Light-year

light yearlight yearslyMlylight-yearsklylightyearlightyearsGlydistance
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 10 12 km) or 5.88 trillion miles (5.88 x 10 12 mi).wikipedia
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Galaxy

galaxiesgalacticgalactic nuclei
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.
As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Parsec

Mpcpckpc
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).
One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length.

Kilometre

kmkilometerkilometers
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 10 12 km) or 5.88 trillion miles (5.88 x 10 12 mi).
light-years

Astronomical unit

AUastronomical unitsAUs
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).
|≈ 15.812507 millionths (15.812507E-6) of a light-year

Minute and second of arc

masarcsecondarc second
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).
an object of diameter 45,866,916 km at one light-year,

61 Cygni

61 Cyg61 Cygni A61 Cyg A
The star was 61 Cygni, and he used a 6.2 in heliometer designed by Joseph von Fraunhofer.
In 1838, Friedrich Bessel measured its distance from Earth at about 10.4 light-years, very close to the actual value of about 11.4 light-years; this was the first distance estimate for any star other than the Sun, and first star to have its stellar parallax measured.

Unit of length

lengthunits of lengthL
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 10 12 km) or 5.88 trillion miles (5.88 x 10 12 mi).
light-year ly ≈ 9460730472580.8 km. The distance that light travels in a vacuum in one Julian year.

Year

myaMamillion years ago
Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.
By convention, the Julian year is used in the computation of the distance covered by a light-year.

Parallax

trigonometric parallaxsolar parallaxmotion parallax
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 km or 6.1 mi).
The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond.

Speed of light

clight speedvelocity of light
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).
Astronomical distances are sometimes expressed in light-years, especially in popular science publications and media.

Mile

mimilesstatute mile
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 10 12 km) or 5.88 trillion miles (5.88 x 10 12 mi).
The radar mile is a unit of time (in the same way that the light year is a unit of distance), equal to the time required for a radar pulse to travel a distance of two miles (one mile each way).

Julian year (astronomy)

yrayears
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).
The Julian year is the basis of the definition of the light-year as a unit of measurement of distance.

Orders of magnitude (numbers)

trillionbillionquadrillion
The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 10 12 km) or 5.88 trillion miles (5.88 x 10 12 mi).
Cosmology: 1 is Archimedes' estimate in The Sand Reckoner of the total number of grains of sand that could fit into the entire cosmos, the diameter of which he estimated in stadia to be what we call 2 light years.

Oort cloud

Öpik-Oort CloudÖpik–Oort cloud20,000–50,000 AU
The Oort cloud, named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years).

Sloan Great Wall

Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.
The wall measures 1.38 billion light-years (1.30 m) in length, located approximately one billion light-years away.

Sirius

SothisDog StarSirius B
At a distance of 2.6 parsecs (8.6 ly), as determined by the Hipparcos astrometry satellite, the Sirius system is one of Earth's near neighbours.

Gliese 581

Gliese planetary system
Gliese 581 is a star of spectral type M3V (a red dwarf) at the center of the Gliese 581 planetary system, about 20 light years away from Earth in the Libra constellation.

Milky Way

galaxyMilky Way Galaxyour galaxy
The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly).

Andromeda Galaxy

AndromedaM31Andromeda Nebula
The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way.

Large Magellanic Cloud

LargeLMCGreater Magellanic Cloud
At a distance of about 50 kiloparsecs (≈163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (~16 kpc) and the possible dwarf irregular galaxy known as the Canis Major Overdensity.

Triangulum Galaxy

M33TriangulumM33 Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light-years (ly) from Earth in the constellation Triangulum.

Metre

metermmetres
= 9460730472580800 metres (exactly)
Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.

Virgo Cluster

VirgoVirgo Cluster of galaxiesVirgo Cl.
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc)

Astronomical constant

astronomicalastronomical constantsIAU (1976) System of Astronomical Constants
Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.

Quasar

quasarsquasi-stellar objectQSO
Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.
The highest redshift quasar known is ULAS J1120+0641, with a redshift of 7.085, which corresponds to a comoving distance of approximately 29 billion light-years from Earth (these distances are much larger than the distance light could travel in the universe's 13.8 billion year history because [[Metric expansion of space#How are distances between two points measured if space is expanding?|space itself has also been expanding]]).