system
Given the following post, respond as if you're commenting on Reddit:
human
What will happen when the center of the Milky Way galaxy ceases to exist?

The earth rotates around the sun, and the sun rotates around the center of the Milky Way galaxy. The center of the Milky Way galaxy is a giant black hole that serves as the acceleration force for our solar system. 

What will it feel like for humans on earth when that black hole dies?
gpt
Supermassive black holes (SMBHs) are indeed very massive, but they only make up a tiny fraction of the mass of a galaxy. Typically, a supermassive black hole is about 1/1000th of the mass of the central bulge of a galaxy. 

(Aside: This correlation is currently thought to be because both the black hole and the bulge form from the same gas. If some gas flows into the centre of a galaxy, ~99.9% of it collapses and forms into stars along the way, while the other ~0.1% ends up going all the way to the black hole. So both get "fed" together, but the black hole ends up with ~0.1% of the mass, because ~0.1% of the gas makes it that far to the centre.)

The central bulge of a galaxy is often only a fraction of the galaxy's mass. And that's not even counting dark matter, which is another factor of 5-10 or so. Really, the SMBH is such a tiny fraction of the galaxy's mass, that you could remove it and it wouldn't affect our orbit in any measurable way. We are really orbiting within the combined gravitational field of all the stars and gas and dark matter that makes up the galaxy.

SMBHs are important though, because of their *local* effect. Although it doesn't directly affect the orbits of most stars in the galaxy, it has a very dramatic effect on *nearby* gas and stars, especially within like 10s of light-years. They have a very sharp and deep potential energy well. So you can orbit around a black hole *very* quickly, and that means there's a lot of kinetic energy to play with. As fast-moved gas flows inwards, it collides with other gas flows, and converts kinetic energy into heat. This settles down into a very hot "accretion disc" of plasma orbiting around the black hole. This accretion disc is the important part, because it's so hot and bright that its radiation does affect the galaxy, heating up nearby gas and potentially dispersing it and preventing star formation. The light is intense enough that its pressure can drive outflows of gas out of the galaxy. The rapidly rotating highly ionised plasma also generates a strong magnetic field, that can launch a jet into intergalactic space at close to the speed of light.

This is what makes SMBHs important - they do not control the orbits of a galaxy, but they are powerful engines that convert huge amounts of energy into light and heat and outflows and jets, which can affect the formation of stars and stir up gas even beyond the galaxy itself.