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X-rays and gamma rays show the universe at its hottest and most violent, the realm of gamma-ray bursts, white dwarfs, neutron stars and black holes

X-rays and gamma rays are the most energetic electromagnetic waves, with wavelengths of a fraction of a nanometre or less.

Observations at these wavelengths show the universe at its hottest and most violent. This is a realm of gamma-ray bursts, of gas at temperatures of hundreds of millions of degrees swirling around the remnants of dead stars, and of fascinating objects such as white dwarfs, neutron stars and black holes.

Burst astronomy

Gamma rays have wavelengths shorter than 0.01 nanometres and are emitted during radioactive decay, or by particles travelling near the speed of light. The first gamma-ray burst was detected in 1967 by satellites monitoring atmospheric nuclear weapons testing.

Most bursts probably occur when a massive, fast-spinning star collapses to form a black hole, sending out a narrow beam of intense radiation, while shorter bursts may be emitted when two neutron stars merge. Bursts typically last a few seconds, with a longer-lived X-ray and optical afterglow, but can release as much energy as our sun will radiate in a 10-billion-year lifetime. They are visible even from the edge of the visible universe: recently, rays were seen from a galaxy 13 billion light years away, meaning they were emitted just 600 million years after the big bang.

As with X-rays, gamma rays are absorbed by the Earth’s atmosphere. A dedicated space mission, NASA’s telescope, has studied over 500 bursts since it was launched in 2004, while ground-based instruments such as in Namibia, in the Canary Islands and in Arizona keep an eye out for light from showers of short-lived subatomic particles created when energetic gamma rays collide with atoms in the Earth’s atmosphere.

X-ray suns

Ordinary stars emit huge amounts of X-rays, as the American T. R. Burnight discovered in 1948 when he launched a captured German V2 rocket containing a roll of photographic film towards the sun. These come mainly from our star’s corona, the outer envelope of hot plasma that is most easily seen during a total eclipse, and also from particularly active regions of the sun’s disc.

Solar X-ray missions such as NASA’s , launched in 1995, and Yokhoh, a joint mission by Japan, the UK and the US launched in 1991, have been able to observe solar flares as they develop. The most powerful of these flares can result in coronal mass ejections where a huge bubble of highly energetic particles and magnetic field lines bursts away from the sun. These can potentially disrupt communications when they hit Earth, and also present a radiation hazard to astronauts on any future crewed interplanetary missions.

Death stars

Cosmic X-rays are absorbed by oxygen and nitrogen in Earth’s atmosphere, so X-ray telescopes must be put into orbit. The first compact X-ray source, in the constellation of Scorpio, was found during rocket observations of the moon in 1962. In 1970, the first dedicated X-ray satellite, NASA’s , was launched.

Many X-ray sources are binary star systems in which gas being shed by a dying star spirals into its companion – a dead, compact remnant of what was once a star. As it does so, it heats up and emits X-rays.

In Sco X-1 the companion object is a neutron star, the remnant of a star 10 times the mass of our sun. Other systems have larger, white-dwarf companions. But measurements in 1971 of the unseen companion’s orbital wobble in one X-ray source, in the constellation Cygnus (pictured below), showed it was too heavy for a white dwarf or neutron star. It had to be a black hole – the first observational evidence of the existence of such a body.

X-rays are also emitted from the hot inner edges of discs of material accreting around supermassive black holes in active galactic centres and quasars (see p iv). Surveys by NASA’s Chandra X-ray observatory and the European Space Agency’s XMM-Newton satellite, both launched in 1999, have pinpointed thousands of such sources. One X-ray spectral line from highly ionised iron has been particularly informative: in some cases, it provides .