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Comment by astletron | Hacker News Reader

"Nighttime will be a different story. The brightness of Betelgeuse’s supernova is about the same as the quarter moon."

This will last maybe a couple of weeks, but we don't expect it for 100K years.

No, but with billions of stars in our own galaxy, and more beyond, wouldn't (my misunderstanding of) the probability of witnessing a past supernova event during one's lifetime be higher than it feels like it is?

Not really. There are a huge number of stars, but most of them are moving away so fast that the light from their death wouldn't ever reach us, and their light would be shifted in to parts of the spectrum we can't see. Similar to Olbers' Paradox - http://en.wikipedia.org/wiki/Olbers%27_paradox

> most of them are moving away so fast that the light from their death wouldn't ever reach us

Correct me if I'm wrong, but I believe that's mistaken. Given enough time, even things moving away from us at the speed of light will eventually be seen by us. http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope

> ...galaxies that are more than approximately 4.5 gigaparsecs away from us are expanding away from us faster than light. We can still see such objects because the Universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated, there will never come a time that we will see the light from such objects being produced today (on a so-called "space-like slice of spacetime") and vice versa because space itself is expanding between Earth and the source faster than any light can be exchanged.

https://en.wikipedia.org/wiki/Metric_expansion_of_space

The rate of expansion of space-time is currently accelerating so the ant on a rubber rope analogy does not hold.

Edit: from the ant on a rubber rope article you linked

> However, the metric expansion of space is accelerating. An ant on a rubber rope whose expansion increases with time is not guaranteed to reach the endpoint.[3] The light from sufficiently distant galaxies may still therefore never reach Earth.

This is complicated. Given the accelerating expansion of the universe, there's an eventual "horizon" where light beyond that limit will never manage to get here, simply because the empty space between here and there will expand faster than the light can make up the distance. My understanding is that under current cosmological models, a likely scenario is that eventually (on the order of multiple billion years), everything beyond our local gravitationally-bound group of galaxies (us, Andromeda, and a few hangers-on) will be 100% invisible forever. Kinda makes you wonder what distant-future astronomers might think of the old tales of "galaxy clusters" or "cosmic structure formation".

Moreover, A good number are in the Milky way or the Andromeda galaxy, which is bound to collide with ours; plus the respective satellite galaxies of the Milky Way + Andromeda.

Will Univers' expansion make even them "disappear"?

First of all, the "pastness" of looking out into space doesn't really affect probability of witnessing them.

Second, supernova occur quite frequently: http://en.wikipedia.org/wiki/List_of_supernovae That's not even a complete list, just the "important" ones. The odds of one occurring within our lifetime that is naked-eye visible are really quite good. The odds of it being spectacular much less so. It's likely to just be a point in the sky that wasn't there last week and won't be there next week.

It depends in part on what you mean by "witness" a supernova. We discover supernovae all the time (hundreds a year), but pretty much all of those are too faint to be seen with the naked eye. There's a decent chance of seeing a supernova visible with the naked eye in your lifetime, but a pretty small chance of one as bright as betelgeuse.

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