Light year

[guāng nián]

Light walks a year in vacuum

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This entry is made by Institute of Physics, Chinese Academy of Sciences, National Astronomical Observatory, Chinese Academy of Sciences Participate in editing and review Science Popularization China · Science Encyclopedia authentication.

Light year (ly), a unit of length, commonly used to indicate astronomical distance, is about 9.46 × 10 twelve Kilometers (accurate value is 9460730472580800 meters). Light year is the distance that light travels through a Julian year (365.25 days) in a vacuum. Because of the word "year", it is often mistaken for a time unit.

The fastest aircraft in the world can reach a speed of 11260 kilometers per hour (on November 16, 2004, the highest speed record of the National Aeronautics and Space Administration (NASA) aircraft was 11260 kilometers per hour). According to this speed, it will take 95800 years for the aircraft to cross a light year. The passenger plane of Changyi is about 885 km/h, so it will take 1.22 million years for Changyi to travel 1 light year. At present, the fastest man-made object is Juno, which reached the polar orbit of Jupiter on July 5, 2016 (launched on August 5, 2011), with a maximum speed of 73.61 km/s (about 265000 km/h). It will take about 4073 years for such a speed to cross the distance of 1 light year.

Light years are generally used to describe the distance between stars or galaxies in non professional scenes such as popular science books and science fiction. Professional astronomers often use the unit second difference (parsec, abbreviated as pc, about 3.26 light years) from astronomy as the distance unit in astronomy. It is the distance when the angle of an astronomical unit is one angular second.

Chinese name: Light year
Foreign name: light-year

Definition: Light walks a year in vacuum
Symbol: ly
Conversion: 1ly=9.46×10 twelve km

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definition

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According to the definition of the International Astronomical Union (IAU), light year is light speed (299792458 m/s) and Julian year (365.25 days=31557600 seconds). These constants were included in the IAU (1976) astronomical constant system, which was launched in 1984.^[2] Before 1983 (specifically 1968~1983), IAU (1964) astronomical constant system including solar year (the average time of the earth's circle around the sun, about 365.242189 days) and measuring the speed of light (rather than the current definition of the speed of light) was briefly used.^[3]

Stars within 12.5 light years from the sun^[1]

Unit conversion

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1 light year=9460730472580800 meters ≈ 63241.077 AU ≈ 0.306601 second difference

Astronomical unit and second difference are common distance units in astronomy. Astronomical unit is defined as the average distance from the sun to the earth, and second difference is defined as the circle radius corresponding to an arc with an arc length of 1 astronomical unit and a radian of 1 angular second.

history

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The term "light year" was invented by Friedrich Bessel, a German astronomer, who first measured the distance of stars other than the sun in 1838. At that time, the largest unit of space distance was Astronomical unit (AU), the radius of the Earth's orbit around the sun (about 150 million kilometers). He observed the star 61 Cygni with the 160mm heliometer designed by Joseph von Fraunhofer, and calculated the distance from the star 61 Cygni to 660000 astronomical units (9.9 × 10 thirteen Km). Bessel calculated that it would take 10.3 years for light to cross this distance.^[4] At that time, he realized that readers might prefer to imagine how long light traveled than huge numbers, but he did not use light years as a unit. He may object to using light year as the standard distance unit, because multiplying with the uncertain parameter of light speed will reduce the accuracy of parallax data.

In Bessel's time, people's understanding of the speed of light was not perfect. French scientists Fizeau and Foucault measured the speed of light differently in 1849 and 1862, respectively. At that time, the speed of light was not considered as the basic constant of nature, and the propagation process of light in the ether or space was still mysterious. Now, since the unit "meter" is defined according to the speed of light and seconds, the speed of light has become a constant (299792458 meters/second). In 1851, the German popular science writer Otto Ulle used "light year" in an article on popular astronomy^[5] Otto Ule explained the strangeness of the name of the distance unit ending in "year" by comparing it with the walking time (the distance of one hour walking, German Wegstunde). Eddington, a British astronomer, claimed that light year is an inconvenient and insignificant unit, but sometimes it is used for science popularization to science and technology.^[6] Although modern astronomers usually prefer to use the second gap, light years are also widely used to measure the scale of interstellar and intergalactic space.

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Generally, the distance expressed in light years includes the distance between stars in a certain area, such as the distance between stars belonging to a spiral arm or globular cluster. The diameter of the galaxy itself varies from thousands to hundreds of thousands of light years, and it is millions of light years away from neighboring galaxies and galaxy clusters. a quasar And the Sloan Great Wall and other celestial structures are billions of light years away from the Earth.

