X-ray radiation
/ EKS-ray /
Most people meet X-rays at the dentist or the hospital, where they slip through soft tissue but are stopped by bone, letting doctors see inside the body. In astronomy, X-rays come from the most violent places in the universe. They are a high-energy form of light, with wavelengths much shorter than ultraviolet — measured in fractions of a nanometer — and each X-ray photon carries hundreds to thousands of times the energy of a visible photon.
Producing X-rays takes extreme conditions: gas heated to millions of degrees, or particles accelerated to enormous energies. So the X-ray sky is a map of cosmic violence — gas spiraling toward a black hole or neutron star and heating up as it falls, the million-degree corona of the Sun, the searing shock waves of exploding stars, and vast clouds of superheated gas filling clusters of galaxies. Where the optical sky is calm and starry, the X-ray sky glows where matter is being torn apart, crushed, or shocked.
X-rays from space are completely absorbed by Earth's atmosphere, so X-ray telescopes must orbit above it. They also cannot be focused by ordinary mirrors, because X-ray photons would punch straight through; instead they are guided by nested mirrors at glancing angles, like skipping a stone across water. This makes X-ray observatories such as Chandra and XMM-Newton remarkable feats of engineering, and they are our primary window onto black holes, neutron stars, and the hot gas that holds galaxy clusters together.
A star orbiting an unseen companion suddenly pours out X-rays. The explanation: the companion is a black hole or neutron star, and gas pulled off the visible star is heated to millions of degrees as it spirals in, glowing brilliantly in X-rays just before it disappears.
X-rays trace the universe's most violent, hottest places — accretion onto black holes and neutron stars.
Astronomical X-rays are produced by hot gas and energetic particles, not by anything radioactive in the everyday sense. And they reveal heat and violence, not the calm starlight we see at night — the X-ray sky looks completely different from the optical one.