Is it true that the stars we see are already dead?

Question posed by Jo

Artist's impression of a dead star called a 'pulsar'. Credit to NASA/JPL-Caltech

Light is quick stuff
Light is really, really fast*, but it still takes time to get from one place to another. When you 'see' anything, what you are really doing is using your eyes to detect the light that is either given off or reflected by it. If you look at the clock across the room, you're not seeing it as it is now, but as it was a tiny, tiny fraction of a second ago. Of course, light is so fast that over such a short distance it doesn't really make a difference.

So next time you're able to, take a look at the moon, and think about how far the light that it is reflecting has to travel before it hits your eye. It's a lot further away than that clock was, and it still takes only a little bit more than one second to get to you: that means that when you see the moon, you're not seeing what it is like now; you're seeing what it was like a second ago.

Now imagine looking at the Sun (do not actually look at the sun- this could potentially blind you). The light given off by the Sun travels further still- it takes about eight minutes to get to Earth, so when Earth-based telescopes look at the Sun, they're seeing it as it was a whole eight minutes ago. Put another way, something could happen on the surface of the Sun- a sunspot pops up, for example- and we wouldn't know about it for eight minutes.

Let's go further out, to the next closest star to us, Proxima Centauri. That's so far away that it takes light four years to get here (it's four 'light years' away). It's not likely to do so, but if it popped out of existence, exploded or turned into a giant frog right now, we wouldn't know anything about it for another four years.

Now think about all the other stars up there- most of them are much, much further away: hundreds, thousands, millions and billions of light years away. If a star is a million light years away, and it died a million years ago, we'll see it die now. If another star died four million years ago, but is a hundred million light years away, we'll still be able to see it for another 96 million years.

The image below shows the results of the Hubble Ultra Deep Field experiment. The Hubble Space Telescope was pointed at a tiny patch of sky that appeared to be empty to any ground-based telescopes, and the shutter left open for just under one million seconds (about 11 days). This is what it saw:

Each spot of light in this image is an entire galaxy. There are around ten thousand of them here, some of which were around only a few hundred million years after the universe first came into being. Most of these, although we can see them now, are long gone, and possibly even replaced by other younger galaxies that we won't be able to see for many billions of years.

Are they all dead?
No. Some of  the stars that you can see at night may be long gone, with the light from their death yet to reach us. But plenty more are still burning bright right now, although we see them as younger than they actually are. As well as that, new stars are being born all the time- there will be many burning at this moment that we can't see yet because their light hasn't had time to reach us.

So yes, some are dead, but some are very much alive, and some are there but yet to be seen!




* It's the fastest thing in the universe.