system
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human
Timescale of Nebula formation?

Basically title; for instance we’re due to see the supernova of  Betelgeuse in around the next million years, and we will be able to see that happen and watch whatever happens afterwards. If that’s a nebula forming , what sort of timescale does it happen over ? Like how long does it take for the supernova to end and the nebula to be fully formed ? I’m unsure of the timescale of the end of a stars life and what happens afterwards if anyone could tell me that would be epic thanks.
gpt
A supernova remnant is quite a short-lived type of nebula. What happens is that the gas from a supernova remnant quite rapidly dissipates into the *interstellar medium*, the general mix of turbulent gas that the stars of the Milky Way are swimming through. The gas is expanding very quickly, and its density drops downto the background within a pretty short time. A "short time" is less than a million years (maybe even less than 100,000?) - most of the supernova remnants we see are only thousands to tens of thousands of years old. There are also very young ones, like SN1987a, which we have seen evolving for about 34 years.

By comparison, a molecular cloud is quite a different type of nebula. Within the interstellar medium, gas sometimes gathers together and starts to cool down. Gas cools down by particle collisions, and if there's some part of the gas that has a higher density, there's slightly more collisions because there's more particles, which means the gas cools down slightly faster. Cooler gas has a lower pressure, which means that it has less resistance against gravity. Denser gas also has stronger gravity. So the gas will contract, making it denser. This means the particles can collide even more often, and cool down even faster. So you get a collapse all the way down into a *molecular cloud* - a turbulent cloud of gas that's cool enough to form molecules. This will continue to fragment and collapse and form a star cluster. The molecular cloud then gets blown away by the radiation, winds, and supernovae of the new stars that are formed. Typically a molecular cloud might last for 10s of millions of years. So the nebulae that form stars are very different from the nebulae that are formed by supernovae.

As for what you'll actually see after Betelgeuse explodes - the Crab Nebula is a good comparison here. This is a supernova that went off about 1000 years ago, and is expanding at 1500 km/s, which is fairly typical. It's over 10x further away than Betelgeuse, and after 1000 years it's maybe 1/3rd the size of a full moon on the sky. So, on a historical scale (rather than an astronomical scale), it's reached a reasonable size in a not ridiculous amount of time. Betelgeuse is much closer, and you might expect it to reach the size of the full moon after maybe a century, and 1/10th of the size of the full moon after maybe 10 years. So definitely quick enough and big enough that a human with binoculars could see it change significantly over their lifetime.

The bright "star" of the supernova itself will last for week or months, depending on where you set the cutoff for "too dim to count as a supernova"