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Black holes and the M-\sigma relation

Let's make one thing clear before we go ahead with the post, Black holes exist and every galaxy in the universe has a black hole at it's center. Though black holes cannot be observed literally, thanks to the fact that they are black, they can be inferred from other things happening around them. And the evolution of a black hole and the galaxy are deeply intertwined with the one driving the other and vice-versa. In most of the cases, the mass of the black hole at the center of a galaxy depends on the size of the galaxy itself and large galaxies have been observed to have super massive black holes at their center.

So, now that we know that black holes indeed exist, let's look at the various ways in which they are studied. There are studies of hyper velocity stars moving in highly elliptical orbits around the black hole, the mass of the black hole i.e it's gravitational potential driving their hyper velocity. The study of the orbit will help understand the mass of the black hole itself. The M-\sigma relation is another way to infer the mass of the black hole from the velocity dispersion of the galaxy's bulge. One can refer to the wiki page or a more serious paper on the M-\sigma relation for quasars. Velocity dispersion is also used to construct a faber-jackson relation for elliptical galaxies.

For whatever reasons, the first questions we get from students who attend our popular astronomy sessions on campus are always on black holes, followed by ones on worm holes and then aliens. While it is an interesting topic, there is a lot of real and interesting science behind and involving black holes. You can read up on hawking radiation which explains energy radiated by black holes and on quasars, which are basically black holes with an accretion disk around them, radiating with the strength of a thousand suns. Actually more.

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