Light-year

For other uses, see light year (disambiguation).

A light-year or lightyear, symbol ly, is the distance light travels in vacuum in one Julian year.

Numerical value

A light-year is equal to

• 9,460,730,472,580.8 km (about 9.461 Pm)
• about 5,878,625,373,183.61 statute miles
• about 63,241.077 AU (often approximated to 63,240 AU)
• about 0.306 601 394 parsecs

The actual, exact length of the light-year depends on the length of the reference year used in the calculation, and there is no wide consensus on the reference to be used. The figures above are based on a reference year of exactly 365.25 days (each of exactly 24 hours). This is the value recommended by the IAU. However, other reference years are often used (eg. Yahoo's calculator uses a smaller value than the IAU), such that the light-year is not an appropriate unit to use when extremely high precision is required.

The IAU style guide [1] recommends the use of calendar years, specifically Julian (and not Gregorian) calendar years of 365.25 days or exactly 31,557,600 seconds. This gives the light-year an exact value of 9,460,730,472,580,800 meters, again about 9.461 Pm).

The light-year is often used to measure distances to stars. In astronomy, the preferred unit of measurement for such distances is the parsec, which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit perpendicular to the line of sight to the observer. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and compared with, observational data. However, outside scientific circles, the term light-year is more widely used.

Units related to the light year are the light-minute and light-second, the distance light travels in a vacuum in one minute and one second, respectively. Since the speed of light is defined as 299,792,458 metres per second, a light-second is exactly 299,792,458 m in length and a light-minute is exactly 17,987,547,480 m. In contrast to the light-year, the lengths of the light-minute and light-second are fixed with 100% precision.

(For a list of lengths on the order of one light-year, see the article 1 E15 m.)

Distances in light-years

Distances less than a light-year

Distances measured in fractions of a light-year usually involve objects within a star system.

• One astronomical unit (the distance from the Sun to the Earth) is 1.58 × 10^-5 light-years.
• The most distant space probe, Voyager 1, was 1.50 × 10^-3 light-years away from Earth in September 2004. It took Voyager 27 years to cover that distance.

Light-years and kilolight-years

Distances measured in light-years include distances between nearby stars, such as those in the same spiral arm or globular cluster.

One kilolight-year, abbreviated "kly", is one thousand light-years, or 307 parsecs. Kilolight-years are typically used to measure distances between parts of a galaxy.

• The Oort cloud is approximately 2 light-years in diameter.
• The nearest known star (other than the Sun), Proxima Centauri is 4.21 light-years away.
• The center of our galaxy, the Milky Way, is about 26 kilolight-years away. The Galaxy is about 98 kilolight-years across.

Megalight-years and gigalight-years

One megalight-year, abbreviated "Mly", is one million light-years, or 306,601 parsecs. Megalight-years are typically used to measure distances between neighboring galaxies and galaxy clusters.

One gigalight-year, abbreviation "Gly", is one billion light-years — one of the largest distance measures used. One gigalight-year equals 306.601 million parsecs, or roughly one-thirteenth the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Gigalight-years are typically used to measure distances to supergalactic structures, such as clusters of quasars or the Great Wall.

Examples of astronomical distances

• Reflected sunlight from the Moon's surface takes 1.2 seconds to travel the 4.04 × 10⁻⁸ light years to Earth.^{[citation needed]}
• It takes approximately 8.31 minutes for light to travel from the Sun to the Earth (a distance of 1.58 × 10⁻⁵ light-years).^{[citation needed]}
• The most distant space probe, Voyager 1, was 13 light hours (only 1.5 × 10⁻³ light years) away from Earth in September 2004. It took Voyager 27 years to cover that distance.^[1]
• The nearest known star (other than the Sun), Proxima Centauri is 4.22 light years away.^[2][3]
• The center of our galaxy, the Milky Way, is about 8 kiloparsecs (26,000 light years) away. The Galaxy is about 100,000 light years across.^[4][5]
• The Andromeda Galaxy is approximately 2.5 megalight-years away.
• The Triangulum Galaxy (M33), at 3.14 megalight-years away, is the most distant object visible to the naked eye.
• The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
• The Great Attractor lies at a distance of somewhere between 150 and 250 megalight years (the latter being the most recent estimate).
• The Sloan Great Wall (not to be confused with the Great Wall) has been measured to be approximately one gigalight-year distant.
• The comoving distance from the Earth to the edge of the visible universe is about 46.5 gigalight-years in any direction; this is the comoving radius of the Observable universe.

More references are being provided in the respective articles.

Notes

1. ^ NASA pressrelease (05-131) 2005-05-24: Voyager Spacecraft Enters Solar System's Final Frontier
2. ^ NASA: Cosmic Distance Scales - The Nearest Star
3. ^ Encyclopedia of Astrobiology, Astronomy, and Spaceflight: Proxima Centauri (Gliese 551)
4. ^ F. Eisenhauer, R. Schoedel, R. Genzel, T. Ott, M. Tecza, R. Abuter, A. Eckart, T. Alexander: A Geometric Determination of the Distance to the Galactic Center, Astrophys.J. 597 (2003) L121-L124
5. ^ McNamara, D. H.; Madsen, J. B.; Barnes, J.; Ericksen, B. F.: The Distance to the Galactic Center, The Publications of the Astronomical Society of the Pacific, Volume 112, Issue 768, pp. 202-216.

See also

• Conversion of units#Length
• Orders of magnitude (length)

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