How come black holes are so massive?

[JacksSmirkingRevenge]

I mean, when they first form they can't have more mass (and therefore gravitational pull) than the star they started out as, can they?
Is it something to do with density?

[Bella Fortuna]

Are you just being dense?

[JacksSmirkingRevenge]

Might have known you would gravitate here.

[Bella Fortuna]

It just sucked me in...

[JacksSmirkingRevenge]

[FBM]

I mean, when they first form they can't have more mass (and therefore gravitational pull) than the star they started out as, can they?
Is it something to do with density?

The star it started out as had to be a pretty massive one to begin with. Once it collapses, it starts stealing shit from other stars.

[JacksSmirkingRevenge]

But why would that happen once it becomes a black hole and not before? It's still about the same mass (or less, presumably) with (presumably) the same gravitational pull and still a similar distance away from said other stars as it was before.
I don't geddit.

[FBM]

I think it may help if you think about it as a star in reverse. When it's still a star, it's pushing shit out constantly. A black hole is sucking shit in constantly. A black hole warps space and sucks shit in because of its incredible density. When it's still a star, the mass may be similar, but the density isn't enough to warp space.

[JacksSmirkingRevenge]

Hmmm....Like, say, a lead ball on a rubber sheet vs something like a beach ball of the same weight (but much greater size) on a rubber sheet?

[JacksSmirkingRevenge]

Also, do they constantly draw space-time in (or something) or is space-time merely being warped?
Surely, if it were just being warped then light would be able to find it's way away from the black hole (but take longer than it ordinarily would because it's got further to travel), but...if it were being drawn in faster than light then after a certain point (the event horizon) light would not be able to escape. Am I thinking along the right lines here? (It seems to me to tie in with the notion of a shrinking observable universe where the expansion becomes faster than light which can't make the journey back to an observer.)

I need some sleep.

[Calilasseia]

in effect, it is a matter of density. The sort of large O class supergiant stars that eventually produce black holes after a supernova event, contain a lot of mass (some of them contain 150 times the mass of the Sun, for example), but that mass is diffuse, and occupies a huge volume. Take for example, the hypergiant star VY Canis Majoris, which is an eminent candidate for a black hole forming supernova event in the future. This star has a mass of about 20 solar masses, but occupies an immense volume - if you replaced the Sun with VY Canis Majoris at the centre of the Solar System, the incandescent gas envelope would extend out to beyond the current orbit of Jupiter.

A supernova event is in the making when such stars, which consume their nuclear fuel extremely rapidly compared to much smaller stars, progresses from hydronge fusion, through helium fusion, and then carbon and oxygen fusion. Eventually, silicon fusion is ignited (at a temperature of 3.5 billion Kelvins!), and this process produces a nickel-iron core at the centre of the star. Since self-sustaining fusion ceases to be possible for iron and heavier elements (any such fusion reactions consume energy instead of liberating energy, as is the case with lighter elements), the star has reached a dead end. Once that iron core reaches a mass exceeding the Chandrashekhar Limit (around 1.4 solar masses), it's game over for the star. What's more, once silicon fusion is ignited, the process leading to the formation of an iron core is extremely rapid - it takes just 24 hours to reach the dead-end stage of a 1.4 solar mass iron core.

At this point, you have to delve into the minutiae of supernova mechanics to work out what's going on, but that iron core, when it exceeds the 1.4 solar mass limit, collapses in upon itself. Electron degeneracy pressure is no longer able to withstand the attractive force of gravity, and the whole core collapses to a volume not much bigger than that of the Earth. This collapse occurs at 25% the speed of light.

It's not known precisely what conditions differentiate neutron star formation from black hole formation, though it's hypothesised that a little mass excess over the 1.4 solar mass limit could be sufficient.

Now, once a black hole has formed, the problem you have is not that the black hole thus formed contains more than 1.4 solar masses. The problem is that said mass is compacted into a volume smaller than the Schwarzchild radius, which can be thought of as the radius within which the escape velocity exceeds the speed of light. Any object passing that black hole at a distance, will experience the same gravitational effects that it would if it passed by a normal 1.4 mass star at the same distance. What changes is what happens when such an object approaches closely to the black hole. At short distances, the gravitational pull is enormous, and consequently, any matter entering a certain region around the black hole will find itself being inexorably attracted more and more powerfully toward the black hole. Eventually, that matter will fall in, and add to the mass of the black hole, making the black hole even more powerfully gravitationally attractive to outside matter.

Now, in the case of black holes far from the centre of a galaxy, there are relatively few opportunities for such growth, though such opportunities have been observed through relevant astronomical instruments. A black hole at the centre of a galaxy has far more numerous opportunities to increase its mass via gravitational attraction and accretion of infalling mass, because stars and interstellar gas are crowded much more closely together. As a black hole in such a location gains mass, those opportunities grow, until the black hole has acquired seemingly ridiculous amounts of mass, and cleared the space around it of material.

Hope this proves useful.

[Rum]

Also binary systems where one star sucks another one in resulting in enough mass to prevent the escape of light.

(Also known a a 'cosmic blowjob'...no really..)

[Clinton Huxley]

That's the thing with black holes, their bums look big in everything

[JimC]

But why would that happen once it becomes a black hole and not before? It's still about the same mass (or less, presumably) with (presumably) the same gravitational pull and still a similar distance away from said other stars as it was before.
I don't geddit.

Actually a fair bit less mass, since in a supernova, a fair amount of the outer core is blasted into space.

The inner part of the core collapses, and if it's over a certain mass, it continues to collapse, forming a singularity. The intense concentration of mass warps space such that nothing may escape - the core of the original massive star occupies a vastly more constricted space...

However, one could still orbit it at a respectable distance with no ill effects, other than the intense radiation released if any in-falling matter spirals in...

[Clinton Huxley]

Also, space isn't empty, so over time an amount of matter would fall into the hole. Lost socks, that kind of thing.