Last updated
Map showing the stars that lie within 12.5 light-years of the Sun [1]
General information
Unit system astronomy units
Unit oflength
Symbolly [2]
1 ly [2] in ...... is equal to ...
    metric (SI) units   9.4607×1015  m
   9.4607  Pm
    imperial & US units   5.8786×1012  mi
    astronomical units    63241  au
   0.3066  pc

The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). [note 1] 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] Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.

Unit of length Reference value of length

A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.

The kilometre, or kilometer is a unit of length in the metric system, equal to one thousand metres. It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the road network of the United Kingdom where the statute mile is the official unit used.

Mile Unit of length

The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.


The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc). [2]

Galaxy astronomical structure

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Popular science is an interpretation of science intended for a general audience. While science journalism focuses on recent scientific developments, popular science is more broad-ranging. It may be written by professional science journalists or by scientists themselves. It is presented in many forms, including books, film and television documentaries, magazine articles, and web pages.

Astrometry part of astronomy, covers star positions and their movements

Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the Milky Way.


As defined by the IAU, the light-year is the product of the Julian year [note 2] (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s). [note 3] 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. The IAU recognized abbreviation for light-year is ly, [2] although other standards like ISO 80000 use "l.y." [5] [6] and localized symbols are frequent, such as "al" in French (from année-lumière) and Spanish (from año luz), "Lj" in German (from Lichtjahr), etc.

The Gregorian calendar is the calendar used in most of the world. It is named after Pope Gregory XIII, who introduced it in October 1582. The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.

An astronomical constant is a physical constant used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964 and in 1976. In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decided to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates. The set of constants is widely reproduced in publications such as the Astronomical Almanac of the United States Naval Observatory and HM Nautical Almanac Office.

1 light-year  = 9460730472580800 metres (exactly)
9.461 petametres
9.461 trillion kilometres
5.878625 trillion miles
63241.077 astronomical units
0.306601 parsecs

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 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530×1015 m (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]

A tropical year is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Because of the precession of the equinoxes, the seasonal cycle does not remain exactly synchronized with the position of the Earth in its orbit around the Sun. As a consequence, the tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars.

Simon Newcomb American astronomer

Simon Newcomb was a Canadian–American astronomer, applied mathematician and autodidactic polymath, who was Professor of Mathematics in the U.S. Navy and at Johns Hopkins.

Clabon Walter (Cla) Allen FRAS, was an Australian astronomer, director of the University of London Observatory and author of Astrophysical Quantities.

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources [13] [14] is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405×1015 m, [15] [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


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 6.2-inch (160 mm) heliometer 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 (1.50×108 km or 9.30×107 mi). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9×1013 km or 6.1×1013 mi). Bessel added that light employs 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 resented expressing distances in light-years because it would deteriorate 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; 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, however, in 1851 in a German popular astronomical article by Otto Ule. [21] The paradox of a distance unit name ending on "year" was explained by Ule by comparing it to a hiking road 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] Eddington called 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 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)ValueItem
10−94.04×10−8 lyReflected sunlight from the Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly 350000 to 400000 kilometres).
10−61.58×10−5 lyOne astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance. [25]
1.27×10−4 lyThe Huygens probe lands on Titan off Saturn and transmits images from its surface, 1.2 billion kilometres from Earth.
5.04×10−4 ly New Horizons encounters Pluto at a distance 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.
10−32.04×10−3 lyThe most distant space probe, Voyager 1, was about 18 light-hours away from the Earth as of October 2014. [26] It will take about 17500 years to reach one light-year (1.0×100 ly) at its current speed of about 17 km/s (38000 mph) relative to the Sun. On September 12, 2013, NASA scientists announced that Voyager 1 had entered the interstellar medium of space on August 25, 2012, becoming the first manmade object to leave the Solar System. [27]
2.28×10−3 lyVoyager 1 as of October 2018, nearly 20 light-hours from the Earth
1001.6×100 lyThe Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 au, with its outer edge at 100000 au.
2.0×100 lyMaximum extent of the Sun's gravitational dominance (Hill sphere/Roche sphere, 125000 au). Beyond this is the deep ex-solar gravitational interstellar medium.
4.22×100 lyThe nearest known star (other than the Sun), Proxima Centauri, is about 4.22 light-years away. [28] [29]
8.6×100 ly 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.
1.19×101 ly HD 10700 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. [30] [31]
2.05×101 ly Gliese 581, a red-dwarf star with several detectable exoplanets.
3.1×102 ly Canopus, second in brightness in the terrestrial sky only to Sirius, a type A9 bright giant 10700 times more luminous than the Sun.
1033×103 ly A0620-00, the nearest known black hole, is about 3000 light-years away.
2.6×104 lyThe centre of the Milky Way is about 26000 light-years away. [32] [33]
1×105 lyThe Milky Way is about 100000 light-years across.
1.65×105 ly 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.
1062.5×106 lyThe Andromeda Galaxy is approximately 2.5 million light-years away.
3×106 lyThe Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye.
5.9×107 lyThe nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away.
1.5×1082.5×108 lyThe Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).
1091.2×109 lyThe 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.
2.4×109 ly 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.
4.57×1010 lyThe 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.

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 299792458 metres or 131557600 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. [34] [35] The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. [36]

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

See also


  1. One trillion here is taken to be 1012 (one million million, or billion in long scale).
  2. One Julian year is of exactly 365.25 days (or 31557600 s based on a day of exactly 86400 SI seconds) [3]
  3. The speed of light is precisely 299792458 m/s by definition of the metre.

