How are stars and black holes born?
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This is a SUPER BASIC LAYMANS explanation.
**Star birth:**
A nebula is a gaseous cloud of lighter elements like hydrogen, the building blocks of stars. Nebulae are often referred to as stellar nurseries because they are where stars are born.
Gravity causes the elements in the nebula to swirl together into clumps. Eventually, the mass of “dust” becomes so large that gravity starts to compress it, and the compression heats it up. This continues until there is so much heat and pressure that nuclear fusion can occur. Fusion starts, and hydrogen atoms are fused into helium atoms, releasing a massive quantity of energy and heat, and the star is “lit.” Over time, as the star ages, lighter elements are fused into heavier elements, which is how these heavier elements exist in the first place.
**Black Hole:**
An EXTREMELY massive star is reaching the end of its life. It is running out of elements to fuse, so the energy that pushes outward from the center of the star no longer matches the gravitational force of all the mass that made up the star. The immense gravitational force causes the star to collapse inwards.
The result of this collapse depends on the mass of the star, but the most massive stars collapse inwards to the point that the matter becomes extraordinarily densely packed. This now tiny object is so dense and has so much mass that its gravitational force is extremely powerful. In fact, the gravitational forces are so powerful that not even light can escape. This is a black hole.
Stars coalesce from interstellar dust, until the density/pressure/temperature are sufficient to begin fusion.
Black holes require a star of sufficient mass to ‘die’. Once the hydrogen fuel runs out, the outward pressure from the reaction drops, and the star collapses under its own gravity. A sufficient mass will continue to crush the matter – sometimes it stops at crushing when the protons & electrons are squished together, leaving a neutron star, but if more mass is present, even the neutrons are crushed, creating a black hole.
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
Most of the interstellar dust is hydrogen gas. The current dust also has heavy elements from the remains of ancient stars that died and created heavy elements. So that part mostly goes into planet formation, but of course it increases the forming star’s metalicity.
But most of the gas is hydrogen. That hydrogen can end up accumulating around a point which will be the gravitational center of the forming solar system. As more and more hydrogen accumulates the force of gravity gets stronger. This pulls the hydrogen towards the middle compressing it, accelerating it and heating it up. Near the center this creates a very hot and high pressure environment and in that environment hydrogen fusion can start. For smaller stars its going to be pp fusion and for large stars its a CNO cycle. You can read up on them its not that complicated. But the point is that hydrogen plasma fuses and makes helium which release energy.
When that happens this energy in the form of radiation (light) will exert an outward pressure and that pressure holds against gravity and keeps the star from collapsing. Because the energy output of the star keeps it from collapsing under its own gravity, the mass of the star also defines its temperature. More massive stars must burn hotter.
As the star is fusing its hydrogen in the core it “burns away” this fuel leaving helium. When the star runs out of hydrogen to fuse, the energy output decreases so it cant hold against gravity, the core gets compressed. Gravity accelerates the particles heating them up and the pressure is also increased creating a more extreme environment. Now helium can start to fuse. The star starts to release more energy pushing away its outer layers. For a smaller star this is the start of the red giant phase. As gravity compresses the core the star starts to fuse heavier and heavier elements, until it reaches iron. Iron fusion doesn’t release energy so gravity takes over. For a red giat the outer layers get spread out and the shrinking core remains. The iron atoms reach their maximum density and that holds against gravity. We now have a white dwarf and it will cool and turn into a black dwarf. (The material of a white dwarf can be more intricate, like carbon maybe in the for of diamond.)
For a more massive star, the death is more violent. As heavier and heavier elements fuse they produce so much energy that it blows off the outer layers, thats a supernova explosion. Once that happens that incredibly massive core starts to collapse and not even the atoms can hold gravity back this time. As protons and electrons are accelerated by gravity they turn into neutrons. Neutrons are fermions so the Pauli exclusion principle applies stating that no to fermions can occupy the same state. This is neutron degeneracy pressure which holds against gravity. The result is a stellar core that is as dense as a nucleus, made from neutrons. Its a neutron star.
For an even more massive star with a core that is at least 3 times the mass of the sun not even neutron degeneracy pressure can hold back gravity. The usual explanation is that the rapid collapse forces neutrons to occupy vastly different momentum values which means they can occupy the same positions. So the formed neuron star collapses further and there isn’t any other mechanism that can hold gravity. So all the material theoretically shrinks into a point of infinite density. But this is where General Relativity starts to break down so we dont really know whats actually going on after gravity wins against the neutrons. We are left with an event horizon. A black hole.
The very early universe was a nearly-uniform cloud of hydrogen. Because of tiny “wrinkles” in this cloud, small spots of hydrogen clumped together and their gravity pulled in other, smaller clumps. Eventually (over hundreds of thousands of years), the clump gets big enough that its gravity crushes the hydrogen in its core so hard that fusion starts and stars are born.
That is how the first generation of stars were born. The Sun is a second-generation star, created from an enormous cloud of hydrogen and heavier elements created by the lives and deaths of the first generation.
What happens when a star dies depends on how big it is. If it was big enough that it had enough fuel to start fusion of iron, then it dies spectacularly. There is no more radiation pushing outward against gravity, so the star implodes, crushing its core. If the star was still big but below a certain threshold, the core gets smashed down into the densest stuff that can still be called matter: neutronium. The outer layers bounce off the neutronium core, causing a supernova that blasts bajillions of tons of material out into the universe, leaving behind a neutron star.
If the star was bigger, then the inward crush of all that falling matter is enough to compress the neutronium past its breaking point. That breaking point also breaks physics as we know it, creating a point in space-time so dense that its escape velocity is greater than the speed of light. That singularity is what we call a black hole.
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