Also can someone explain the specifics of what causes the star to compress and reverse its gravity to begin a black hole. Thank you thank you thank you. ----stary
Well, as someone else said, gravity does not reverse. Gravity is a one-way good deal that just pulls(or attracts). Anyway....
Anything that has mass also carries with it a gravitational field. Stars have an enormous amount of mass therefore they have some pretty hefty gravity. As you read this, you are orbiting a puny little star that's 93,000,000 miles away. That's a lot of gravity!!
Without some other force to keep that huge amount of mass from crushing itself by virtue of it's own gravity, you need some other source of energy.
Stars burn hydrogen, and they burn a lot of it. The nuclear fire that burns inside a star creates extremely high temperature and pressure. This nuclear fire pushes outward and keeps the star in a state of DYNAMIC STABILITY. Gravity is trying to squish it, nuclear fire won't let it.
This goes on for a while. Thousands, millions, billions of years.........however long is dependent on how massive the star is. The more massive it is, the sooner the star will use up it's fuel and leave the MAIN SEQUENCE. More massive=less time because the amount of fuel it burns(therefore creating the appropriate amount of temperature and pressure) goes up in a disproportionate amount compared to the mass. Did that make sense???
Eventually, the star burns it's fuel up and starts to collapse because the nuclear fire that's held it in a state of dynamic stability for so long no longer has the hydrogen to keep it going. Burning hydrogen fuses it and makes helium.....burning helium makes....blah, blah...on and on until it just keep burning and burning until all you have left is iron. When it gets to that point, no more energy can be obtained from nuclear reactions. The outward push will stop, but the gravity of it's own mass will still be there. At this point it collapes into something different than what it was before. What it contracts to is dependent on the mass that's doing the collapsing.
What makes a black hole is density. If the contracting mass is high enough, it will get by the next few steps and be a black hole.
If matter is compressed enough the electrons get squished and form a sort of free-flowing electronic fluid. When the mass of matter contracts to this point and does not compress farther, you get a white dwarf star.
If a star is massive enough to begin with, the increased density of the electronic fluid isn't enough to keep it from collapsing either and it will continue to shrink into a neutron star(made of neutronic fluid, or neutronium).
If a star is yet even too massive for neutronium to withstand the crushing forces, it will then cruise right by the white dwarf and neutron star stages and become a black hole.
By the same token, if you could take some matter and squish it enough, it would turn into a black hole. A star is just the most common(albeit large) example of how to come up with that much compression. Some people think there's a whole pack of miniature black holes running around from the big bang because the violence of the explosion should have compressed matter to the state of density required for black hole formation.
There's more stuff.......red giants, nova's, supernova's, that swartsguy radius......but I'm tired of typing for now.