Supermassive stars

My star is bigger than your star.

The early universe supermassive black hole problem, as it is sometimes called, is so severe that the community has dreamt up several rather extreme solutions to it. One idea is that some of the first generation of stars were themselves supermassive (a thousand to a million times the mass of our sun).

Is such a thing even possible?

We don’t know. But it doesn’t seem completely impossible. There are several different ways such a star might form. For an example with pretty pictures, see this recent paper on turbulent cold flows. The white circles show the formation sites of two of these supermassive stars:

But once they form, can they survive? Of particular danger to a supermassive star is the general relativistic radial instability. In a normal star, the effects of general relativity are minor, but here, they can cause the star to collapse. From the point of view of building a supermassive black hole, that is exactly what we want, because the star collapses into a black hole!

But what if it doesn’t?

In 2014, a group showed that the star might instead explode in a supernova 1000 times more energetic than normal:

We took another look at this supernova with a more realistic code and found that it might indeed explode. More recently, we conducted a rigorous study of the general relativistic instability that causes the explosion and determined that not only is the explosion possible, but it has a much wider mass range than previously thought.

Another possible way of making a supermassive star is by merging two specific types of galaxies, which for some technical reasons is thought to result in a dense disk at the center of one of the galaxies, which then becomes a supermassive star. The interesting thing about this scenario is that it can happen for any type of gas (the above required metal free gas). This means that you could in theory have a metal enriched supermassive star! We showed that these can also explode (via something called the rapid proton capture process) and that these explosions could explain anomalous nitrogen abundance in the luminous high redshift galaxy, GN-z11.