Are the stars we see already dead?
Is the night sky full of ghosts?
No. But.
Everything in Astronomy is huge, like the unfathomable depths of the ocean. Our brains are not built to understand Astronomy. The Sun is big. Saturn is far away. The space is cold? Galaxies are big. It is quite like asking you to think about the concept of infinity. You can learn how to operate with it, but you will never understand it.
|
Yet we believe the stars are further away than they really are. If you ask an average person about the distance to the stars, you will hear things like “millions of light-years away”, “thousands of light-years away”. In fact the stars in our sky are much closer, some as close as just a handful of light-years away. Not like we could understand what a distance of even one light-year means anyway, but the difference matters, and you will see why in a moment.
|
How far are the stars?At a mere 4.24 light-years away, Proxima Centauri is the closest star to the Earth (other than the Sun). At least today. It is not easy to pin down the distance to the farther stars that we can see with the naked eye, but it is something like ten thousand light-years from us.
How long do the stars live?Stars are giant nuclear reactors. The bigger the star, the higher the pressure and temperature at its core, and the more efficient nuclear fusion will be. So the bigger the star, the faster it consumes its fuel and the shorter it will live.
The biggest stars live as little as one million years. The smaller stars can live much longer, around 10¹¹ years* (much longer than the current age of the Universe!)
*That is a one followed by 11 zeroes, like this: 100000000000. But it is easier to compare 10⁹ and 10¹⁰ than it is to compare 1000000000 and 10000000000.
|
You can probably see where this is going now. The stars we can see in the sky are usually dozens of light-years away. A few of them make it into the thousands of light-years away. But they live much, much longer than that. The giants of the sky live for millions of years. And only a fraction of the stars have such short lives.
In short, expecting a star you can see in the sky to be dead is like receiving a letter and expecting the writer to be dead. Sure, it can happen. It WILL eventually happen if we keep looking at the stars every night. But even in that case it will be one, maybe two dead stars in the night sky. The night is very much alive.
|
BUT
If you take a telescope, or even if you learn where to find the Andromeda galaxy with the naked eye, then things are different. Galaxies are much farther away. The Andromeda galaxy, our closest neighbour, is 2.5 million light-years away. And other bright galaxies of our skies are tens of millions of light-years away.
|
So with galaxies it is not a matter of "if". The biggest stars in the galaxies we see today are well dead. You will not be able to see individual stars in the galaxies with a telescope (not before you actually see them die in a supernova) but you can be sure that part of the light you see is coming from stars that have already exploded into supernovae. And there you have the ghosts in the sky.
|
And yet another xkcd comic on this subject!
|
Mouse-over text reads:
"Anyway, that's a common misconception. Geese live for a long time; all the ones we can see will probably keep flying around for billions of years before they explode."
|