If a star burnt out, at several million light years away, how long would it take for humans to notice.

Exactly the same number of years as its distance in light years. So, for example, if we see a star supernova in a galaxy four million light years away, we are seeing what actually happened four million years ago.

There is a correction required if the object is so far away that the expansion of the Universe changes the distance between the object and us during the time it takes the light to get here; but for that to make any significant difference the object has to be hundreds of millions of light years away. For distances of mere millions of light years or less, the fact that light covers one light-year in one year (which is how we define a light-year) means that the two numbers (distance in light-years and time in years) are the same.


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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