Suppose a star is 100 light-years away from the Earth. Therefore, the light coming from the star (which carries its information) takes roughly 100 years to reach Earth. However, if in the meantime the star dies, we would still be able to see that star in the sky. Is that truly right? If yes, how often does it happen when we look up in the sky (any guesses)?
Answer
Yes this is correct. However the kind of event you are thinking about rarely happens for stars in our Galaxy. The reason for this is that the size of our Galaxy is about 100,000 light years across, but the ages of most stars is measured in millions or billions of years. So, the travel time of the light to us is a tiny fraction of the stellar lifetime. For this event to occur, you need to see a star near the end of its life. Such an object is Betelgeuse in the constellation Orion. It is a massive supergiant that may be somewhere in the last 100,000 years of its life. It is however only about 700 light years away, so there is less than a 1% chance that this has already exploded and the light is on its way. It is however your best bet of seeing (with the naked eye) the progenitor of a star that is now dead.
If you go further afield though, to objects in other galaxies, then this issue does arise. Gamma ray bursts are thought to signal the deaths of very massive stars. They are seen in galaxies billions if light years away, but the lifetimes of the stars that produced them may have been a million years. In these cases we don't even see the long-dead star that exploded with big telescopes.
Somewhere in between we can see (with telescopes) the brightest massive stars in nearby other galaxies like that in Andromeda, at distances of millions of light years. Here, these stars are very likely to have already exploded as supernovae, as they have lifetimes of a few million years too, but we still see them "as they were".
No comments:
Post a Comment