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.

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]

Map showing the stars that lie within 12.5 light-years of the Sun[1]
General information
Unit systemastronomy units
Unit oflength
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


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.

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]

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

  • "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]


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) 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 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.
10−3 2.04×10−3 ly The 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 ly Voyager 1 as of October 2018, nearly 20 light-hours 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 Sun's gravitational dominance (Hill sphere/Roche sphere, 125000 au). Beyond this is the deep ex-solar gravitational interstellar medium.
4.22×100 ly The 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.
103 3×103 ly A0620-00, the 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.
1.5×1082.5×108 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.
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 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.


  1. ^ The Universe within 12.5 Light Years: The Nearest Stars
  2. ^ a b c d e International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013
  3. ^ IAU Recommendations concerning Units, archived from the original on 2007-02-16
  4. ^ "Selected Astronomical Constants" 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. ^ P. Kenneth Seidelmann, ed. (1992), Explanatory Supplement to the Astronomical Almanac, Mill Valley, California: University Science Books, p. 656, ISBN 978-0-935702-68-2
  8. ^ Basic Constants, Sierra College
  9. ^ Marc Sauvage, Table of astronomical constants, archived from the original on 2008-12-11
  10. ^ Robert A. Braeunig, Basic Constants
  11. ^ C. W. Allen (1973), Astrophysical Quantities (third ed.), London: Athlone, p. 16, ISBN 978-0-485-11150-7
  12. ^ Arthur N. Cox, ed. (2000), Allen's Astrophysical Quantities (fourth ed.), New York: Springer-Valeg, p. 12, ISBN 978-0-387-98746-0
  13. ^ Nick Strobel, Astronomical Constants
  14. ^ KEKB, Astronomical Constants
  15. ^ Thomas Szirtes (1997), Applied dimensional analysis and modeling, New York: McGraw-Hill, p. 60, ISBN 9780070628113
  16. ^ Sun, Moon, and Earth: Light-year
  17. ^ Comins, Neil F. (2013), Discovering the Essential Universe (fifth ed.), W. H. Freeman, p. 365, ISBN 978-1-4292-5519-6
  18. ^ Hassani, Sadri (2010), From Atoms to Galaxies, CRC Press, p. 445, ISBN 978-1-4398-0850-4
  19. ^ Deza, Michel Marie; Deza, Elena (2016), Encyclopedia of Distances (fourth ed.), Springer, p. 620, ISBN 978-3-662-52843-3
  20. ^ Bessel, Friedrich (1839). "On the parallax of the star 61 Cygni". London and Edinburgh Philosophical Magazine and Journal of Science. 14: 68–72. Retrieved 4 April 2014. 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. Retrieved 4 April 2014.
  22. ^ Diesterweg, Adolph Wilhelm (1855). Populäre Himmelskunde u. astronomische Geographie. p. 250.
  23. ^ The Student and Intellectual Observer of Science, Literature and Art. 1868. Retrieved 1 November 2014.
  24. ^ "Stellar movements and the structure of the universe". Retrieved 1 November 2014.
  25. ^ "Chapter 1, Table 1-1", IERS Conventions (2003)
  26. ^ WHERE ARE THE VOYAGERS?, retrieved 14 October 2014
  27. ^ NASA Spacecraft Embarks on Historic Journey Into Interstellar Space, retrieved 14 October 2014
  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. Retrieved 1 November 2014.
  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.
  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
  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 from the original on 2009-09-02
  35. ^ Junor, W.; Biretta, J. A. (1994), "The Inner Light-Month of the M87 Jet", in Zensus, J. Anton; Kellermann; Kenneth I., 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 Planetarium Accessed October 2010.
  37. ^ David Mermin (2009). It's About Time: Understanding Einstein's Relativity. Princeton, New Jersey: Princeton University Press. p. 22. ISBN 978-0-691-14127-5.

External links

  • The dictionary definition of light-year at Wiktionary
Angular diameter

The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture (of a lens). The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side. Angular radius equals half the angular diameter.

Astronomical unit

The astronomical unit (symbol: au, ua, or AU) 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 (93 million miles). 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.

Bok globule

In astronomy, Bok globules are isolated and relatively small dark nebulae, containing dense cosmic dust and gas from which star formation may take place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across (about 4.5×1047 m3). They contain molecular hydrogen (H2), carbon oxides and helium, and around 1% (by mass) silicate dust. Bok globules most commonly result in the formation of double- or multiple-star systems.Bok globules were first observed by astronomer Bart Bok in the 1940s. In an article published in 1947, he and Edith Reilly hypothesized that these clouds were "similar to insect's cocoons" that were undergoing gravitational collapse to form new stars, from which stars and star clusters were born.