Related Research Articles

Astronomical unit mean distance between Earth and the Sun, common length reference in astronomy

The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres, or about 150 million kilometres. The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

Polaris star in the constellation Ursa Minor

Polaris, designated α Ursae Minoris, commonly the North Star or Pole Star, is the brightest star in the constellation of Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years, while calculations by other methods derive distances around 30% closer.

Parsec unit of length used in astronomy

The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 648000/π astronomical units. One parsec is equal to about 3.26 light-years or 31 trillion kilometres (31×1012 km) or 19 trillion miles (19×1012 mi). The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun.

Speed of light speed at which all massless particles and associated fields travel in vacuum

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second. It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.

61 Cygni binary star in the Cygnus constellation

61 Cygni is a binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 years. Of apparent magnitude 5.20 and 6.05, respectively, they can be seen with binoculars in city skies or with the naked eye in rural areas without photopollution.

Hubbles law observation in physical cosmology

Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that:

  1. Objects observed in deep space—extragalactic space, 10 megaparsecs (Mpc) or more—are found to have a redshift, interpreted as a relative velocity away from Earth;
  2. This Doppler shift-measured velocity of various galaxies receding from the Earth is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.
Albireo star in the constellation Cygnus

Albireo is the traditional name for the double star also designated Beta Cygni, although the International Astronomical Union now regards the name as only applying to the brightest component. Although designated 'beta', it is fainter than Gamma Cygni, Delta Cygni, and Epsilon Cygni and is the fifth-brightest point of light in the constellation of Cygnus. Appearing to the naked eye to be a single star of magnitude 3, viewing through even a low-magnification telescope resolves it into its two components. The brighter yellow star makes a striking colour contrast with its fainter blue companion.

Andromeda Galaxy spiral galaxy within the Local Group

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky in which it appears, the constellation of Andromeda.

Messier 87 Galaxy in the Virgo Galactic Cluster

Messier 87 is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it has a large population of globular clusters—about 12,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.

The astronomical system of units, formally called the IAU (1976) System of Astronomical Constants, is a system of measurement developed for use in astronomy. It was adopted by the International Astronomical Union (IAU) in 1976, and has been significantly updated in 1994 and 2009.

Centaurus A peculiar galaxy in the constellation Centaurus

Centaurus A or NGC 5128 is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target, although the galaxy is only visible from low northern latitudes and the southern hemisphere.

S Doradus Variable star in the Large Magellanic Cloud

S Doradus is located 160,000 light years away, and is one of the brightest stars in the Large Magellanic Cloud (LMC), a satellite of the Milky Way. It is a Luminous Blue Variable and one of the most luminous stars known, but so far away that it is invisible to the naked eye.

Messier 32 Elliptical galaxy in the constellation Andromeda

Messier 32 is a dwarf "early-type" galaxy located about 2.65 million light-years from Earth, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil in 1749. M32 measures 6.5 ± 0.2 thousand light-years in diameter at the widest point.

Messier 106 Galaxy

Messier 106 is an intermediate spiral galaxy in the constellation Canes Venatici. It was discovered by Pierre Méchain in 1781. M106 is at a distance of about 22 to 25 million light-years away from Earth. M106 contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole has been demonstrated from radio-wavelength observations of the rotation of a disk of molecular gas orbiting within the inner light-year around the black hole. NGC 4217 is a possible companion galaxy of Messier 106. A Type II supernova was observed in M106 in May 2014.

53 Arietis is a variable star in the northern constellation of Aries. 53 Arietis is the Flamsteed designation; it also bears the variable star designation UW Arietis. It is a B-type main sequence star with a stellar classification of B1.5 V and mean apparent magnitude of 6.10, which is near the lower limit for naked eye visibility. Based upon an annual parallax shift of 3.92 mas, the estimated distance to this star is roughly 800 light-years.

Theta Cygni Star in the constellation Cygnus

Theta Cygni is a star in the northern constellation of Cygnus. It has an apparent visual magnitude of 4.5, so it can be seen from the northern hemisphere with the naked eye in sufficiently dark skies. Based upon parallax measurements, it is at a distance of about 59.8 light-years from the Earth. It is suspected of hosting an extrasolar planet.

16 Cygni Bb or HD 186427 b is an extrasolar planet approximately 69 light-years away in the constellation of Cygnus. The planet was discovered orbiting the Sun-like star 16 Cygni B, one of two solar-mass (M) components of the triple star system 16 Cygni. It makes one revolution every 799 days and was the first eccentric Jupiter and planet in a triple star system to be discovered.

SS Cygni star

SS Cygni is a variable star in the northern constellation Cygnus. It is the prototype of the subclass of dwarf novae that show only normal eruptions. It typically rises from 12th magnitude to 8th magnitude for 1–2 days every 7 or 8 weeks. The northerly declination of SS Cygni makes the star almost circumpolar from European and North American latitudes, allowing a large proportion of the world's amateur astronomers to monitor its behavior. Furthermore, since the star lies against the rich backdrop of the Milky Way band, the telescope field of view around SS Cygni contains an abundance of useful brightness comparison stars.

NML Cygni red hypergiant star

NML Cygni or V1489 Cygni is a red hypergiant (RHG) or red supergiant (RSG) in the constellation Cygnus. It is one of the largest stars currently known, and is also one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way. If placed at the center of the Solar System, its surface would extend past the orbit of Jupiter or Saturn.


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