Post
by Calilasseia » Fri Apr 12, 2013 6:10 am
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.