Since a star's light takes so long to reach us, how do we know that the star is still there?
Published: April 2, 2017
We don't know that it is still there. For all we know, the North Star or any other star could have been ingested by a passing black hole years ago. (Although this is highly unlikely, it is possible.) When you look at a star, you are looking at the light that came from the star. Because stars are so far away, it takes years for their light to reach us. Therefore, when you look at a star, you are actually seeing what it looked like years ago. It is entirely possible that some of the stars you see tonight do not actually exist anymore.
Keep in mind that nothing in the universe can travel faster than light. This means that there is no way for us to know about the present state of stars. The most recent information we can ever have about a star is the information in the light that it emitted years ago. For close stars, the information we are now receiving is only a few years old. For the most distant stars in our observable universe, the information we are now receiving is billions of years old. For these distant stars, it is quite likely that they look very different from what we see in the light that we are now receiving. However, there is no way for us to know the present state of a star or for the present state of a star to affect us in any way. Therefore, even if a star that we see in the sky does not really exist anymore, this fact means nothing to us at the current moment. Therefore, it is more of a philosophical question whether a certain star really exists anymore or not. Scientifically, its light is still reaching us and therefore the star still exists from our viewpoint.