Scale (ly)	distance	example
ten -9	4.04×10 -8 ly	It takes 1.2 to 1.3 seconds for the sunlight reflected from the moon surface to reach the earth surface (about 350000 to 400000 kilometers).
ten -6	1.58×10 −5 ly	An astronomical unit (the distance from the sun to the earth). It takes about 499 seconds (8.32 minutes) for light to travel this distance.
	1.27×10 −4 ly	The Huygens probe that landed on Titan transmits images from 1.2 billion kilometers away from Earth.
	5.04×10 −4 ly	New Horizon collided with Pluto at a distance of 4.7 billion kilometers from the Earth, and communication took 4 hours and 25 minutes to reach the Earth.
ten −3	2.04×10 −3 ly	As of October 2014, the most distant space probe, Voyager 1, was about 18 light hours (130 AU, 19.4 billion kilometers) away from the Earth. 26] At the current speed of about 17 km/s (61 200 km/h) relative to the sun, it will take about 17500 years to reach a light year. On September 12, 2013, NASA announced that Voyager 1 entered interstellar space, becoming the first man-made object to leave the solar system.
ten −3	2.28×10 −3 ly	As of October 2018, Voyager 1 is nearly 20 light hours away from the Earth (14.4 AU, 21.6 billion kilometers).
ten zero	1.6×10 zero ly	The diameter of the Oort cloud is about two light years. It is speculated that its internal boundary is 50000 AU and its external boundary is 100000 AU.
	2.0×10 zero ly	The maximum range of the main influence of solar gravity (Hill sphere/Roche sphere, 125000 AU). In addition, there is the extrasolar gravitational interstellar medium.
	4.24×10 zero ly	Bilin star, the nearest known star except the sun, is about 4.24 light-years away.
	8.6×10 zero ly	Sirius, the brightest star in the night sky. Its mass is twice that of the sun, and its luminosity is 25 times that of the sun. Due to its close distance, it is brighter than those stars with higher luminosity.
	1.19×10 one ly	Planet HD 10700 e is a candidate for a habitable exoplanet. Its mass is 6.6 times that of the Earth, and it is located in the habitable zone of Tau star in the constellation Cetus.
	2.05×10 one ly	Gliese 581 is a red dwarf star with several detectable exoplanets.
	3.1×10 two ly	The brightness of the old star in the sky is only second to Sirius, and the A9 bright giant star is 10700 times as bright as the sun.
ten three	3×10 three ly	A0620-00 is the second closest black hole known, about 3000 light-years away.
	2.6×10 four ly	The center of the Milky Way Galaxy is about 26000 light years away from the Earth.
	1×10 five ly	The Milky Way is about 100000 light years in diameter.
	1.65×10 five ly	R136a1 is located in the Large Magellanic Cloud. Its luminosity is 8.7 million times that of the sun. It is the highest known star with an apparent magnitude of 12.77.
ten six	2.5×10 six ly	Andromeda galaxy, about 2.5 million light-years away.
	3×10 six ly	Triangle galaxy (M33), about 3 million light-years away, is the most distant object visible to the naked eye.
	5.9×10 seven ly	The nearest galaxy cluster, the Virgo Galaxy Cluster, is about 59 million light-years away.
	1.5×10 eight ～ 2.5×10 eight ly	The giant source is located at a distance of 150 million to 250 million light years (the latter is the latest estimate).
ten nine	1.2×10 nine ly	Sloan Great Wall, about 1 billion light years away.
	2.4×10 nine ly	3C 273, the brightest quasar in the optical band, has an apparent magnitude of 12.9, second only to R136a1, and is about 2.4 billion light-years away.
	4.57×10 ten ly	The co motion distance from the Earth to the edge of the visible universe is about 45.7 billion light years, which is also the co motion radius of the observable universe.

Relevant units

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The distance between objects in a stellar system is usually only a small part of a light year, usually expressed in astronomical units. However, by multiplying the time unit by the speed of light, a smaller unit of length can also be produced. For example, the light second commonly used in astronomy and relativistic physics is equal to 299792458 meters or 1/31557600 light years. Units such as light minutes, light hours and light days are sometimes used in popular science publications. The light moon is about one twelfth of the light year and is occasionally used for approximate measurements. The Hayden Planetarium in New York, USA, more accurately defines the light month as a 30 day light travel time.^[7] Because light is in one nanosecond The word "light ruler" is sometimes used as an informal unit of time measurement.

For more distant objects, astronomers sometimes use red shift Z represents the distance, which means that when photons travel through the universe Cosmic expansion And make photon wavelength Multiplier of increase.

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