What does the X-ray Universe look like?

Credit: NASA/CXC/SAO

Only the hottest cosmological objects and the most energetic events generate X-rays. Studying them teaches astronomers about the violent Universe, where stars explode, black holes devour matter and galaxies collide.

Bullet Cluster
Dark and normal matter: Energetic X-rays (pink) are emitted by hot normal matter that has piled up where two clusters of galaxies have collided. The dark matter (blue) has mysteriously passed through the crash undisturbed.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Solar cycle in X-ray
X-ray cycle: During the peak of its 11-year activity cycle (foreground left and right), the Sun emits many more X-rays than during solar minimum (background).
Credit: David Chenett/Joseph B. Gurman/Loren W. Acton
Hot halo: X-rays from extremely hot gas and from neutron stars and black holes in the Antennae Galaxies are colour-coded blue in this composite image.
Credit: NASA, ESA, SAO, CXC, JPL-Caltech, and STScI.

The hottest objects and the most violent events in the Universe produce high-energy X-rays: solar flares, neutron stars, cosmic shock waves, extremely hot gas in galaxy clusters, colliding galaxies and matter spiralling into black holes. The human eye cannot see X-rays. In fact, your retina would be destroyed by this energetic cosmic radiation. Astronomers use representative colours to create images of the X-ray Universe, based on observations by Earth-orbiting space telescopes.

Andromeda in X-ray
Glowing Andromeda: X-ray binary stars show up as blue and white dots in this composite image of the Andromeda Galaxy. The orange colour represents infrared light from warm dust.
Credit: ESA/Herschel/PACS/SPIRE/J. Fritz, (U. Gent); X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, (MPE)
Tycho Brahe supernova
Explosion aftermath: Million-degree hot gas is colour-coded green and red in this X-ray image of Tycho’s supernova remnant – the expanding remains of a stellar explosion in 1572.
Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.

Our ears cannot hear the high pitch of a dog whistle. In the same way, our eyes cannot see X-rays coming from space. The pictures here show what you would see if you had X-ray vision.