Re: Could a galaxy form from the death of a star in a nebula?

Hi Jerry,

Astronomers have found much evidence for supermassive black holes in the centers of galaxies. By supermassive, we mean that their masses range from about a million to over a billion times the mass of our Sun (astronomers use the mass of the Sun as a mass unit, the Solar mass).

There is a great deal of speculation about how these black holes formed. It seems unlikely, though, that they formed by ordinary supernovae from massive stars. Those should leave black holes with masses of a few Solar masses. If such a black hole were surrounded by a gaseous nebula, it would, as you suggest, accrete matter. In fact, one way that we can "see" black holes is by looking for the signs of gas being heated up to very high temperatures as it spirals into the black hole.

But to accrete millions or billions of Solar masses of material would require a very long time, longer than we think it took for supermassive black holes to form. We believe that quasars, for example, are powered by supermassive black holes accreting gas in their centers. We see quasars that date back to when the universe was only a couple of billion years old, it's hard to imagine any way that a black hole that started with only a few Solar masses could accrete so much material in that short a time.

There's an interesting relationship between supermassive black holes and their galaxies--the mass which we infer for the black hole is almost directly related to the mass of the bulge of the galaxy. Galaxies are divided into disk and bulge components. The disk is the rapidly spinning, frisbee-shaped region where the youngest stars are, while the bulge has older stars which move more at random, and is more concentrated to the center than the disk. Some galaxies are almost all disk, others can be essentially all bulge, while galaxies like our own have both components.

The relationship between the black hole and bulge mass might indicate that the black holes acted as "seeds" for the formation of the galaxies. Perhaps, the bigger the black hole the bigger the bulge of the galaxy which formed around it. Or, they started out as smaller seeds, around which small galaxies formed, and the small galaxies then merged to form larger galaxies. When the galaxies merged, their central black holes would also eventually merge to form a larger one. Both of these pictures have the black holes forming either before the galaxies, or at the same time. How that could have happened is the subject of much speculation and research.

You can find more information about black holes in galaxies at the BBC web site, at Victoria University in New Zealand, and in an article done for Stardate at the University of Texas.