Light-year

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A light-year, alternatively spelled light year (ly or lyr[1]), is a unit of length used to express astronomical distances and is equal to exactly Script error: No such module "val"., which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days).[2] Despite its inclusion of the word "year", the term should not be misinterpreted as a unit of time.[3]

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications.[3] The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years).[2]

Definitions

As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian yearTemplate:Refn (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light (Script error: No such module "val".).[note 1] Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.[4] From this, the following conversions can be derived:

1 light-year   = Script error: No such module "val". metres (exactly)
c.Script error: No such module "Check for unknown parameters". Script error: No such module "val". petametres
c.Script error: No such module "Check for unknown parameters". Script error: No such module "val". trillion (short scale) kilometres (Script error: No such module "val". trillion miles)
c.Script error: No such module "Check for unknown parameters". Script error: No such module "val". astronomical units
c.Script error: No such module "Check for unknown parameters". Script error: No such module "val". parsec

The abbreviation used by the IAU for light-year is "ly",[2] International standards like ISO 80000:2006 (now superseded) have used "l.y."[5][6] and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (from année-lumière, año luz and anno luce, respectively), "Lj" in German (from Lichtjahr), etc.

Before 1984, the tropical 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.[7] The product of Simon Newcomb's J1900.0 mean tropical year of Script error: No such module "val". ephemeris seconds and a speed of light of Script error: No such module "val". produced a light-year of Script error: No such module "val". (rounded to the seven significant digits in the speed of light) found in several modern sources[8][9][10] was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,[11] which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).[12]

Other high-precision values are not derived from a coherent IAU system. A value of Script error: No such module "val". found in some modern sources[13][14] is the product of a mean Gregorian year (365.2425 days or Script error: No such module "val".) and the defined speed of light (Script error: No such module "val".). Another value, Script error: No such module "val".,[15] is the product of the J1900.0 mean tropical year and the defined speed of light.

Script error: No such module "anchor".Abbreviations used for light-years and multiples of light-years are:

  • "ly" for one light-year[2]
  • "kly" [16] or "klyr"[17] for a kilolight-year (1,000 light-years)
  • "Mly" for a megalight-year (1,000,000 light-years)[18]
  • "Gly"[19] or "Glyr"[20] for a gigalight-year (Script error: No such module "val". light-years)

History

The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a Script error: No such module "convert". heliometre designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at Script error: No such module "convert".. In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be Script error: No such module "convert".. Bessel added that light takes 10.3 years to traverse this distance.[21] 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 the aether or space was still enigmatic.

The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule.[22] 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.[23] In 1868 an English journal labelled the light-year as a unit used by the Germans.[24] Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.[25]

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

Usage of term

Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.

