Week 4: Black Holes

I recently read a brief article about one of the largest known supermassive black holes, located within an elliptical galaxy called NGC 4889. The black hole at the center of this galaxy has a mass about 21 billion times the mass of the Sun--as a point of comparison, the black hole at the center of the Milky Way, Sagittarius A*, is 5000 times smaller, weighing in at "only" 4 million times the Sun's mass. As I was looking over the article, I encountered some black hole terminology that I was mostly familiar with, but wanted to find out a bit more about, so this post is an attempt to better understand black holes (at least qualitatively).

Supermassive black hole: These are the largest type of black hole, generally found at the center of galaxies. While our own black hole is considered supermassive, there are clearly a lot of black holes that are even more massive. Andromeda's, for example, is around 25-50 times more massive than Sagittarius A*, despite the fact that Andromeda and the Milky Way are fairly close in mass. Interestingly, larger black holes tend to have lower densities than smaller black holes, meaning that the forces at the event horizon of a supermassive black hole are actually less than those at the event horizon of a smaller black hole.

Event horizon: While the event horizon has a more technical quantum mechanical definition having to do with the passage of information, it's essentially the point at which a black hole's gravity is so strong that not even light can escape. This boundary is defined by the black hole's mass, with more massive black holes having larger event horizons. The volume enclosed within the event horizon is actually what's used to calculate the density of black holes, which is part of the reason why more massive black holes are considered less dense. While event horizons are important in physics, I think Douglas Adams' application of the event horizon in The Restaurant at the End of the Universe may be even more important:

Many years ago this was a thriving, happy planet – people, cities, shops, a normal world. Except that on the high streets of these cities there were slightly more shoe shops than one might have thought necessary. And slowly, insidiously, the number of the shoe shops were increasing. It’s a well-known economic phenomenon but tragic to see it in operation, for the more shoe shops there were, the more shoes they had to make and the worse and more unwearable they became. And the worse they were to wear, the more people had to buy to keep themselves shod, and the more the shops proliferated, until the whole economy of the place passed what I believe is termed the Shoe Event Horizon, and it became no longer economically possible to build anything other than shoe shops. Result – collapse, ruin and famine.

Accretion disk: An accretion disk is formed when diffuse material spirals around a star, black hole, or other massive object. The gravitational force on the material causes it to increase in temperature, compress, and emit electromagnetic radiation. The frequency of the radiation that's emitted from an accretion disk can help astronomers determine what type of body is at the center of the disk. Final fun fact: the study of accretion disks is called discoseismology, which, I would imagine, was the dream job of many an astronomer in the 70s.

Honestly I thought this search would give me more to work with.

Sources
http://www.iflscience.com/hubble-takes-new-look-gigantic-black-hole-and-its-quiet-host
https://en.wikipedia.org/wiki/Supermassive_black_hole
https://en.wikipedia.org/wiki/Andromeda_Galaxy
https://en.wikipedia.org/wiki/Milky_Way
https://telescoper.wordpress.com/2010/07/16/the-shoe-event-horizon/
The Restaurant at the End of the Universe, Douglas Adams 
https://en.wikipedia.org/wiki/Accretion_disk