How are stars and black holes born?
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Stars are a kind of joint physical/nuclear reaction.
You have a lot of stuff – mostly hydrogen because that’s what most stuff in the universe is. So much stuff that it gravitates together into a ball. So much stuff that that ball compresses in on itself, packing in tighter and tighter because of gravity.
Now, that stuff will keep getting packed tighter and tighter together because of gravity. Eventually, it might reach an equilibrium in the form of a neutron star or white dwarf. If there is enough stuff, it will pack so tightly together that its gravity visually bends light that gets within a certain distance of it so that light can’t escape or pass by it – a black hole.
But before that any of that can happen, something else happens. Most of this stuff is hydrogen, and when you pack hydrogen under a lot of pressure, it begins to heat up and fuse together to form Helium.
This fusion reaction pushes a lot of energy outwards. Some of it is heat, some of it is light. And a lot of it is a physical outward force that counteracts gravity. This is what a star is – a big ball of hydrogen that has enough pressure in its core to cause fusion. That fusion pushes back against gravity, preventing the star from collapsing in on itself from the gravitational force it exerts on itself. A star is born when enough mass is clumped together to get to that point.
Eventually, that star will run out of hydrogen. It will start fusing helium, then lithium, and so on. If Eventually, it will get to a point where it no longer produces enough outward force from fusion to prevent itself from collapsing under its own gravity. These stars go supernova, expelling its outer material and collapsing down to a black hole or neutron star.
Smaller stars merely shed their outer shell less violently and remain as white dwarfs, slowly cooling down over millions of years.
Smaller star
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