Stars burn through a process of nuclear fusion, which, in the main, fuses hydrogen atoms together to form helium. As the hydrogen ‘fuel’ gets used up, the star condenses and heats up even more and the star starts to produce heavier elements such as lithium and beryllium (numbers 3 and 4 on the periodic table), all the way to carbon (number 6) and oxygen (number 8), fusing each element in turn to keep on burning.
Small stars may stop there but big stars can go on fusing subsequent elements all the way up to iron (number 26).
Stars don’t like fusing much beyond iron and when they’ve finished burning all their fuel, which creates an outward pressure, they’re at the mercy of gravity.
The outer layers of the star might explode in a supernova and the core will collapse in a way that depends on the original mass of the star.
Lighter stars, up to about 10 solar masses, become white dwarfs. Such stars are hot when they form but no fusion is taking place and they eventually burn out to become a black dwarf. This accounts for about 97% of stars in our galaxy.
If the original star is between 10 and 30 times the mass of the Sun it can become a neutron star. These are incredibly dense stars. A thimble full of their material can weigh nearly a billion tons and they rotate many hundreds of times a second.
If the original star is more massive still, it can collapse into a black hole.
During the process of forming the black hole, there is supposed to be a gap. If the core is up to 50 times the mass of the Sun or over 130 times the mass of the Sun, the black hole will form. But the physics of a core between 50 and 130 times the mass of the Sun should go into a runaway process that instead creates an explosion called a pair-instability supernova.
There should be no black holes in the 50 to 130 solar mass range. That’s what the theory says and we’ve never observed a black hole in that range. Or at least that’s been the case up until now.
Astrophysicists monitoring the LIGO and Virgo gravitational-wave detectors may now have found a black hole in the range where they’re not supposed to exist and I link to the article describing this potential observation.
There are a couple of $100 bottles of wine at stake here. Physicists like a wager, particularly about black holes it seems, and some thought there was a way black holes could indeed form in the forbidden range. Bets were made and wallets are at the ready to pay out.
Note that none of this has anything to do with the supermassive black holes at the centres of galaxies. These can be millions or billions of times the mass of the Sun and are (probably) formed in a different way, maybe shortly after the universe was sneezed out of the Great Green Arkleseizure's nose.