I’ve always been told that massive stars undergo supernova events, while smaller ones like the sun just grow into red giants, to then just shrink back to a white dwarf. My point is: how do we know red supergiants aren’t just regular “Suns” that are just on their giant phase?
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Let me expand on the excellent answer by /u/Aramike :
TL;DR Antares and Betelgeuse are red giants, just very big ones. They haven’t gone nova or super-nova because they aren’t yet old enough or haven’t hit anything big enough to set the nova off.
First, Antares and Betelgeuse **are** old stars, nearing the end of their life. The exact recipe of nuclear fusion going on inside them matches our understanding of how old stars behave. They just started off bigger than our Sun. Hence the name Red Super-Giant.
Virtually all stars we can see are almost entirely hydrogen. The fusing hydrogen becomes helium, some of that helium becomes other elements higher up on the periodic table. This process takes time and the speed it happens depends highly on how big the star is. (big stars grow other elements faster than small stars)
Most stars in a given section of a given galaxy formed around the same time. So they all started with the same balance of hydrogen, helium and other stuff they collected from the clouds they formed from. That “other stuff” is from the ashes of previous, long gone stars that blew up in novas or super-novas.
For weird reasons I won’t get into here, astronomers call ALL of those other elements “metals”. If the astronomers look at the colour of the light a star gives off, they can determine just how much of these “metals” and how much of each, the star has. If you have an estimate for how much “metal” got included when the star first formed, you can figure out how much the star has made on its own. You can also confirm your estimate of the size of the star that you made in other ways.
A star goes nova when the amount of matter it has suddenly increases. This always seems to be the result of a binary star system colliding. When stars orbit each other, over billions of years they get closer and closer until they merge. Neither Antares or Betelgeuse have a stellar neighbour close enough to have collided yet. Antares is part of a stellar group that hasn’t spiraled into each other so far. Betelgeuse is a lone “runaway star” that is cruising through the galaxy all on its own.
A super-nova can occur when two very massive stars hit, or when a very very old and massive star goes through a radical change. For example, when the very old massive star finally cools enough that gravity can overwhelm the temperature pressure and the star collapses. There are different classes of collapse, depending on how big the star started off .
If it collapses into a white dwarf or a neutron star, eventually that object will end in a super-nova. But if the star is big enough when it gets old, it can collapse faster than it can blow up. When that happens, you end up with a black hole.
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