.306601parsec

Before 1984, thetropical year(not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.[8]The product ofSimon Newcomb'sJ1900.0mean tropical year of31556925.9747ephemeris secondsand a speed of light of299792.5 km/sproduced a light-year of9.460530×1015m(rounded to the sevensignificant digitsin the speed of light) found in several modern sources[9][10][11]was probably derived from an old source such asC. W. Allen's 1973Astrophysical Quantitiesreference work,[12]which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).[13]

Other high-precision values are not derived from acoherentIAU system. A value of9.460536207×1015mfound in some modern sources[14][15]is the product of a mean Gregorian year (365.2425 days or31556952s) and the defined speed of light (299792458m/s). Another value,9.460528405×1015m,[16]is the product of the J1900.0 mean tropical year and the defined speed of light.

Abbreviations used for light-years and multiples of light-years are

  • "ly" for one light-year[2]
  • "kly" for a kilolight-year (1,000 light-years)[17]
  • "Mly" for a megalight-year (1,000,000 light-years)[18]
  • "Gly" for a gigalight-year (1,000,000,000 light-years)[19]

History[edit]

The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, byFriedrich Besselin 1838. The star was61 Cygni,and he used a 160-millimetre (6.2 in)heliometredesigned byJoseph von Fraunhofer.The largest unit for expressing distances across space at that time was theastronomical unit,equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni'sparallaxof 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9×1013km; 6.1×1013mi). Bessel added that light takes 10.3 years to traverse this distance.[20]He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

The speed of light was not yet precisely known in 1838; the estimate of its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through theaetheror space was still Enigma tic.

The light-year unit appeared in 1851 in a German popular astronomical article byOtto Ule.[21]Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name.[22]In 1868 an English journal labelled the light-year as a unit used by the Germans.[23]Eddingtoncalled the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.[24]

Although modern astronomers often prefer to use theparsec,light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.

Usage of term[edit]

Distances expressed in light-years include those betweenstarsin the same general area, such as those belonging to the samespiral armorglobular cluster.Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies andgalaxy clustersby millions of light-years. Distances to objects such asquasarsand theSloan Great Wallrun up into the billions of light-years.

List of orders of magnitude forlength
Scale (ly) Value Item
10−9 4.04×10−8ly Reflected sunlight from theMoon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly350000to400000kilometres).
10−6 1.58×10−5ly Oneastronomical unit(the distance from theSunto the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.[25]
1.27×10−4ly TheHuygensprobelands onTitanoffSaturnand transmits images from its surface, 1.2 billion kilometres from Earth.
5.04×10−4ly New HorizonsencountersPlutoat a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−3 2.04×10−3ly The most distantspace probe,Voyager 1,was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014.[26]It will take about17500years to reach one light-year at its current speed of about 17 km/s (38000mph,61 200 km/h) relative to the Sun. On 12 September 2013, NASA scientists announced thatVoyager 1had entered theinterstellar mediumof space on 25 August 2012, becoming the first manmade object to leave theSolar System.[27]
2.28×10−3ly Voyager 1as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.
100 1.6×100ly TheOort cloudis approximately two light-years in diameter. Its inner boundary is speculated to be at50000au≈ 0.8 ly, with its outer edge at100000au≈ 1.6 ly.
2.0×100ly Approximate maximum distance at which an object can orbit theSun(Hill sphere/Roche sphere,125000au). Beyond this is the deep ex-solar gravitationalinterstellar medium.
4.24×100ly The nearest knownstar(other than the Sun),Proxima Centauri,is about 4.24 light-years away.[28][29]
8.6×100ly Sirius,the brightest star of the night sky. Twice as massive and 25 times moreluminousthan the Sun, it outshines more luminous stars due to its relative proximity.
1.19×101ly Tau Ceti e,an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of starTau Ceti.[30][31]
2.05×101ly Gliese 581,a red-dwarf star with several detectable exoplanets.
3.1×102ly Canopus,second in brightness in the terrestrial sky only to Sirius, a type A9bright giant10700times more luminous than the Sun.
103 3×103ly A0620-00,the second-nearest knownblack hole,is about3000light-years away.
2.6×104ly Thecentreof theMilky Wayis about26000light-years away.[32][33]
1×105ly TheMilky Wayis about100000light-years across.
1.65×105ly R136a1,in theLarge Magellanic Cloud,the most luminous star known at 8.7 million times theluminosityof the Sun, has an apparent magnitude 12.77, just brighter than3C 273.
106 2.5×106ly TheAndromeda Galaxyis approximately 2.5 million light-years away.
3×106ly TheTriangulum Galaxy(M33), at about 3 million light-years away, is the most distant object visible to the naked eye.
5.9×107ly The nearest largegalaxy cluster,theVirgo Cluster,is about 59 million light-years away.
1.5×108–2.5×108ly TheGreat Attractorlies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).
109 1.2×109ly TheSloan Great Wall(not to be confused withGreat WallandHer–CrB GW) has been measured to be approximately one billion light-years distant.
2.4×109ly 3C 273,optically the brightestquasar,of apparent magnitude 12.9, just dimmer thanR136a1.3C 273 is about 2.4 billion light-years away.
4.57×1010ly Thecomoving distancefrom the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comovingradiusof theobservable universe.This is larger than theage of the universedictated by thecosmic background radiation;seeherefor why this is possible.

