So many astronomical objects emit x-rays that the bigger surprise may be the objects that do not. This is a measure of just how violent our universe is. In truth, while the regions producing x-rays must be much hotter than the surface of the Sun, these regions are far from the hottest places in the universe, and are not nearly as hot as the mechanisms producing the energy would imply. For instance, the energy released when a free-falling object strikes the surface of a neutron star is about 15% of its rest-mass energy, which implies a temperature of several-hundred MeV, which is around 105 times the temperatures characteristic of x-ray regions. Clearly other physics besides the available amount of energy is behind the physics of x-ray emission.
Main sequence stars of all sizes produce x-rays. Stars the size of the Sun and smaller have convective outer layers that generate magnetic fields. The magnetic fields generated by a star are carried way to the tenuous outer regions of the star's atmosphere, where they dissipate their energy. This tenuous region is called the star's corona, and the energy dumped by the magnetic fields into this region keep the temperature of the corona very high, typically over 1 million degrees Kelvin. The Sun has a corona that has been studied in great detail. Many other stars have corona that are visible to current x-ray observatories. (continue)
Very massive stars are also seen in the x-ray, but these stars do not have a corona. Instead, a massive star drive a strong stellar wind out of their atmospheres. This wind is pushed by the star's radiation. Instabilities in the wind give rise to shock waves that heat the wind sufficiently to radiate x-rays.
Isolated neutron stars have only two sources of energy: the energy trapped in their cores (no energy is generated at the core of a neutron star), and the energy associated with their rotation. An interesting feature of neutron stars is that the energy in their cores is released most effectively as neutrinos. Despite this cooling mechanism, some energy does still diffuse to the surface of a neutron star, where it is radiated away as black-body radiation. The temperature at the surface of a young neutron can be over 1 million degrees, and despite the small surface area, the x-ray flux from the surface is sufficient to make the star visible to modern observatories. (continue)
Some isolated neutrons stars are visible because their rapid spin and strong magnetic field generate a massive electric field that accelerates particles to high velocities. These systems, which are generally called radio pulsars, because they were first seen at radio frequencies, can produce substantial x-rays.
Compact binary stars are among the most brilliant of x-ray sources. These x-ray producing systems fall into two groups: binary systems where one star is a degenerate (white) dwarf, and systems where one star is a neutron star or a black hole-candidate; all of these systems have a main-sequence star or a giant star as a companion. The system with a degenerate dwarf star is called cataclysmic variables, while the system with a neutron star or black-hole candidate is called an x-ray binary. The x-ray binaries are further subdivided based on their specific properties.
All of these systems are visible at x-ray energies because the compact star is pulling material from its companion star onto its surface. Usually this material flows through an accretion disk to the compact star. The x-rays are produced by the material in the inner regions of the accretion disk and by the material that has fallen onto the surface of the compact object, if that object is a white dwarf or neutron star. On neutron stars, additional x-rays are produced through the nuclear fusion of hydrogen and helium deposited onto the neutron star's surface through accretion. Our observational knowledge of black-hole candidates was won principally through the study of x-ray binaries. (continue)
Supernovae create high-velocity shock waves that move into the interstellar medium. These shock waves can heat the interstellar medium to sufficiently high temperatures to cause the emission of x-rays. Many supernovae shock waves have been observed with modern x-ray telescopes.
Active galaxies are galaxies with bright, highly variable cores. These galaxies are believed to be powered by very massive black holes surrounded by massive accretion disks. The accretion disks are not only responsible for the highly-variable radiation seen from these objects, but also for relativistic jets of material that shoot away from these galaxies. These systems are visible at x-ray energies, and are believed to account for a diffuse x-ray background seen by x-ray instruments with low angular resolution. (continue)
The cooling flow is the peculiar name given to hot gas trapped in a cluster of galaxies. It has this name, because the gas at the very center of the cluster, at the bottom of the gravitational potential well, is rapidly cooling through the release of radiation, causing the gas away from the center to flow towards the center. The temperature of the gas in a cooling flow is very high, so that these systems release x-rays. Astronomers are particularly interested in the composition of the gas, so the atomic x-ray lines produced by a cooling flow is under intense investigation. (continue)