HOW DO WE X-RAY THE UNIVERSE?
X-rays and Gamma rays are very high frequency light that we cannot see - extremely dangerous to life. Luckily for us, the Earth’s atmosphere protects us. But unfortunately for astronomers, it does mean that we need to put telescopes into space if we want to study them. Early X-ray telescopes were sent up on rockets and balloons for a couple of hours at a time, but nowadays we use hi-tech satellites such as the European Space Agency’s X-ray observatory XMM-Newton (see picture) that run for several years. X-ray images such as the one shown on page one are taken with CCD cameras, like very advanced and expensive versions of the digital cameras available today.

IF BLACK HOLES ARE BLACK, HOW CAN WE SEE THEM?
If you were to take any star in the sky, and squash it into a small enough space, you would get a black hole. Once you get close enough to a black hole, the gravity is so strong that nothing, not even light, can escape. This point of no return is called the event horizon; for our Sun the event horizon is about 2 miles, some 460,000 miles below its surface. We know of two kinds of black hole. One kind is millions of times heavier than the Sun, and is found snacking on stars at the heart of many galaxies, while the other kind is "only" about 10-100 times heavier than the Sun and is formed by the supernova death of the very heaviest stars.

Although we can’t see black holes themselves, we can see their effects on surrounding objects. Most stars exist in pairs, so if one turns into a black hole it can feed off the other one: the gravity of the black hole pulls gas from the companion, forming an accretion disc of material that spirals into the black hole, feeding the monster (see picture on page 2). X-ray binaries with black holes in them can be many times more powerful than ones with neutron stars because the black holes are 10-100 times heavier: they can be up to 50 million times more powerful than the Sun! The giant black holes at the centres of galaxies perform the same trick, swallowing nearby gases and stars. In galaxies like ours, the beast is docile, but in others, the black holes are VERY active, spewing out great jets of material at a respectable fraction of the speed of light.

An active galaxy with huge jets of material being ejected by a black hole.
(STScI/NASA)

These Active Galactic Nuclei (AGN) produce more energy than the rest of the galaxy put together! They often look like stars because some are so bright that they can be seen from billions of light years away. They are bright in X-rays, visible light and radio band. The X-rays in AGNs change the most - we reckon that the quickest variability is 50 seconds per million Suns of mass in the black hole, so for a black hole as heavy as 100 million Suns, we’d expect to see changes over 1-2 hours.