Light year facts for kids
A light-year (symbol: ly) is a way to measure huge distances in space. It is the distance that light travels in one Earth year. Light moves incredibly fast, about 300,000 kilometers (186,000 miles) every second!
Because light travels so fast, a light-year is a very long distance. It's about 9.46 trillion kilometers (or 5.88 trillion miles). Even though its name has "year" in it, a light-year is a measure of distance, not time.
Astronomers use light-years because the universe is so vast. Stars and galaxies are often hundreds, thousands, or even millions of light-years away from us.
Imagine a star that is 100 light-years away from Earth. The light we see from that star today actually left it 100 years ago. This means when we look at distant objects in space, we are looking back in time!
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History of the Light-Year Unit
The idea of the light-year came about after scientists first measured the distance to a star far away from our Sun. This happened in 1838, thanks to a scientist named Friedrich Bessel. He measured the distance to a star called 61 Cygni.
At that time, the biggest unit for measuring space distances was the astronomical unit (AU). One AU is the distance from the Sun to the Earth. Bessel found that 61 Cygni was about 660,000 AU away. He also noted that light would take about 10.3 years to travel this distance.
Bessel didn't use "light-year" as an official unit. He might have worried that using the speed of light, which wasn't perfectly known then, would make his measurements seem less accurate.
The term "light-year" first appeared in 1851 in a German astronomy article by Otto Ule. He explained that even though it had "year" in its name, it was a distance, like how a "walking hour" describes distance walked in an hour.
Today, astronomers often use a unit called the parsec. However, light-years are still very popular for talking about the huge distances between stars and galaxies.
How We Use Light-Years
Light-years help us understand the enormous sizes and distances in space. For example, we use them to describe how far apart stars are within the same area, like in a spiral arm or a globular cluster.
Galaxies themselves can be thousands to hundreds of thousands of light-years across. They are separated from other galaxies and galaxy clusters by millions of light-years. Some of the most distant objects we know, like quasars, are billions of light-years away!
Here are some examples of distances in space, measured in light-years:
| Scale (ly) | Distance | What it is |
|---|---|---|
| 10−9 | 4.04×10−8 ly | Light from the Moon takes about 1.2 to 1.3 seconds to reach Earth. |
| 10−6 | 1.58×10−5 ly | One astronomical unit (distance from the Sun to the Earth). Light takes about 8.32 minutes to travel this far. |
| 1.27×10−4 ly | The Huygens probe landed on Titan (a moon of Saturn) and sent images from 1.2 billion kilometers away. | |
| 5.04×10−4 ly | The New Horizons spacecraft flew past Pluto at 4.7 billion kilometers away. Signals took 4 hours and 25 minutes to reach Earth. | |
| 10−3 | 2.04×10−3 ly | The Voyager 1 probe, our most distant spacecraft, was about 18 light-hours away from Earth in 2014. It would take about 17,500 years for it to travel one light-year at its current speed. |
| 2.28×10−3 ly | Voyager 1 in October 2018, nearly 20 light-hours (21.6 billion km) from Earth. | |
| 100 | 1.6×100 ly | The Oort cloud, a huge cloud of icy objects around our Solar System, is about two light-years wide. |
| 2.0×100 ly | The maximum reach of the Sun's gravity. Beyond this, other stars' gravity is stronger. | |
| 4.24×100 ly | Proxima Centauri, the closest known star to our Sun, is about 4.24 light-years away. | |
| 8.6×100 ly | Sirius, the brightest star we see in the night sky. It's very bright because it's relatively close to us. | |
| 1.19×101 ly | HD 10700 e, a planet outside our solar system that might be able to support life. | |
| 2.05×101 ly | Gliese 581, a red dwarf star with several planets orbiting it. | |
| 3.1×102 ly | Canopus, the second brightest star in our sky, is a very luminous giant star. | |
| 103 | 3×103 ly | A0620-00, the second-closest known black hole, is about 3,000 light-years away. |
| 2.6×104 ly | The center of our own Milky Way galaxy is about 26,000 light-years away. | |
| 1×105 ly | Our Milky Way galaxy is about 100,000 light-years across. | |
| 1.65×105 ly | R136a1, a super bright star in the Large Magellanic Cloud, is 8.7 million times brighter than the Sun. | |
| 106 | 2.5×106 ly | The Andromeda Galaxy, our closest large galaxy neighbor, is about 2.5 million light-years away. |
| 3×106 ly | The Triangulum Galaxy (M33), about 3 million light-years away, is the farthest object you can see with just your eyes. | |
| 5.9×107 ly | The Virgo Cluster, a large group of galaxies, is about 59 million light-years away. | |
| 1.5×108 – 2.5×108 ly | The Great Attractor, a huge area of space with a lot of gravity, is between 150 and 250 million light-years away. | |
| 109 | 1.2×109 ly | The Sloan Great Wall, one of the largest known structures in the universe, is about one billion light-years long. |
| 2.4×109 ly | 3C 273, the brightest quasar we can see, is about 2.4 billion light-years away. | |
| 4.57×1010 ly | The edge of the part of the universe we can see is about 45.7 billion light-years away in any direction. |
Calculating a Light-Year
To calculate the distance of one light-year, we multiply the speed of light by the number of seconds in a year.
Here's how it's calculated: 
Where:
- 299,792,458 is the number of meters light travels in one second.
- The first 60 is the number of seconds in a minute.
- The second 60 is the number of minutes in an hour.
- 24 is the number of hours in a day.
- 365.25 is the average number of days in a year (including leap years).
Related Units of Distance
Because space is so big, scientists also use other "light" units for different distances:
- Light-second: This is the distance light travels in one second. It's about 300,000 kilometers (186,000 miles). For example, our Moon is about 1.25 light-seconds away from Earth.
- Light-minute: This is the distance light travels in one minute. It's about 18 million kilometers (11.16 million miles). Our Sun is about 8 light-minutes away from Earth.
- Light-month: This is the distance light travels in one month (usually considered 30 days). It's roughly one-twelfth of a light-year.
In Spanish: Año luz para niños
See also
