Is there any time in space?

time and room How do you know if a star we see at night still exists?

Lights out, star dead

If you take it very strictly, then in the end you really don't know. The only way stars can "tell" us, so to speak, that they no longer exist is for them to stop shining. If we imagine that a star is 1,000 light years away and went out yesterday, then theoretically we would not find out on Earth for 1,000 years.

But there are a few ways to estimate the probability. For example, most of the stars that we see in the immediate vicinity - by that I mean our Milky Way galaxy - should still exist at this moment. The Milky Way is 100,000 light years across. So at the Milky Way we are looking at most 100,000 years into the past. That sounds like a lot, but it is only a fraction of the time these stars have even existed. Because most of them have been around for billions of years. That is ten thousand times as long as it takes the light from them to reach us. This means that the probability that the star went out in this relatively short time in which its light was on its way to us is, statistically speaking, relatively small.

But you can estimate it a little more precisely for the individual stars. Because stars, just like humans, have a typical life cycle. They glow because they fuse hydrogen to helium. And at some point the hydrogen is used up. Now it depends on how big the star is. Because the bigger the star, the faster the hydrogen is “burned up” and the shorter the life expectancy of the star.

Measure the size of stars based on their luminosity

The astronomers can look at the sky and determine: How big is the star? You can measure that mainly by its luminosity. Small stars usually still have a lot of time ahead of them, large stars have a short life and they will soon come to an end. And how much of your life has already passed can often be seen from the color of the light. If the light is reddish, the end of the star is closer than if it shines bright and bluish.

So if there are stars up there that no longer exist at the moment, are they more reddish ones?

Yes, there is a prime example of this, that is the star Betelgeuse in the constellation Orion. It's relatively close, a little over 400 light-years away, and it's very large, 20 times the size of the Sun. And the astronomers find that its luminosity has been decreasing for years, that is, they are not giving it much time. That is, it should be over sometime within the next 10 to 20,000 years. Then it will briefly inflate itself into a supernova and become extremely bright. And although Betelgeuse is so far away, you would definitely notice it on earth. It is said that a light phenomenon will then be perhaps as bright as the moon. But that could be in 10,000 years, maybe its time was over yesterday, only that we haven't noticed it because we can still see its light. We would then only see that after 400 years.

The text and audio of this post are licensed under the CC BY-NC-ND 4.0 Creative Commons license.