Light-year

light yearlight yearsMlylight-yearslyklylightyearGlylightyearsdistance
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 non-specialist 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 approximately equal to 31 trillion kilometres (3.1 km) or 19 trillion miles (1.9 mi), and equates to about 3.26 light-years.

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).

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).
The astronomical unit is too small to be convenient for interstellar distances, where the parsec and light-year are widely used.

61 Cygni

61 Cygni A61 Cyg61 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 lengthDistance
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).

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).

Year

myaMay
Because it includes the word "year", the term light-year may be 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 speedspeed of light in vacuum
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

miRoman milemiles
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.

Star

starsstellarmassive star
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.
The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique.

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).

Oort cloud

Oort cloud objectÖpik-Oort CloudÖpik–Oort cloud
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.

Friedrich Bessel

Friedrich Wilhelm BesselBesselBessel, Friedrich
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.
In 1838 Bessel announced that 61 Cygni had a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicated that the star is 10.3 ly away.

Canopus

Alpha Carinaeα Carinaea first magnitude star
Canopus is the brightest star in the southern constellation of Carina, and is located near the western edge of the constellation around 310 light-years from the Sun.

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

Milky Way Galaxygalaxyour 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 (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), 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

M33TriangulumMessier 33
The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum.

Virgo Cluster

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

Metre

metermmetres
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.