Astronomy problem

[Argy]

I was presented this problem in my astronomy class today. It's not homework, but I'm having a hard time coming to a definite answer. It's not a trick question.

Imagine that you are observing the light from a distant star that was located in a galaxy 100 million light years away from you. By analysis of the starlight received, you are able to tell that the image we see is of a 10 million year old star. You are also able to predict that the star will have a total lifetime of 50 million years, at which point it will end in a catastrophic supernova.

Has the supernova already occurred? If so, when did it occur?

I know that it has already occurred, but when is what I am having trouble determining. If someone could explain it, I would be very grateful.

 

[Murgatroyd]

Assuming a universally applicable timeline (i.e. not a trick question), here's the answer with explanation:

As of 100 million years ago, the star was 10 million years old.

The star went nova when its age was 50 million years.

Therefore, it went nova 40 million years later, which was 60 million years ago.

 

[Argy]

Thanks. That was actually my latest conclusion, but I couldn't quite explain how I got to it in simple terms.