Active galactic nuclei

Giant black holes at the centres of the galaxies

It appears that centres of most galaxies possess a gigantic black hole that is millions of times, or even hundreds of millions of times, heavier than the Sun. We can discover these black holes and weigh them by studying how the stars at the centres the galaxies move. Most of these black holes are actually rather peaceful, and there is very little going on in their surroundings. However the nuclei of some galaxies are extremely bright, and sometimes one may even see jets shooting out from the nuclei.

In these nuclei the balck holes are absorbing, accreting, gas from its surroundings. The gas forms an accretion disc around the black hole. The gas in this disc is heated by friction until it becomes so hot that it starts to radiate both ordinary light and X-rays, and by some still unknown process it can even generate a jet of the gas that stretches far outside of the host galaxy.

A curious feature with many active galaxies is that the activity is not limited to just the galactic nucleus. Outside of the nucleus one will often find that new stars are being formed in much larger quantities than is normal in galaxies. One speaks of that the galaxy is going through a starburst. Apparently these two processes are connected in some way, though it is still uncertain how they are connected.