Related units[edit]

Distances between objects within astar systemtend to be small fractions of a light-year, and are usually expressed inastronomical units.However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, thelight-second,useful in astronomy, telecommunications and relativistic physics, is exactly299792458metres or1⁄31557600of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used inpopular sciencepublications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[34][35]TheHayden Planetariumspecifies the light month more precisely as 30 days of light travel time.[36]

Light travels approximately one foot in ananosecond;the term "light-foot" is sometimes used as an informal measure of time.[37]

See also[edit]

Notes[edit]

  1. ^One Julian year is exactly 365.25 days (or31557600sbased on a day of exactly86400SIseconds)[4]
  2. ^The speed of light is precisely299792458m/sby definition of the metre.

References[edit]

  1. ^"The Universe within 12.5 Light Years – The Nearest stars".atlasoftheuniverse.Retrieved2 April2022.
  2. ^abcdeInternational Astronomical Union,Measuring the Universe: The IAU and Astronomical Units,retrieved10 November2013
  3. ^abBruce McClure (31 July 2018)."How far is a light-year?".EarthSky.Retrieved15 October2019.
  4. ^IAU Recommendations concerning Units,archived fromthe originalon 16 February 2007
  5. ^"Selected Astronomical ConstantsArchived2014-07-26 at theWayback Machine"inAstronomical Almanac,p. 6.
  6. ^ISO 80000-3:2006 Quantities and Units – Space and Time
  7. ^IEEE/ASTM SI 10-2010, American National Standard for Metric Practice
  8. ^P. Kenneth Seidelmann, ed. (1992),Explanatory Supplement to the Astronomical Almanac,Mill Valley, California: University Science Books, p. 656,ISBN978-0-935702-68-2
  9. ^Basic Constants,Sierra College
  10. ^Marc Sauvage,Table of astronomical constants,archived fromthe originalon 11 December 2008
  11. ^Robert A. Braeunig,Basic Constants
  12. ^C. W. Allen (1973),Astrophysical Quantities(third ed.), London: Athlone, p. 16,ISBN978-0-485-11150-7
  13. ^Arthur N. Cox, ed. (2000),Allen's Astrophysical Quantities(fourth ed.), New York: Springer-Valeg, p. 12,ISBN978-0-387-98746-0
  14. ^Nick Strobel,Astronomical Constants
  15. ^KEKB,Astronomical Constants,archived fromthe originalon 9 September 2007,retrieved5 November2008
  16. ^Thomas Szirtes (1997),Applied dimensional analysis and modeling,New York: McGraw-Hill, p. 60,ISBN978-0-07-062811-3
  17. ^Comins, Neil F. (2013),Discovering the Essential Universe(fifth ed.), W. H. Freeman, p. 365,ISBN978-1-4292-5519-6
  18. ^Hassani, Sadri (2010),From Atoms to Galaxies,CRC Press, p. 445,ISBN978-1-4398-0850-4
  19. ^Deza, Michel Marie;Deza, Elena(2016),Encyclopedia of Distances(fourth ed.), Springer, p. 620,ISBN978-3-662-52843-3
  20. ^Bessel, Friedrich (1839)."On the parallax of the star 61 Cygni".London and Edinburgh Philosophical Magazine and Journal of Science.14:68–72.Bessel's statement that light employs 10.3 years to traverse the distance.