This hypothesis was difficult to verify due to the observational difficulties of establishing what was happening inside a dense dark cloud that obscured all visible light emitted from within it. An analysis of near-infrared observations published in 1990 confirmed that stars were being born inside Bok globules. Further observations have revealed that some Bok globules contain embedded warm sources, some contain Herbig–Haro objects, and some show outflows of molecular gas. Millimeter-wave emission line studies have provided evidence for the infall of material onto an accreting protostar.

It is now thought that a typical Bok globule contains about 10 solar masses of material in a region about a light-year or so across, and that Bok globules most commonly result in the formation of double- or multiple-star systems.Bok globules are still a subject of intense research. Known to be some of the coldest objects in the natural universe, their structure and density remains somewhat a mystery. Methods applied so far have relied on column density derived from near-infrared extinction and even star counting in a bid to probe these objects further.

Bok globules that are irradiated by ultraviolet light from hot nearby stars exhibit stripping of materials to produce a tail. These types are called "cometary globules" (CG).

Buzz Lightyear

Buzz Lightyear is a fictional character in the Toy Story franchise. He is a toy Space Ranger superhero according to the movies and an action figure in the franchise. Along with Sheriff Woody, he is one of the two lead characters in all three Toy Story movies. He also appeared in the movie Buzz Lightyear of Star Command: The Adventure Begins and the television series spin-off Buzz Lightyear of Star Command.

He is voiced by Tim Allen in the Toy Story films, a few video games, and the Buzz Lightyear movie, Patrick Warburton in the TV series, and by Pat Fraley for the video games and the Buzz Lightyear attractions in Disney theme parks.

Eskimo Nebula

The Eskimo Nebula (NGC 2392), also known as the Clownface Nebula or Caldwell 39, is a bipolar double-shell planetary nebula (PN). It was discovered by astronomer William Herschel in 1787. The formation resembles a person's head surrounded by a parka hood. It is surrounded by gas that composed the outer layers of a Sun-like star. The visible inner filaments are ejected by a strong wind of particles from the central star. The outer disk contains unusual, light-year-long filaments.

NGC 2392 lies more than 2,870 light-years away, and is visible with a small telescope in the constellation of Gemini.

Gregory Alan Isakov

Gregory Alan Isakov is a singer-songwriter born in Johannesburg, South Africa. He and his family emigrated to the United States in 1986 and he was raised in Philadelphia, Pennsylvania. He currently resides in Boulder, Colorado.Isakov's music combines indie and folk, featuring instruments such as the guitar and banjo — and he is widely known for the songs "The Stable Song", "Big Black Car", "If I Go, I'm Goin" and "Raising Cain". Isakov has released seven albums, most recently Evening Machines in October 2018.

Messier 106

Messier 106 (also known as NGC 4258) 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.


NGC 2 is a spiral galaxy in the constellation Pegasus, discovered by Lawrence Parsons, 4th Earl of Rosse on 20 August 1873, and was described as "very faint, small, south of NGC 1." It lies slightly to the south of NGC 1. It is a faint spiral galaxy of apparent magnitude 14.2.NGC 2 is about 115,000 light years in diameter, but is 3 to 5 more luminous than the Milky Way as it is quite compact. AGC 102559, a 60,000-light year across galaxy, is the closest galaxy to NGC 2, being only 1.8 mly from it. Although it is apparently quite close to NGC 1, the latter is closer and unrelated to NGC 2.

NGC 40

NGC 40 (also known as the Bow-Tie Nebula and Caldwell 2) is a planetary nebula discovered by William Herschel on November 25, 1788, and is composed of hot gas around a dying star. The star has ejected its outer layer which has left behind a smaller, hot star with a temperature on the surface of about 50,000 degrees Celsius. Radiation from the star causes the shed outer layer to heat to about 10,000 degrees Celsius, and is about one light-year across. About 30,000 years from now, scientists theorize that NGC 40 will fade away, leaving only a white dwarf star approximately the size of Earth.

NGC 4261

NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. The galaxy is a member of its own galaxy group known as the NGC 4261 group.The active galactic nucleus (AGN) contains a 400-million-solar mass supermassive black hole (SMBH) with an 800-light-year-wide spiral-shaped disk of dust fueling it.The galaxy is estimated to be about 60 thousand light-years across, and a jet eminating from it is estimated to span about 88 thousand light-years.

NGC 6881

NGC 6881 is a planetary nebula, located in the constellation of Cygnus. It is formed of an inner nebula, estimated to be about one fifth of a light-year across, and symmetrical structure that spread out about one light-year from one tip to the other. The symmetry could be due to a binary star at the nebula's centre.

