# 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**

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]]).