Not to hijack this thread:
I know that novae and supernovae are rare, and may take a huge number of years to become visible from earth. But considering the number of galaxies we can see, and the number of stars in each galaxy . . . why isn’t the night sky filled with their light?
This is a variation on Oblers’ Paradox.
It is. Supernovae happen very, very frequently in the deep sky as a whole - but in galaxies so far away, they are very dim.
Each supernova will approximately double the brightness of a distant galaxy -but since most galaxies are completely invisible to all but the biggest telescopes, this doesn’t add much light to the sky which we can see.
Too late to edit, but Cecil did a column: Why is the Night Sky Dark?
Apparently there are thirty supernovae per second in the visible universe;
Which is itself a variation of Olber’s paradox, I presume…
No need to cite Olbers’ (note) paradox.
The Universe contains a small amount of energy per unit of space. While stars (and supernovas) seem quite bright, the average over very large distances isn’t all that great. (Think hundreds of millions of light years across.)
Note that our “region” gets the same energy from neighboring regions as they send to us. I.e., at a large scale, our vicinity is as bright as the neighboring ones. So our region isn’t lit up all that much nor is any other region.
Olbers’ paradox assumes an unlimited supply of energy per unit of space. Which is a major blunder.
Metric expansion of space redshifts them out of the visible spectrum, lowering their energy and making them harder to detect. That’s why you don’t see them that much.
It does not. It assumes an unlimited supply of unchanging space.
Doesn’t matter, see my post. You don’t need expansion of the Universe to explain any of this. Just elementary Thermodynamics.
“Unchanging” in the sense of “always able to produce more energy forever and ever.” I.e., an infinite amount of energy available per unit of space.
Same thing.
No, that is inherent in the assumption of a static, infinite universe. If the universe is infinite and has existed for an infinite amount of time, then by definition that means the universe has an infinite supply of energy per unit volume.
You’re arguing past one another. Olber asked how it could not be that each square area of the sky was the visual representation of an infinite number of stars that were placed, at varying distances, in that segment of view.
That included many assumptions. Space was assumed to be infinite, for there was no reason to think otherwise. Each section of space would contain an approximately equal number of stars (or clumps of stars like galaxies) because there was no reason to think otherwise. The light from all those stars would reach us eventually. because space was static. Even though the more distant stars would be less bright the growing number in the enlarging cone of space would make up for that. The universe was timeless, so there had always been enough time for the light to reach us.
So you are all right, but each of you is giving only a fraction of the total number of assumptions that are hidden.
It only takes a modest amateur telescope to see a supernova every few months to every couple of years depending the size of your telescope and your observing skill and determination.