Scientists have found very bright source of light which they call quasar and the are found to be supermassive black holes. So these black holes are so massive that they cannot be formed by a supernova. So how are these formed?
That's it, in a nutshell. However, there are some various ideas floating about. Here's a (long) list of some:
From this page:
- Collapse of massive gas clouds
- Merger of lots of stellar-mass black holes
- Growth of a stellar-mass black hole to astronomical (pun intended) proportions
- Core collapse of a cluster of stars
- Primordial black holes coalescing
- Accretion of primordial gas left over from the Big Bang
- The death of really, really massive stars (Note: This would only be the beginning of a supermassive black hole; much more accretion would have to take place. In fact, this can be said of many of the solutions presented.)
But the truth is (and I'll be blunt here) that scientists don't have as much evidence as they'd like for any of these ideas. There are lots of models (just see the ideas above) for the formation of these black holes based on observations of quasars, active galactic nuclei and normal galaxy centers, but it's difficult, if not impossible, to properly test them.
Normal black holes form from the mass of a dying star, so for a black hole to have more mass than the star it got it mass from something must be up. To understand why black holes in Quazars are so large, we need to understand what a Quazar is. Black holes often consume nearby stars, and during consumption, the stardust orbits the black hole and accelerates to speeds close to the speed of light. Friction with other stardust particles cause immense heat and emission of electromagnetic radiation. The black hole gains more mass by consuming nearby stars until it becomes big enough to make a real quazar, the kind that can be seen.