Light-year

Jai Ganesh · 2021-12-14 20:03:15

Light-year

A light-year, alternatively spelled lightyear, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46 × {10}^{12} km) or 5.88 trillion miles (5.88×1012 mi). 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). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.

The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry: it is the distance at which one astronomical unit subtends an angle of one second of arc.

Definitions

As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s). Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this, the following conversions can be derived. The IAU recognized abbreviation for light-year is ly, although other standards like ISO 80000 use "l.y." and localized abbreviations are frequent, such as "al" in French (from année-lumière), Spanish (from año luz) and Italian (from anno luce), "Lj" in German (from Lichtjahr), etc.

1 light-year = 9460730472580800 metres (exactly)
≈ 9.461 petametres
≈ 9.461 trillion kilometres (5.879 trillion miles)
≈ 63241.077 astronomical units
≈ 0.306601 parsecs

Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530 × {10}^{15} m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).

Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207 × {10}^{15} m found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405 × {10}^{15} m, is the product of the J1900.0 mean tropical year and the defined speed of light.

Abbreviations used for light-years and multiples of light-years are

* "ly" for one light-year
* "kly" for a kilolight-year (1,000 light-years)
* "Mly" for a megalight-year (1,000,000 light-years)
* "Gly" for a gigalight-year (1,000,000,000 light-years)

History

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. The star was 61 Cygni, and he used a 160-millimetre (6.2 in) heliometre designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9×{10}^{13} km; 6.1×1013 mi). Bessel added that light takes 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resented expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

The speed of light was not yet precisely known in 1838; its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.

The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.

Although modern astronomers often prefer to use the parsec, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.

Usage of term

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. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years.

Related units

Distances between objects within a star system tend to be small fractions of a light-year, and are usually expressed in astronomical units. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly 299792458 metres or 1⁄31557600 of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time.

Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time.

Other definitions of a light-year

a) Llight-year, in astronomy, is the distance traveled by light moving in a vacuum in the course of one year, at its accepted velocity of 299,792,458 metres per second (186,282 miles per second). A light-year equals about 9.46073 × {10}^{12} km (5.87863 × {10}^{12} miles), or 63,241 astronomical units. About 3.262 light-years equal one parsec.

b) A light-year is a measurement of distance and not time (as the name might suggest). A light-year is the distance a beam of light travels in a single Earth year, or 6 trillion miles (9.7 trillion kilometers).

On the scale of the universe, measuring distances in miles or kilometers doesn't cut it. In the same way that you may measure the distance to the grocery store in the time it takes to drive there ("The grocery store is a 15-minute drive away"), astronomers measure the distances of stars in the time it takes for light to travel to us. For example, the nearest star to our sun, Proxima Centauri, is 4.2 light-years away.

How far is a light-year?

Unlike the speed of your car when running errands, the speed of light is constant throughout the universe and is known to high precision. In a vacuum, light travels at 670,616,629 mph (1,079,252,849 km/h). To find the distance of a light-year, you multiply this speed by the number of hours in a year (8,766). The result: One light-year equals 5,878,625,370,000 miles (9.5 trillion km). At first glance, this may seem like an extreme distance, but the enormous scale of the universe dwarfs this length.

Why use light-years?

Measuring in miles or kilometers at an astronomical scale would be extremely cumbersome and impractical. Starting in our cosmic neighborhood, the closest star-forming region to us, the Orion Nebula, is a short 7,861,000,000,000,000 miles away, or more simply, 1,300 light-years away. The center of our galaxy is about 27,000 light-years away. The nearest spiral galaxy to ours, the Andromeda galaxy, is 2.5 million light-years away. Some of the most distant galaxies we can see are billions of light-years from us.

Measuring in light-years also allows astronomers to determine how far back in time they are viewing. Because light takes time to travel to our eyes, everything we view in the night sky has already happened. In other words, when you observe something 1 light-year away, you see it as it appeared exactly one year ago. We see the Andromeda galaxy as it appeared 2.5 million years ago. The most distant object we can see, the cosmic microwave background, is also our oldest view of the universe, occurring just after the Big Bang some 13.8 billion years ago.

Alternatives to light-years

Astronomers also use parsecs as an alternative to the light-year. Short for parallax-second, a parsec comes from the use of triangulation to determine the distance of stars. To be more specific, it is the distance to a star whose apparent position shifts by 1 arcsecond (1/3,600 of a degree) in the sky after Earth orbits halfway around the sun. One arcsecond is equal to 3.26 light-years.

Like degrees, the light-year can also be broken down into smaller units of light-hours, light-minutes or light-seconds. For instance, the sun is more than 8 light-minutes from Earth, while the moon is just over a light-second away. Scientists use these terms when talking about communications with deep-space satellites or rovers. Because of the finite speed of light, it can take more than 20 minutes to send a signal to the Curiosity rover on Mars.

Whether it's light-years or parsecs, astronomers will continue to use both to measure distances in our expansive and grand universe.

Jai Ganesh · 2021-12-19 14:51:27

Light-year Part-2

History

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. The star was 61 Cygni, and he used a 160-millimetre (6.2 in) heliometre designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660,000 astronomical units (9.9 × {10}^{13} km; 6.1 × {10}^{13} mi). Bessel added that light takes 10.3 years to traverse this distance.[20] He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resented expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

The speed of light was not yet precisely known in 1838; its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.

The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde).

A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.

Although modern astronomers often prefer to use the parsec, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space.

What Is a Light-Year?

For most space objects, we use light-years to describe their distance. A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km). That is a 6 with 12 zeros behind it!

Looking Back in Time

When we use powerful telescopes to look at distant objects in space, we are actually looking back in time. How can this be?

Light travels at a speed of 186,000 miles (or 300,000 km) per second. This seems really fast, but objects in space are so far away that it takes a lot of time for their light to reach us. The farther an object is, the farther in the past we see it.

Our Sun is the closest star to us. It is about 93 million miles away. So, the Sun's light takes about 8.3 minutes to reach us. This means that we always see the Sun as it was about 8.3 minutes ago.

The next closest star to us is about 4.3 light-years away. So, when we see this star today, we’re actually seeing it as it was 4.3 years ago. All of the other stars we can see with our eyes are farther, some even thousands of light-years away.

A chart explaining how far away certain objects are from Earth. The Sun is 8.3 light-minutes away. Polaris is 320 light-years away. Andromeda is 2.5 million light years away. Proxima Centauri is 4.3 light-years away. The center of the Milky Way is 26,000 light-years away. GN-z11 is 13.4 billion light-years away.

Stars are found in large groups called galaxies. A galaxy can have millions or billions of stars. The nearest large galaxy to us, Andromeda, is 2.5 million light-years away. So, we see Andromeda as it was 2.5 million years in the past. The universe is filled with billions of galaxies, all farther away than this. Some of these galaxies are much farther away.

In 2016, NASA's Hubble Space Telescope looked at the farthest galaxy ever seen, called GN-z11. It is 13.4 billion light-years away, so today we can see it as it was 13.4 billion years ago. That is only 400 million years after the big bang. It is one of the first galaxies ever formed in the universe.

Learning about the very first galaxies that formed after the big bang helps us understand what the early universe was like.

light_year.jpg?w=650

fredwire · 2022-06-06 11:44:36

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Last edited by fredwire (2022-06-09 01:32:32)

Jai Ganesh · 2022-06-06 15:03:54

Hi fredwire,

Thanks!

Welcome to the forum!

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#1 2021-12-14 20:03:15

Light-year

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Re: Light-year

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