Dear Straight Dope:
Is it true that all the stars we can see in the sky are so far away that they are all actually burned out, and that we are just seeing their glow?
SDStaff DavidB replies:
To tell you the truth, there is no way to know for certain. But the likely answer to whether they are all burned out is “no.”
Allow me to explain. Light, like anything else, travels at a finite speed. In this case, that would be the speed of light — the fastest anything can travel, as far as we know (it’s not just a good idea, it’s the law). So when we look up at the stars, we’re seeing light that left those stars a long time ago and traveled a long distance since. How long? Well, the nearest star other than our own sun is Proxima Centauri, which is 40,000,000,000,000 kilometers away. Light from that star takes over four years to reach us, so we say it’s four light-years distant from Earth. (A light-year is a measurement of distance, not time, and represents the distance covered by light in a year.) So when we look up and see Proxima Centauri, we are seeing it as it was over four years ago.
Most stars are much farther away than that. The farthest star astronomers have found is over 12 billion light years away! That means we’re seeing it as it was 12 billion years ago, and have no clue what it is like now. Has it blown up or burned out? In 12 billion years, the odds are pretty good that it has, but we have no way of knowing, and won’t be able to know what it looks like “now” until 12 billion years in the future! At that point, the question will be moot as far as you and I are concerned.
This is true even of our own sun. It takes several minutes for light from the sun to reach Earth. If the sun winked out tomorrow at 5 a.m., we wouldn’t know it right away. The whole concept can be a little confusing, but it’s not unlike seeing lightning and then hearing thunder several seconds later. Light travels faster than sound, so you see the lightning, but you don’t hear the sound it made at that time until the sound gets to you.
So, getting back to the question — we can never be 100% certain that all the stars we see are still there, because we can only see them as they were when they emitted the light we see. It’s likely some of the more distant ones are gone, but most of the closer ones are probably still around.
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