Hubble - greatest discoveries
Black Holes, Quasars, and Active Galaxies:
How Hubble found black holes at the heart of all large galaxies
Black holes are objects so dense, and with so much mass, that even light cannot escape their gravity. It is in the study of supermassive black holes that Hubble has made its biggest contribution.
It is impossible to observe black holes directly, and astronomers had no way to test their theories until Hubble started it work. The high resolution of Hubble made it possible to see the effects of the gravitational attraction of some of these objects on their surroundings. Hubble has also proved that supermassive black holes are most likely present at the centres of most, if not all, large galaxies. This has important implications for the theories of galaxy formation and evolution.
As black holes themselves, by definition, cannot be observed, astronomers have to study their effects on their surroundings. These include powerful jets of electrons that travel many thousands of light years from the centres of the galaxies. Matter falling towards a black hole can also be seen emitting bright light and if the speed of this falling matter can be measured, it is possible to determine the mass of the black hole itself. This is not an easy task and it requires the extraordinary capabilities of Hubble to carry out these sophisticated measurements. Hubble observations have been fundamental in the study of the jets and discs of matter around a number of black holes. Accurate measurements of the masses have been possible for the first time. Hubble has found black holes 3 billion times as massive as our Sun at the centre of some galaxies. While this might have been expected, Hubble has surprised everyone by providing strong evidence that black holes exist at the centres of all large and even small galaxies. Hubble also managed not only to observe the jets created by black holes but also the glowing discs of material surrounding a supermassive black hole.
Furthermore, it appears that larger galaxies are the hosts of larger black holes. There must be some mechanism that links the formation of the galaxy to that of its black hole and vice versa. This has profound implications for theories of galaxy formation and evolution and is an ongoing area of research in astronomy.
Black holes and the quasar connection
Before Hubble, quasars were considered to be isolated star-like objects of a mysterious nature. Hubble has observed several quasars and found that they all reside at galactic centres. Today most scientists believe that supermassive black holes at the galactic centres are the "engines" that power the quasars. They also believe that quasars, radio galaxies and the centres of so-called active galaxies just are different views of more or less the same phenomenon: a black hole with energetic jets beaming out from two sides. When the beam is directed towards us we see the bright lighthouse of a quasar. When the orientation of the system is different we observe it as an active galaxy or a radio galaxy. This unified model has gained considerable support through a number of Hubble observational programmes.
This is one of nine articles highlighting some of the greatest discoveries made by the Hubble Space Telescope. Read more in the articles linked from the right-hand menu.