SafekipediaLight-year
Adapted from Wikipedia · Discoverer experience
A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances. It is equal to exactly 9460730472580.8 km, which is about 9.46 trillion kilometres or 5.88 trillion miles. 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). Even though it uses the word "year," a light-year is not a unit of time; it is a way to measure huge distances.
The light-year is very useful when talking about distances to stars and other objects in space on a large scale, especially in non-specialist contexts and popular science publications. For example, the nearest star to our Sun, Proxima Centauri, is about 4.24 light-years away. This means that the light we see from Proxima Centauri today actually left that star 4.24 years ago!
In professional astronomy, scientists often use another unit called the parsec (pc), which is about 3.26 light-years. Both units help us understand and discuss the vast distances in our universe.
Definitions
As defined by the International Astronomical Union, the light-year is the distance light travels in one Julian year (365.25 days) at the speed of light (299,792,458 meters per second). This makes it a way to measure huge distances in space.
We use special abbreviations for light-years and their multiples:
- "ly" for one light-year
- "kly" or "klyr" for a thousand light-years
- "Mly" for a million light-years
- "Gly" or "Glyr" for a billion light-years
| 1 light-year | = 9460730472580800 metres (exactly) |
| c. 9.461 petametres | |
| c. 9.461 trillion (short scale) kilometres (5.879 trillion miles) | |
| c. 63241.077 astronomical units | |
| c. 0.306601 parsec |
History
The idea of a light-year started appearing not long after scientists first measured how far a star is from Earth. In 1838, a scientist named Friedrich Bessel figured out the distance to a star called 61 Cygni using a special tool called a heliometre. At that time, the biggest unit people used for space distances was the astronomical unit, which is the distance from the Earth to the Sun.
Later, in 1851, a German writer named Otto Ule used the term “light-year” in an article to help people imagine how far light travels in a year. Even though some scientists thought it wasn’t the best unit, many people still use light-years today to talk about distances between stars and galaxies. Some scientists now prefer using a unit called the parsec, but light-years remain popular.
Usage of term
Distances in space are often measured in light-years. For example, stars that are close to each other, like those in the same spiral arm or globular cluster, might be a few light-years apart. Galaxies, which are huge collections of stars, can be thousands to hundreds of thousands of light-years wide. The space between galaxies or galaxy clusters can be millions of light-years. Even very faraway objects like quasars and the Sloan Great Wall are billions of light-years from us.
Main article: Spiral arm
Main articles: Galaxy cluster
Further information: Quasars
Further information: Sloan Great Wall
| Scale (ly) | Value | Item |
|---|---|---|
| 10−9 | 4.04×10−8 ly | Reflected 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). |
| 10−6 | 1.58×10−5 ly | 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. |
| 1.27×10−4 ly | The 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 of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth. | |
| 10−3 | 2.04×10−3 ly | 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 2014. It will take about 17500 years to reach one light-year at its current speed of about 17 km/s (38000 mph, 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. |
| 2.28×10−3 ly | Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth. | |
| 100 | 1.6×100 ly | The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 au ≈ 0.8 ly, with its outer edge at 100000 au ≈ 1.6 ly. |
| 2.0×100 ly | Approximate maximum distance at which an object can orbit the Sun (Hill sphere/Roche sphere, 125000 au). Beyond this is the deep ex-solar gravitational interstellar medium. | |
| 4.24×100 ly | The nearest known star (other than the Sun), Proxima Centauri, is about 4.24 light-years away. | |
| 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 | 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. | |
| 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. | |
| 103 | 1.56×103 ly | Gaia BH1, the nearest known black hole, is about 1560 light-years away. |
| 3.3×103 ly | A0620-00, the fifth-nearest known black hole, is about 3300 light-years away. | |
| 2.6×104 ly | The centre of the Milky Way is about 26000 light-years away. | |
| 1×105 ly | The 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. | |
| 106 | 2.5×106 ly | The Andromeda Galaxy is approximately 2.5 million light-years away. |
| 3×106 ly | The Triangulum Galaxy (M33), at about 3 million light-years away, is the most distant object visible to the naked eye. | |
| 5.9×107 ly | The nearest large galaxy cluster, the Virgo Cluster, is about 59 million light-years away. | |
| 1.5×108 – 2.5×108 ly | The Great Attractor lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate). | |
| 109 | 1.2×109 ly | 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. |
| 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. | |
| 1.353×1010 ly - 3.38×1010 ly | MoM-z14, is the farthest confirmed galaxy discovered as of 2025, with an apparent magnitude of 20.2. It is about 13.53 billion light-years away in light travel distance, and 33.8 billion light-years away in proper distance. | |
| 4.57×1010 ly | 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 are usually smaller than a light-year and are often measured in astronomical units. We can also create smaller distance units by multiplying time by the speed of light. For example, a light-second is the distance light travels in one second and is equal to 299,792,458 meters, or about 1/31,557,600 of a light-year. Other units like the light-minute, light-hour, and light-day are sometimes used in popular science books and shows. The light-month, which is roughly one-twelfth of a light-year, is also used sometimes for rough estimates. The Hayden Planetarium defines a light-month more precisely as the distance light travels in 30 days.
Light travels about one foot in a nanosecond, and the term "light-foot" is sometimes used informally to measure very short time intervals.
Images
Related articles
This article is a child-friendly adaptation of the Wikipedia article on Light-year, available under CC BY-SA 4.0.
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