A team of astronomers from several universities recently reported the discovery of two black holes that are SO BIG that they have each eaten as much material as 10 billion Suns -- the largest black holes ever found. Each is located in the center of a big galaxy and the two galaxies are both over 300 million light years away.
A black hole is a place where one or more stars have collapsed completely and their gravity overwhelms every other force in the universe. Nothing, not even light, can escape from a black hole -- making falling into one of them a "once-in-a-lifetime experience!" Many black holes exist in splendid isolation, but if a black hole forms in the crowded center of a big galaxy, then watch out! When there are stars (and gas and dust) close enough to the black hole, it can cosume them for lunch -- and thus grow a bit. Like the Blob in the old horror movie, the more a black hole eats, the bigger it gets, the bigger it gets, the more it can eat. As long as there is food available, a black hole can grow.
Astronomers had expected that in the biggest, most crowded galaxies, black holes would grow really, really big. Now, in these two galaxies, which have only catalog numbers for names (NGC 3842 and NGC4889), we see clear evidence of just how "overfed" such black hole monsters can be. (Astronomers measure the mass of such black holes by watching stars close to the black hole whirl around at fast speed due to its enormous gravity.)
What is also interesting about this discovery is its connection to "quasars" -- which were mysterious when first discovered, but are now known to be the energetic (bright) centers of distant galaxies. They are tiny regions at the cores of their galaxies which can outshine the entire galaxy of stars in which they live. Their energy output is truly staggering.
Astronomers have established that these quasars were supermassive black holes that had so much "food" (material to eat) around them, that the swirling ingestion of this material produced fantastic energies -- large amounts of light and other radiation. (The energy had to come from a region outside the black hole -- once anything falls INTO the black hole, no energy can come back out.) But to produce such huge amounts of energy, the quasar black hole must have been very massive indeed.
We believe that we see quasars only in the early days of the universe -- when the giant black holes were supplied with plentiful food, espeially when smaller galaxies fell into bigger ones and were eaten in an act of galaxy cannibalism. Nowadays, such really big black holes will have eaten all the material in their neighborhood and have little left to eat. Thus they are no longer shining as quasars. As one of the discoverers of the monster black holes said, these are black holes "in retirement."
The more quiet black holes of our present epoch are harder to find. So astronomers have been actively searching for really huge "retired" black holes, which once were quasars but now are quiet. It looks like the two we found are candidates for just this kind of status. When, long ago, they acted like quasars, their gigantic mass ensured that the feeding process was a very energetic one.