How far is a light-year?
This is a system called SS433. It's 18,000 light-years away. This image spans half a light-year. (NASA/CXC/U.Amsterdam/S.Migliari et al.)
This is an object called SS 433 – possibly a black hole and neutron star in orbit around each other. This system is located some 18,000 light-years from us. This image spans half a light-year.
When speaking of stars and galaxies, astronomers talk about light-years.
A light-year is the distance that starlight travels in a single year. Light travels at 300,000 kilometers per second – that’s 186,000 miles a second. It’s like circling Earth’s equator 7 and 1/2 times in one second.
Now multiply that number by the number of seconds in a year. And that’s a light-year – 9 and 1/2 trillion kilometers – nearly 6 trillion miles. It’s difficult to comprehend such humongous numbers. So the 20th century astronomer Robert Burnham Jr. came up with an ingenious way to portray a light-year, relating it to our distance from the sun.
The sun-Earth distance is called one astronomical unit by astronomers. There are about 63,000 astronomical units in one light-year. By coincidence, there are also 63,000 inches in a mile. If you scale the distance between our Earth and sun to the length of a single inch, then a light-year on this scale would equal one mile.
Excluding our sun, the closest star is over 4 light years away. So if there’s one inch between the Earth and sun, it’s 4 miles, or 6 kilometers, between us and the nearest star.
Special thanks to Research Corporation, a foundation for the advancement of science.
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