  21. ^Ule, Otto (1851)."Was wir in den Sternen lesen".Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben.1:721–738.
  22. ^Diesterweg, Adolph Wilhelm (1855).Populäre Himmelskunde u. astronomische Geographie.p. 250.
  23. ^The Student and Intellectual Observer of Science, Literature and Art.Vol. 1. London: Groombridge and Sons. 1868. p. 240.
  24. ^"Stellar movements and the structure of the universe".Retrieved1 November2014.
  25. ^"Chapter 1, Table 1-1",IERS Conventions (2003)
  26. ^WHERE ARE THE VOYAGERS?,retrieved14 October2014
  27. ^NASA Spacecraft Embarks on Historic Journey Into Interstellar Space,retrieved14 October2014
  28. ^NASA,Cosmic Distance Scales – The Nearest Star
  29. ^"Proxima Centauri (Gliese 551)",Encyclopedia of Astrobiology, Astronomy, and Spaceflight
  30. ^"Tau Ceti's planets nearest around single, Sun-like star".BBC News.19 December 2012.Retrieved1 November2014.
  31. ^Tuomi, Mikko; Jones, Hugh R. A.; Jenkins, James S.; Tinney, Chris G.; Butler, R. Paul; Vogt, Steve S.; Barnes, John R.; Wittenmyer, Robert A.; O'Toole, Simon; Horner, Jonathan; Bailey, Jeremy; Carter, Brad D.; Wright, Duncan J.; Salter, Graeme S.; Pinfield, David (March 2013)."Signals embedded in the radial velocity noise: periodic variations in the τ Ceti velocities"(PDF).Astronomy & Astrophysics.551:A79.arXiv:1212.4277.Bibcode:2013A&A...551A..79T.doi:10.1051/0004-6361/201220509.S2CID2390534.
  32. ^Eisenhauer, F.; Schdel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T. (2003), "A Geometric Determination of the Distance to the Galactic Center",The Astrophysical Journal,597(2): L121,arXiv:astro-ph/0306220,Bibcode:2003ApJ...597L.121E,doi:10.1086/380188,S2CID16425333
  33. ^McNamara, D. H.;Madsen, J. B.; Barnes, J.; Ericksen, B. F. (2000), "The Distance to the Galactic Center",Publications of the Astronomical Society of the Pacific,112(768): 202,Bibcode:2000PASP..112..202M,doi:10.1086/316512
  34. ^Fujisawa, K.; Inoue, M.; Kobayashi, H.; Murata, Y.; Wajima, K.; Kameno, S.; Edwards, P. G.; Hirabayashi, H.; Morimoto, M. (2000),"Large Angle Bending of the Light-Month Jet in Centaurus A",Publications of the Astronomical Society of Japan,52(6): 1021–26,Bibcode:2000PASJ...52.1021F,doi:10.1093/pasj/52.6.1021,archived fromthe originalon 2 September 2009
  35. ^Junor, W.; Biretta, J. A. (1994), "The Inner Light-Month of the M87 Jet", in Zensus, J. Anton; Kellermann; Kenneth I. (eds.),Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11–12, 1994,Green Bank, WV: National Radio Astronomy Observatory (NRAO), p. 97,Bibcode:1994cers.conf...97J
  36. ^Light-Travel Time and Distance by the Hayden PlanetariumAccessed October 2010.
  37. ^ David Mermin(2009).It's About Time: Understanding Einstein's Relativity.Princeton, New Jersey: Princeton University Press. p. 22.ISBN978-0-691-14127-5.

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