Light-year
Light-year | |
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General information | |
Unit system | astronomy units |
Unit of | length |
Symbol | ly[2] |
Conversions | |
1 ly[2] in ... | ... is equal to ... |
Pm | |
imperial and US units |
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astronomical units |
A light-year, alternatively spelled light year (ly), is a
The light-year is most often used when expressing distances to stars and other distances on a
Definitions
As defined by the
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.1 light-year = 9460730472580800 metres (exactly) ≈ 9.461 petametres≈ 9.461 trillion kilometres (5.879 trillion miles) ≈ 63241.077 astronomical units ≈ 0.306601 parsec
Before 1984, the
Other high-precision values are not derived from a
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
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 160-millimetre (6.2 in) 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 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9×1013 km; 6.1×1013 mi). 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 the aether or space was still enigmatic.
The light-year unit appeared in 1851 in a German popular astronomical article by Otto 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] 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
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.[25] |
1.27×10−4 ly | The 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[update].[26] 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.[27]
|
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, with its outer edge at 100000 au. |
2.0×100 ly | Maximum extent of the 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.[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 | ||
2.05×101 ly | Gliese 581, a red-dwarf star with several detectable exoplanets. | |
3.1×102 ly | bright giant 10700 times more luminous than the Sun.
| |
103 | 3×103 ly | A0620-00, the second-nearest known black hole, is about 3000 light-years away. |
2.6×104 ly | The centre of the Milky Way is about 26000 light-years away.[32][33] | |
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 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 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.
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Related units
Distances between objects within a
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 petametre (examples of distances on the order of one light-year)
- Einstein protocol
- Hubble length
- Orders of magnitude (length)
Notes
References
- ^ "The Universe within 12.5 Light Years – The Nearest stars". www.atlasoftheuniverse.com. Retrieved 2 April 2022.
- ^ a b c d e International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013
- ^ a b Bruce McClure (31 July 2018). "How far is a light-year?". EarthSky. Retrieved 15 October 2019.
- ^ IAU Recommendations concerning Units, archived from the original on 16 February 2007
- ^ "Selected Astronomical Constants Archived 2014-07-26 at the Wayback Machine" in Astronomical Almanac, p. 6.
- ^ ISO 80000-3:2006 Quantities and Units – Space and Time
- ^ IEEE/ASTM SI 10-2010, American National Standard for Metric Practice
- ISBN 978-0-935702-68-2
- ^ Basic Constants, Sierra College
- ^ Marc Sauvage, Table of astronomical constants, archived from the original on 11 December 2008
- ^ Robert A. Braeunig, Basic Constants
- ISBN 978-0-485-11150-7
- ISBN 978-0-387-98746-0
- ^ Nick Strobel, Astronomical Constants
- ^ KEKB, Astronomical Constants, archived from the original on 9 September 2007, retrieved 5 November 2008
- ISBN 978-0-07-062811-3
- ISBN 978-1-4292-5519-6
- ISBN 978-1-4398-0850-4
- ISBN 978-3-662-52843-3
- ^ 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.
- ^ Ule, Otto (1851). "Was wir in den Sternen lesen". Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben. 1: 721–738.
- ^ Diesterweg, Adolph Wilhelm (1855). Populäre Himmelskunde u. astronomische Geographie. p. 250.
- ^ The Student and Intellectual Observer of Science, Literature and Art. Vol. 1. London: Groombridge and Sons. 1868. p. 240.
- ^ "Stellar movements and the structure of the universe". Retrieved 1 November 2014.
- ^ "Chapter 1, Table 1-1", IERS Conventions (2003)
- ^ WHERE ARE THE VOYAGERS?, retrieved 14 October 2014
- ^ NASA Spacecraft Embarks on Historic Journey Into Interstellar Space, retrieved 14 October 2014
- ^ NASA, Cosmic Distance Scales – The Nearest Star
- ^ "Proxima Centauri (Gliese 551)", Encyclopedia of Astrobiology, Astronomy, and Spaceflight
- ^ "Tau Ceti's planets nearest around single, Sun-like star". BBC News. 19 December 2012. Retrieved 1 November 2014.
- S2CID 2390534.
- S2CID 16425333
- doi:10.1086/316512
- doi:10.1093/pasj/52.6.1021, archived from the originalon 2 September 2009
- Bibcode:1994cers.conf...97J
- ^ Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010.
- ^
ISBN 978-0-691-14127-5.
External links
- The dictionary definition of light-year at Wiktionary