Some galaxies are much more active than others. Our Milky Way for instance is reasonably active but not more than that. Let's say that it's a good intermediate in every aspect. If we look into or own Local Group of galaxies, we find that the smaller Triangulum Galaxy (M33) produces four times as many stars as the Milky Way. Just look at all of those gigantic knots in its spiral arms which are all stellar nurseries up to sixty (!) times the diameter of our Orion Nebula!
But M33's just peanuts compared to some of the other galaxies out there. Take M61, for example, one of the largest members of the Virgo galaxy cluster. This is a galaxy roughly the same size as our Milky Way but twice as massive and it produces ten times as many stars. Sometimes starburst activity's caused by the close interaction or even collision with other galaxies, as is the case with M82 for instance. Near M61 we find a couple of smaller companion galaxies, one of which (NGC4301) was easily visible in the same field of view. These companions, however, are certainly not massive enough to trigger such freak activity in its much larger parent and so we have to look for an additional explanation in this case. This explanation's found right in the galaxy's core: a supermassive black hole. Actually, it's not that difficult to see a black hole. As I explained about M77, black holes are indeed "black" because not even light can escape from it. Yet they emit a lot of energy, the result of collisions between the atoms that are squeezed together around the black hole just before they fall into it; the so-called Hawking radiation. Therefore, when you see a galaxy with an unusually bright core that looks almost stellar, you can bet on it that it contains a gigantic black hole.
Another interesting fact about M61 is that its spiral arms are not really curved but more angular-shaped, hence its nickname: Hexagon Galaxy. Over the last century no less than six supernovae have been observed in this galaxy and this puts it on a divided second place with M83. The absolute king of supernovae remains the "Fireworks Galaxy" (NGC6946) in Cygnus with ten so far.