X-Rays

• What Causes X-Rays? X-rays are produced by very energetic events such as the falling of material ("accretion") onto the surface of a compact object (black hole, white dwarf star or neutron star). A neutron star is a dead star in which gravity has squeezed all of the matter into the size of a small city. One tablespoon of a neutron star would weigh as much as the entire human race. A black hole is the extreme case of a compact object, where gravity is so strong not even light can escape from its surface. When a neutron star and an ordinary star are in a binary system (rotating around one another), material falling from the normal star onto the neutron star is heated to millions of degrees and emits X-rays; similar to a rock rolling down a hill and releasing its energy when it hits the bottom (but much more energetic!).
• X-rays also come from acceleration of particles in outer parts of a star's atmosphere or corona. Our Sun is a bright source of X-rays. X-rays are also produced in distant galaxies or Quasars where it it thought that a very massive black hole (billion times more massive than our Sun) is accreting matter in the center.

  No X-rays from space can get through the Earth's atmosphere to the ground, so all X-ray astronomy is carried out with instruments on rockets or satellites.