The most common device for producing X rays is a vacuum tube called a Coolidge tube. It contains two electrodes: a cathode, which is heated to emit electrons, and an anode, which supports a piece of metal called the target. When a high voltage is applied across the two electrodes, the electrons stream toward the target and strike it at high speed, producing X rays. The target is usually made of tungsten or some other metal with a high melting point because the bombardment of the electrons also generates a large amount of heat.

When the electrons strike the target, X rays are produced by two processes. One process occurs as the electrons are slowed down and deflected by the atoms of the target; the kinetic energy (energy of motion) of the electrons is converted directly into X-ray radiation. The radiation, called bremsstrahlung, contains X rays of all wavelengths above a certain minimum wavelength. The higher the voltage of the tube, the greater the kinetic energy of the electrons that strike the target, and the shorter the minimum wavelength of the X rays that can be produced.

The other process involves the electrons of the atoms that make up the target. The electrons can be thought to be arranged about the nucleus, or core, of the atom in a number of shells, or layers, one within the other. The larger the shell, the greater the energy of the electrons that form it. When a high-speed electron from the cathode strikes an atom of the target, it sometimes knocks an electron from out of one of the inner shells. The ejected electron is immediately replaced by an electron from a larger (higher-energy) shell. The energy lost by the electron's dropping down to a shell of lower energy is given off as an X ray.

A variety of astronomical objects are sources of X rays produced naturally by physical processes involving very hot gases or high-energy particles.