X-ray observations are difficult to make since X-rays are absorbed by the Earth's atmosphere before they reach the ground. Early X-ray detectors flew in sounding rockets and high altitude hot air ballons, and modern observatories orbit the Earth as satellites.

X-rays are very energetic, each photon carries a large amount of energy. They are emitted from extreme objects with exotic sounding names such as pulsars, quasars and black holes, although other objects such as the Sun also emit X-rays.

Famous X-ray observatories include Chandra, XMM-Newton, Rosat, and Einstein.