What causes the nuclei of Seyfert Galaxies to be "active?"?

Question

Also, is the Milky Way a Seyfert?

Answer 1

The Milky Way is not a Seyfert Galaxy as far as I know - it's a fairly boring, normal galaxy.

All active galaxies have the same source - a supermassive black hole at the center, on which material (gas, dust, stars) is falling. Essentially the black hole is eating, but it's a messy eater, and emits lots of energy in the process.

Yes, it's true that nothing can escape a black hole. But stuff (including energy) that gets close to the black hole might be flung away at high energies and high speeds. That's what's happening in an active galaxy.

Our galaxy also has a supermassive black hole at its center, but it's not eating anything (much) right now. When we collide with the Andromeda galaxy, however, that might change!

Answer 2

The classification of galaxies as "Seyfert" is many decades old, and for many of those decades it wasn't understood what was happening in the nuclei of Seyferts to power the central, active nucleus. In the past few years, astronomers have come to think that all galaxies have massive black holes in their centers, and that the size of that black hole is proportional to the mass of the galaxy. It is therefore apparent that all, or nearly all, galaxies could have active nuclei if sufficient interstellar matter were dumped on that massive black hole, and that being a "Seyfert" is not an intrinsic property of a galaxy, but a transient phase that may occur in essentially any galaxy, including the Milky Way.

The onset of an active phase for the central black hole depends on a large amount of interstellar medium coming into the immediate vicinity of the black hole---quite a lot of interstellar medium, about 10 million solar masses. This amount of interstellar gas can be found in several giant molecular clouds, or maybe one really big giant molecular cloud. It is not so easy to get a giant molecular cloud to fall all the way into the vicinity of the black hole---these clouds are on orbits, and like all orbiting bodies they tend to conserve energy and angular momentum, at least approximately. In the case of giant molecular clouds, there are processes that can drain off energy and angular momentum, the two main ones being (1) shocks due to collisions with other interstellar clouds or (2) dynamical friction between the cloud and the stars in the galaxy. The first process is greatly enhanced by any disruption in the galaxy, such as a collision with another galaxy. The second process is slow, but goes on regardless of external influences. Therefore, most galaxies whose interstellar medium contains giant molecular clouds (all spirals) will undergo a Seyfert phase at odd intervals of ten million years or so.