X-rays are a form of radiation, just like light, heat, gamma rays, microwaves and the radiation we use to transmit radio and television signals.
Soft X-rays range from 10 to 0.1 nanometres (nm) (about 0.12 to 12 keV). Hard X-rays range from 0.1 nm to 0.01 nm (about 12 to 120 keV).
Stars generate electromagnetic radiation at X-ray and radio broadcast wavelengths as well as visible light. This is the basis of radio and X-ray astronomy.
X-ray astronomy needs to be carried on from spacecraft in space, as the Earth's atmosphere blocks all of the radiation at X-ray wavelengths, to our good luck.
The first X-rays detected in space were found in 1969, with instruments sent into space by Americans using a captured German V2 rocket.
The first distant cosmic X-ray source known was Scorpius X-1, discovered in 1962. This won Riccardo Giacconi the Nobel Prize in Physics in 2002.
Scorpius X-1 emits 10,000 times as much energy in the X-ray range as it does in the vislible light part of the spectrum.
Most X-ray telescopes use charge-coupled devices (CCDs) to build up an image, pixel by pixel.
Black holes are said to give off X-rays, but these are from matter that collides as it falls very fast towards the black hole.