(p.197) Black holes swallow gas at significantly lower rates than predicted. Here is a schematic drawing of how energy is converted as gas falls into the black hole.
Somewhere in this chain of events, the process becomes inefficient, and the black hole doesn’t actually swallow as much gas as it should. We do not yet understand why.
Stars galaxies don’t have fixed orbits like the planets in the solar system. Their orbits change because they are affected by gravity of other stars around them. Since the orbits change, these stars can actually get shredded by the central supermassive black holes. You can see a theoretical simulation of this process in the figure on page 200.
We also observe these effects in real (astronomy) life. Bellow is an image of Sagittarius A, Miliky Way’s supermassive black hole, flaring up in X-rays. The image is taken in X-rays and the red corresponds to low energy, green mediumand blue to high energy X-rays. This particular event was a megaflare – it made the source look 400 times brighter than usual. It occurred on 14th September, 2013. There was another big flare in October 2014 which was 200 times brighter than usual.
Most likely it was a star that has been tidally disrupted and its gas was stretched and swallowed by the black hole the next time that it approached the black hole.
The figure shows this gas, marked as G2 flying past the central part of the galaxy and then what it looks like when the flare occurs.
It is predicted that every few thousand years one star could be destroyed in such a way by the central massive black hole in a quiescent galaxy like our own.