When did a supernova actually occur if it happened in a galaxy 160000 light years away?

Answer 1

If we're just SEEING the supernova now, although it was in a galaxy 160K light years away, then it actually exploded 160,000 years ago. The light has been on its way to us (actually, expanding in all directions!) for 160,000 years.

If it were to explode right now, we wouldn't know about it for another 160,000 years.

This may become an important point some day. The red giant star Betelgeuse, the shoulder of Orion, is about 600 light years away. As a giant star, it burns very quickly and will "soon" explode. ("Soon" is relative to the lives of stars, not people. It may be any time within the next half-million years.) In fact, it may have already exploded - and we wouldn't know it until the light arrives here, hundreds of years later!

Answer 2

If a supernova occurred in a galaxy 160,000 light years away, we would see the event 160,000 years after it happened due to the time light takes to travel that distance. So, if we observe a supernova today in that galaxy, it actually occurred 160,000 years ago.