NGC 7008

NGC 7008 (PK 93+5.2), also known as the Fetus Nebula is a planetary nebula with a diameter of approximately 1 light-year located at a distance of 2800 light years in northern Cygnus. It was discovered by William Herschel in 1787, in Slough, England. NGC 7008 (H I-192) is included in the Astronomical League's Herschel 400 observing program.

NGC 7320

NGC 7320 is a spiral galaxy in the Stephan's Quintet. However, it is not an actual member of the galaxy group, but a much closer line-of-sight galaxy at a distance of about 40 million light years. Other galaxies of Stephan's Quintet are some 300 million light-year distant.

NGC 7662

NGC 7662, also known as the Blue Snowball Nebula or Snowball Nebula, is a planetary nebula located in the constellation Andromeda.

The distance to this nebula is not known with any real accuracy. According to the Skalnate Pleso Catalogue (1951) the distance of NGC 7662 is about 1,800 light years, the actual diameter about 20,000 AU. In a more recent survey of the brighter planetaries, C.R.O'Dell (1963) derived a distance of 1,740 parsecs or about 5,600 light years, increasing the actual size to 0.8 light year, or nearly 50,000 AU. It has a faint central star that is variable, with a magnitude range of 12 to 16. The central star is a bluish dwarf with a continuous spectrum and a computed temperature of about 75,000K. The nuclei of the planetary nebulae are among the hottest stars known.NGC 7662 is a popular planetary nebula for casual observers. A small telescope will reveal a star-like object with slight nebulosity. A 6" telescope with a magnification around 100x will reveal a slightly bluish disk, while telescopes with a primary mirror at least 16" in diameter may reveal slight color and brightness variations in the interior.


NGC 8 is an asterism of two completely unrelated stars (spectral types K6I and G4) in the constellation Pegasus, discovered on 29 September 1865 by Otto Wilhelm von Struve. It is approximately 2.7 arc minutes away from NGC 9.The two stars are completely unrelated to each other, with the whiter, dimmer star being at a distance of 10400+4400−2400 light years, and the yellower, brighter star having a minimum distance of 215,000 light years. While both stars are technically outside of the milky way's galactic disc, the nearer is 6400±2100 light-years south of the 1,000-light-year-thick disc, and the further is not only at least 130,000 light-years south of the disk, but is located entirely outside of the Milky Way itself, being at least 220,000 light-years from the galactic core.


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.The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner. Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.

In August 2015, the IAU passed Resolution B2, which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648000/π astronomical units, or approximately 3.08567758149137×1016 metres (based on the IAU 2012 exact SI definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references.


Peta () is a decimal unit prefix in the metric system denoting multiplication by 1015 (1000000000000000). It was adopted as an SI prefix in the International System of Units in 1975, and has the symbol P.

Peta is derived from the Greek πέντε, meaning "five". It denotes the fifth power of 1000 (10005). It is similar to the prefix penta ("five"), but without the letter n (on the analogy of the prefix tera [from the Greek for "monster"] for 10004 looking like tetra- ["four"] with a letter missing).


1 petametre = 1015 metres

1 petasecond = 1015 seconds (31.7 million years)

1 petahertz = 1015 cycle per second. Visible light is around 0.5 PHz.

1 petabyte = 1015 bytes

the mass–energy equivalence is 89.9 PJ/kg

1 light-year = 9.461 Pm

Project Phoenix (SETI)

Project Phoenix was a SETI project: in this case a search for extraterrestrial intelligence by analyzing patterns in radio signals. It was run by the independently funded SETI Institute of Mountain View, California, USA.

Project Phoenix started work in February 1995 with the Parkes radio telescope located in New South Wales, Australia, the largest telescope in the Southern Hemisphere.

Between September 1996 and April 1998, the Project used the National Radio Astronomy Observatory in Green Bank, West Virginia, USA.

Rather than attempting to scan the whole sky for messages, the Project concentrated on nearby systems that are similar to our own. Project Phoenix's targets comprised about 800 stars with a 200 light-year range.

The Project searched for radio signals as narrow as 1 Hz between 1,000 and 3,000 MHz: a broad bandwidth compared with most SETI searches.

In March 2004 the Project announced that after checking the 800 stars on its list, it had failed to find any evidence of extraterrestrial signals. Project leader Peter Backus remarked that they had been forced to conclude that "we live in a quiet neighborhood".

TT Cygni

TT Cygni is a carbon star. It is 561 parsecs (1,830 ly) away in Cygnus. It has an apparent magnitude of 7.44. It is called a carbon star because it has a high ratio of carbon to oxygen in its surface layers. The carbon was produced by helium fusion, dredged up from inside the star. A shell of carbon monoxide, about half a light year across, was emitted 6,000 years before the star was as it appears from Earth now.

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