How long ago was the light we see from stars emitted?

To figure out how long ago the light we see from stars was emitted, we have to know roughly how far away the star is and use the known speed of light (approximately 3 × 108 meters per second). The Sun is roughly 150 million kilometers from the Earth, from which we can calculate that sunlight reaching the Earth left the Sun roughly eight minutes ago. The distance to the next nearest star is much farther, roughly 410 × 1011 kilometers from Earth. This translates into a time of over four years between when light leaves this star and when it reaches telescopes on Earth. Keep in mind that this is the next nearest star, so all others are even farther away. This also means that, if we see a star explode, it really happened many years ago.


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