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Astronomy/Light through the universe


If an object supernovas 4 million light years from Earth, the light we see is 4 million years old, I get that.  By this time, the object has probably gone cold, right?  If so, how is it we still see the light from the supernova?  If I turn on my flash light for 24 hours, then turn it off, the light disappears, how is the light from the supernova still visible to me?  I'm sure I am missing something here,

You're correct in assuming that the onetime supernova is long "gone" by the time we see it. In fact, depending upon what kind of supernova it is, the onetime star may no longer exist within seconds of the time its light left it.

However, the light it emitted doesn't "disappear" just because the star is no longer there. It just keeps going and going until it runs into something (which most of the time is never, but in the case you've proposed is 4 million years later, when it runs into us). And when it does run into us, we can see that light for however long it lasted. For instance, if it lasted the same 24 hours as your flashlight example, we would see the light for 24 hours. At the start of that 24 hours, we would see the light that was emitted by the flashlight/supernova when it "turned on", and at the end of that 24 hours, we would see the last of the light that was emitted by it, but after that we wouldn't see it any more.

The thing that's confusing you is that you assume that when you turn the flashlight off, the light disappears, in the sense that it no longer exists. But it hasn't completely disappeared. It still exists, but as a beam of light 24 light hours in length, heading in the same direction the flashlight was pointing, for however long it takes the light to run into something. And if the flashlight just happened to be pointed in the direction of something 4 million light years away, then someone there would be able to see it (albeit very faintly) for the same 24 hours that it was originally on.

In the same way, for however long it took the supernova to stop shining -- whether it was days, weeks, or months, when its light finally ran into something, no matter how far away it was (even billions of light years away), whoever was "there" would be able to see the light that was emitted by the supernova for exactly the same length of time that it was being emitted.


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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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