7 upvotes, 4 direct replies (showing 4)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
So black holes are formed from collapsing stars, right? If so, then according to mass conservation, a star's mass equals a black hole's mass, right? So then, if a black hole has enough mass to prevent any lightwaves from escaping, why don't stars?
Comment by curien at 26/04/2023 at 18:43 UTC
20 upvotes, 0 direct replies
Because black holes are much smaller, light can get much closer than it could to the original star. It is only at very close distances -- distances which would have been *inside* the star when it was still around -- that light cannot escape.
If the Sun were suddenly replaced by a black hole of equal mass (never mind the complications involved in that), it wouldn't suck in the Earth or other planets, they'd continue orbiting exactly as they are because the rules of gravity are the same for stars and black holes at these kinds of distances.
Comment by B_zark at 26/04/2023 at 19:27 UTC*
8 upvotes, 0 direct replies
To expand on /u/curien's answer, the key here is density. At a distance outside a star or it's black hole counterpart, the gravity felt is the same. But this isn't true after you cross the threshold of the star's surface. When you are inside the star, the effect of gravity pulling you towards the center of the star decreases since there is less mass towards the center of the star. Note that this is not the same as pressure, the pressure will still be very intense, but gravity will be less so. The further into the star you go, the smaller the force of gravity, but the opposite will be true for a black hole since the density of a black hole is infinite. I.e. as you get closer a blackhole's center, the effect of gravity increases.
Comment by CapWasRight at 26/04/2023 at 19:13 UTC
7 upvotes, 1 direct replies
There are a couple of things to address here.
So black holes are formed from collapsing stars, right?
Small ones, yeah. We're still figuring out the big ones.
If so, then according to mass conservation, a star's mass equals a black hole's mass, right?
Not exactly, some of it usually gets blown out in the process that creates it, and some mass is carried away by energy in that process as well (including gravitational waves, which for mergers can be enough energy to amount to a substantial fraction of the original mass of the system). But yea, they still obey conservation laws, it's just that not all of the mass will be *in the black hole once you're done*. So if you read an article saying a 20 solar mass star left a 2 solar mass black hole, that's probably why.
So then, if a black hole has enough mass to prevent any lightwaves from escaping, why don't stars?
Gravity depends on two things -- the amount of mass involved, and the distance between the mass. Solar mass black holes aren't black holes because of their large mass, they're black holes because of their small radius. If you can somehow figure out how to get it to collapse to a small enough size, the Sun would become a black hole. Or the Earth. Or your pet hamster. It's just that no natural processes cause that to happen to Earths or hamsters, but there are such processes for massive stars.
Comment by Fit-Personality-6549 at 18/06/2023 at 00:05 UTC
1 upvotes, 0 direct replies
Well blackholes are just dead stars which are very dense that even light can't escape from its gravitational force...
And also suppose there is a star and a blackhole and they both have same amount of mass... The star would be much more bigger compared to the blackhole cause stars aren't as dense as blackholes...
If you wanna learn more about it try understanding the concept of escape velocity.