Question: Hi :) How are black holes formed?

Sara Imari Walker answered on 17 Jun 2011:

Hello rachel!! Great question!! Black holes are fascinating aren’t they? There are two common kinds of black holes that are formed very differently.

The first are supermassive black holes at the centers of galaxies. At the center of most every galaxy (at least ones we know enough about to say) there is a black hole that helps to keep the entire galaxy gravitationally bound. These black holes are huge! On order of thousands to billions of times the mass of our Sun. We are not exactly sure how these black holes form, but we think it has something to do with the galaxy formation process.

The second kind are probably the ones you are more interested in. They certainly are the more well known ones. These are black holes that have masses in the range of a few times the mass of the sun to upwards of 100 times it mass. These black holes are actually formed from dying stars!! Stars much more massive than our Sun die in a very interesting way. They actually explode in what we call a supernova (type II supernova if you want the technical jargon). The explosion is very complex. All stars fuse Hydrogen to form Helium in their interior for most of their lifetime – this is exactly the process that powers even our own Sun. However, once Hydrogen stops burning in the stellar interior the outward radiation pressure can no longer sustain the star against the inward force of gravity so the star implodes. The implosion is halted by Helium ignition: implosion increases the pressure and temperature in the interior and the star starts burning Helium (Hydrogen burning produces helium but initially the interior of the star is not hot enough to burn helium). For very massive stars, this process repeats several times; every time burning of a nuclei species ceases the core collapses and the collapse is halted by the ignition of a further process involving more massive nuclei (He first, than Carbon, Neon, Oxygen, Silicon) at higher temperatures and pressures. The star starts to look like a layered onion! Eventually nickel and iron are produced in the core. The iron-nickel core cannot undergo fusion. The pressure of the star causes it to implode, but no fusion ignites to create pressure to counteract the implosion!! For massive enough stars the collapse due gravity is unstoppable. A black hole is formed. Pretty wild isn’t it?