List of orders of magnitude for length
Scale (ly) Value Item
10−9 Script error: No such module "val". Reflected sunlight from the Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly Script error: No such module "val". to Script error: No such module "val". kilometres).
10−6 Script error: No such module "val". One astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.[26]
Script error: No such module "val". The Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.
Script error: No such module "val". New Horizons encounters Pluto at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−3 Script error: No such module "val". The most distant space 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 2014Script error: No such module "Check for unknown parameters"..[27] It will take about Script error: No such module "val". years to reach one light-year at its current speed of about 17 km/s (Script error: No such module "val"., 61 200 km/h) relative to the Sun. On 12 September 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on 25 August 2012, becoming the first manmade object to leave the Solar System.[28]
Script error: No such module "val". Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.
100 Script error: No such module "val". The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at Script error: No such module "val". ≈ 0.8 ly, with its outer edge at Script error: No such module "val". ≈ 1.6 ly.
Script error: No such module "val". Approximate maximum distance at which an object can orbit the Sun (Hill sphere/Roche sphere, Script error: No such module "val".). Beyond this is the deep ex-solar gravitational interstellar medium.
Script error: No such module "val". The nearest known star (other than the Sun), Proxima Centauri, is about 4.24 light-years away.[29][30]
Script error: No such module "val". Sirius, the brightest star of the night sky. Twice as massive and 25 times more luminous than the Sun, it outshines more luminous stars due to its relative proximity.
Script error: No such module "val". 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 star Tau Ceti.[31][32]
Script error: No such module "val". Gliese 581, a red-dwarf star with several detectable exoplanets.
Script error: No such module "val". Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9 bright giant Script error: No such module "val". times more luminous than the Sun.
103 Script error: No such module "val". A0620-00, the second-nearest known black hole, is about Script error: No such module "val". light-years away.
Script error: No such module "val". The centre of the Milky Way is about Script error: No such module "val". light-years away.[33][34]
Script error: No such module "val". The Milky Way is about Script error: No such module "val". light-years across.
Script error: No such module "val". R136a1, in the Large Magellanic Cloud, the most luminous star known at 8.7 million times the luminosity of the Sun, has an apparent magnitude 12.77, just brighter than 3C 273.
106 Script error: No such module "val". The Andromeda Galaxy is approximately 2.5 million light-years away.
Script error: No such module "val". The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.
Script error: No such module "val". The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.
Script error: No such module "val". – Script error: No such module "val". The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).
109 Script error: No such module "val". The Sloan Great Wall (not to be confused with Great Wall and Her–CrB GW) has been measured to be approximately one billion light-years distant.
Script error: No such module "val". 3C 273, optically the brightest quasar, of apparent magnitude 12.9, just dimmer than R136a1. 3C 273 is about 2.4 billion light-years away.
Script error: No such module "val". The comoving distance from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving radius of the observable universe. This is larger than the age of the universe dictated by the cosmic background radiation; see here for why this is possible.

Related units

Distances between objects within a star system tend to be small fractions of a light-year, and are usually expressed in astronomical units. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly Script error: No such module "val". metres or Template:Sfrac of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.[35][36] The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.[37]

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.[38]

See also

Notes

Template:NoteFoot

References

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  1. ↑ Script error: No such module "Citation/CS1".
  2. ↑ a b c d Script error: No such module "citation/CS1".
  3. ↑ a b Script error: No such module "citation/CS1".
  4. ↑ "Selected Astronomical Constants Script error: No such module "webarchive"." in Astronomical Almanac, p. 6.
  5. ↑ ISO 80000-3:2006 Quantities and Units – Space and Time
  6. ↑ IEEE/ASTM SI 10-2010, American National Standard for Metric Practice
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".
  9. ↑ Script error: No such module "citation/CS1".
  10. ↑ Script error: No such module "citation/CS1".
  11. ↑ Script error: No such module "citation/CS1".
  12. ↑ Script error: No such module "citation/CS1".
  13. ↑ Script error: No such module "citation/CS1".
  14. ↑ Script error: No such module "citation/CS1".
  15. ↑ Script error: No such module "citation/CS1".
  16. ↑ Script error: No such module "citation/CS1".
  17. ↑ Viollier 1994
  18. ↑ Script error: No such module "citation/CS1".
  19. ↑ Script error: No such module "citation/CS1".
  20. ↑ Sanchez et al. 2022
  21. ↑ Script error: No such module "Citation/CS1". Bessel's statement that light employs 10.3 years to traverse the distance.
  22. ↑ Script error: No such module "Citation/CS1".
  23. ↑ Script error: No such module "citation/CS1".
  24. ↑ Script error: No such module "citation/CS1".
  25. ↑ Script error: No such module "citation/CS1".
  26. ↑ Script error: No such module "citation/CS1".
  27. ↑ Script error: No such module "citation/CS1".
  28. ↑ Script error: No such module "citation/CS1".
  29. ↑ Script error: No such module "citation/CS1".
  30. ↑ Script error: No such module "citation/CS1".
  31. ↑ Script error: No such module "citation/CS1".
  32. ↑ Script error: No such module "Citation/CS1".
  33. ↑ Script error: No such module "citation/CS1".
  34. ↑ Script error: No such module "citation/CS1".
  35. ↑ Script error: No such module "citation/CS1".
  36. ↑ Script error: No such module "citation/CS1".
  37. ↑ Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.
  38. ↑ Script error: No such module "citation/CS1".

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External links

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