Despite the title suggests, a light year is not a mesure of time but a measure of distance. A light year is defined as: the distance that light travels in a year.
One light year is approximately 9 1/2 million km.
The nearest star to our Solar System, other than our own Sun, is Proxima Centauri which is approximately 4.2 light years away. To work this out in kilometres = speed of light x number of seconds in a year. Therefore Proxima Centauri is 4.2 x (9.5 x 10^15) which equates to: 40,000,000,000,000 km.
How big is the Milky Way Galaxy?
A light year is approximately 9 1/2 million km. Our galaxy, the Milky Way, is 100,000 light years across.
Milky Way Galaxy image courtesy of science-resources.co.uk
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