The X-ray sky can be divided into 5 broad categories (there are many other ways to divide the X-ray sources; this is just one way). These are:
The corona of our Sun was the first celestial X-ray source to be detected. It's not surprising, since the Sun is very close, in astronomical terms. The corona is a region above the visible surface (called the photosphere) of the Sun with very tenuous and very hot gas. It turns out that some stars have far more active coronae than does our Sun (which is a good thing, particularly for the astronauts who will spend many months in the International Space Station!). Some stars, particularly young stars, rotate more rapidly, which leads to stronger, and more twisted magnetic fields, which makes the corona more active. Scientists still do not understand all the details of how the coronal gas is heated to such high temperatures, though, or why some elements appear to be more common in the corona than in the photosphere.
Binary star systems contain two stars that orbit around their common center of mass. Many of the stars in our Galaxy are part of a binary system. A special class of binary stars is X-ray binaries, so-called because they were first discovered as very strong X-ray sources. X-ray binaries are made up of a normal star and a collapsed star (neutron star or black hole). These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees!). This area is known as the accretion disk.
Cataclysmic variables are like X-ray binaries, except with a white dwarf instead of a black hole or a neutron star. A white dwarf has mass comparable to the Sun but is closer to the Earth in size (neutron stars and black holes are much smaller still, with radii about 1/1000th of that of the Earth). Because the gravitational potential well of a white dwarf is not as deep as for a neutron star or a black hole, a cataclysmic variable is not as X-ray bright as the X-ray binaries. But there are many more cataclysmic variables, some of them relatively close to the Sun. So we can often study the details of the accretion process better in a cataclysmic variable than in an X-ray binary.
When a massive star explodes in a supernova, it expels a large amount of material (often many times more than the mass of the Sun) at thousands of kilometers per second. This high speed gas then collides with the interstellar medium – that's the very tenuous gas and dust in between the stars – and heats it up to millions of degrees. For the next 20,000 years or so, a hot ball of gas is left, glowing in X-rays: this is called a supernova remnant. This material is then used to create the next generation of stars and planets; Earth and the elements that our bodies are made of originated from this same material. Scientists are naturally interested in how elements like carbon, oxygen, and iron are distributed in supernovae, and so they study the supernova remnants. X-ray energies turn out to be a good region of the electromagnetic spectrum in which to do this.
Many galaxies appear to contain a massive black hole – perhaps a million times the mass of the Sun – at their centers, or the nuclei. When they accrete matter (like in X-ray binaries), they become some of the most luminous objects in the universe, with names like quasars and blazars and Seyfert galaxies. Collectively, they are known as 'Active Galactic Nuclei', or AGN. With X-ray spectroscopy, it is possible to study the motion of gas very close to the central black hole, even to the point of verifying the predictions of Einstein's General Relativity Theory. Also of interest: what is the connection between the different types of AGN, and between AGN and normal galaxies?
Clusters of galaxies were first detected as a collection of galaxies in a small patch of sky. When the X-ray telescopes are pointed at the clusters of galaxies, scientists found out that they are strong sources of X-rays. They are seen to originate from gases that fill the entire clusters of galaxies, in and in-between the component galaxies. It turns out that the mass of this gas exceeds that of the stars in the visible galaxies, and that they are held in place by an even larger amount of dark matter. This finding is very important in cosmology – the question of whether the Universe will keep on expanding forever hinges on the amount of matter. The hot gas itself comes from supernova explosion in the early Universe, and can be used to trace the star formation history of the Universe, another important topic in cosmology.
