Let me start by saying don’t worry. Our sun has about five billion years left to live, although the Earth will become inhospitable in about a billion years. By that time we’ll be extinct, have evolved into something else and/or have left our homeworld. The reason for this is that the sun is slowly getting hotter.
So, what will happen to the sun, eventually, long after any of us have to worry about it?
The Sun Will Not Go Nova
With all the talk about Betelgeuse’s odd behavior, it’s worth laying this to rest right away.
Sol is not going to become a nova or supernova. The reason is simple: Our sun is too small. In fact, Sol is a type of star called a main sequence yellow dwarf (G2V).
The “dwarf” part is key here. Dwarf stars don’t have enough mass to go nova. Giant stars are the ones which create nova and supernova explosions. (And all stars are one or the other; dwarfs or giants. There are subgiants, but there’s no “regular” sized star in the terminology we use). Oh, and the sun is also way too small to turn into a black hole.
So, What Will Actually Happen?
Right now, our sun is in the main sequence. While, as mentioned, it’s slowly getting hotter, it’s still stable.
In about five billion years, Sol will run out of hydrogen. The helium-filled core will compress and the star’s atmosphere expand. The yellow dwarf will become a red giant, expanding to consume the inner planets (including the Earth, although by that point it’s likely it will be uninhabitable).
This stage will last about a billion years, while the sun uses up remnant hydrogen in its outer core. Then the helium in the core will fuse into heavy elements and the star will shrink and become a white dwarf, only the size of the Earth. Said white dwarf will be in the middle of a planetary nebula, although a very faint one.
And it will sit there for a good long while, slowly cooling. After a few more billions of years all that will be left is a brown dwarf, a remnant that no longer shines.
But again, we don’t have to worry about this. If we get off this rock, then our descendants may mourn the sun, but they probably won’t be anything like we are now if they exist.
Kind of morbid, but hey, at least we don’t have to worry about the sun blowing up on us in the near future.