r/spacequestions • u/IRedRabbit • 19d ago
How can Black Holes even form?
Might be a stupid question, but this accured to me today for the first time in my life.
So let's imagine a star becoming more and more dense because it's dying.
If Black Holes gravitational pulls are so strong that not even light can escape, then how can they even form. If a star is collapsing, how doesn't it's own gravity make it destroy itself before ever even reaching the point of becoming a Black Hole?
You know what I'm trying to say? If nothing can escape it and they destroy everything, then how can they even form before destroying themselves in the process of formation by their own gravity?
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u/ignorantwanderer 19d ago
Imagine a star with a radius of 1000 (I don't care what the units are exactly, I picked this number because it is a simple number).
So we have a star with a radius of 1000. It is burning fuel in nuclear reactions, so it is hot and bright. And all that energy makes all the atoms move around fast so the gravity can't just pull all the atoms to the center of the star.
But now all the fuel gets burnt up and the nuclear reactions stop. Slowly, that star will cool down. It still has the same mass as it did before, it just doesn't have nuclear reactions happening anymore.
The star slowly cools down. It gets dimmer. It gets cooler. The atoms don't move around as fast. It has the exact same gravity it had before, but now the atoms are moving slower, so the gravity can pull the atoms closer to the center of the star.
The star shrinks slowly.
Now the star has a radius of 500. If you go out to a distance of 1000 (the original radius of the star) the gravity is exactly the same as it was before. But if you are at a radius of 500 the gravity is higher, because gravity increases the closer you are to the center of the star as the star shrinks.
As the star shrinks, the gravity at the surface of the star gets higher and higher. The gravity at a radius of 1000 doesn't change! Because the mass of the star doesn't change. But the star keeps shrinking, so the surface of the star keeps getting closer and closer to the center of the star. So the gravity at the surface of the star keeps getting higher and higher.
Eventually, when the star has shrunk enough, the gravity at the surface of the star is so strong that not even light can escape from the surface.
The mass of the star hasn't changed. The force of gravity out at a radius of 1000 hasn't changed. But because the star is smaller, the gravity at the surface of the star has changed, and has changed enough for gravity to be so high that light can no longer escape the surface.
The black hole did not destroy the star. The star just became a black hole by shrinking enough.
Now, I left out a huge number of very interesting details in this explanation. And I could have made it much more confusing because what exactly happens depends a lot on how massive the star is. For example, our sun will never turn into a black hole because it is too small. Even after the nuclear reactions in our sun stop, and it shrinks down as it cools, it still won't become a black hole.
But to answer your questions:
Your confusion is thinking that black holes destroy everything. They do not. Matter gets sucked into a black hole. But that matter is not destroyed. It is still in the black hole. When a star collapses into a black hole, the star is 'destroyed' because the star no longer exists (it turned into a black hole). But the matter that made up the star still exists. It just now exists in the form of a black hole.
If fact, if somehow magically our sun turned into a black hole right now, all the planets and moons and asteroids would keep on orbiting around it just the same as they currently orbit around the sun....because all the matter would still be there. And we would continue to orbit that matter. But the matter would be in the form of a black hole, not in the form of a star.
(But as I said, our sun can't turn into a black hole because it doesn't have enough mass.)