Light-year

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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.

[edit] 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.)

[edit] Distances in light-years

[edit] 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.

[edit] 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.

[edit] 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.

• The Triangulum Galaxy (M33), at a little under 2.6 megalight-years away, is the most distant object visible to the naked eye.
• The Andromeda Galaxy is 2.5 megalight-years away.
• The nearest large galaxy cluster, the Virgo Cluster, is about 59 megalight-years away.
• The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 2.1 gigalight-years away.
• The particle horizon (observable part) of the universe has a radius of about 13 gigalight-years.

[edit] Miscellaneous facts

• Reflected sunlight from the Moon's surface takes 1.2 seconds to travel the 4.04 × 10⁻⁸ light years to Earth.
• 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).
• 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]
• Continuing at the speed above, it would take Voyager 18000 years to cover a single light year, and 76000 years to cover the distance to Proxima Centauri.
• 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 Triangulum Galaxy (M33), at 3.14 million light years away, is the most distant object visible to the naked eye.
• The nearest large galaxy cluster, the Virgo Cluster, is about 60 million light years away.
• The particle horizon (observable part) of the universe has a radius of about 46 billion light years, but light from the edge of the observable universe was emitted only 13.7 billion years ago (the age of the universe).^{[citation needed]} The figures differ because distant objects have continued to recede from us due to cosmological expansion (see Hubble's law).
• In the Disney movie Toy Story one character was named Buzz Lightyear. Buzz referring to Buzz Aldrin - one of the first men on the moon, and Lightyear referring to astronomical distance.

[edit] See also

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

[edit] External links