Active Galaxies
Written by Fraser Cain
Some galaxies are quiet. They shine because of the combined light of all their stars. But other galaxies are active. In addition to the radiation from their stars, they have a very bright core. In some cases, the centers of active galaxies can outshine the entire galaxy that they're within.
For a long time, active galaxies were a mystery. Astronomers called them "quasi-stellar objects", or quasars. They could tell they were very far away, even billions of light-years away, and yet they seemed to shine from a single, star-like point in the sky.
We now know that radiation streaming out of an active galaxy is actually coming from a supermassive black hole at the center of the galaxy. This is an enormous black hole with hundreds of millions of times the mass of the Sun, blasting out radiation. The radiation isn't coming from the black hole itself, but it's actually coming from an accretion disk of material that surrounds the black hole. This disk of material is the stuff that's waiting to be destroyed by the black hole. The incredible gravitational force of the black hole compresses it down until it behaves like the core of a star, pouring out radiation.
Another feature of active galaxies are huge jets of material that pour out of the galactic nuclei. These are created and channeled by the powerful magnetic forces surrounding the black hole, and can extend out into space for tens of thousands of light-years.
There are several different types of active galaxies, but these are really the same thing, seen from different perspectives. It all depends on the orientation of the galaxy. When the galaxy is perpendicular to us, we see the galaxy edge on, and the jets moving above and below the galaxy. This is a Radio or Seyfert Galaxy, with much of the radiation blocked by a thick torus of gas and dust around the center of the galaxy. When we see the disk at an angle, sort of the way the Andromeda Galaxy looks from our perspective, we see a quasar – we can see over the torus of gas and dust, to see much of the radiation. And when we're seeing the galaxy perfectly face on, that's a blazar. One of the galaxy's jets is pointed directly towards us.
It's entirely likely that our own Milky Way was once an active galaxy, and whenever material falls into the supermassive black hole at the center of the galaxy, it will become active again.
We have written many articles about galaxies for Universe Today. Here's an article about how many active galaxies are shrouded from view.
Want more resources on galaxies? Here's a link to the Messier catalog's section on Galaxies. And here's NASA's World Book on galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
Filed under: Astronomy
