When we observe stars in the night sky, we are actually seeing the light that was emitted by those stars in the past. Light travels at a finite speed, and it takes time for the light from a distant star to reach our eyes or telescopes. This means that the stars we see are not necessarily in their current state.
The distance between Earth and the star determines how far back in time we are observing the star's light. For example, if a star is located 100 light-years away, the light we see from that star left the star's surface 100 years ago. If the star were to suddenly disappear or change in some way, we wouldn't immediately know about it because the last light we received from it would continue to travel to Earth for another 100 years.
Therefore, we can say that we see stars as they were in the past, with the time delay depending on their distance from us. The farther away a star is, the longer the light takes to reach us, and thus the greater the time difference between the current state of the star and the light we observe.
It's important to note that while we can observe distant stars and galaxies from millions or even billions of light-years away, our ability to see stars that no longer exist is limited. If a star ceased to exist before its light reached us, we would never see its disappearance because the light would never reach us.