Over the last century our window on the universe has been widened by a staggering amount. From observations in the visible region of the electromagnetic spectrum, which spans one octave, astronomers have extended their view in two different directions. We are now able to study the cosmos in the long-wavelength radio and microwave regions right up to the ultrashort wavelengths of X-rays and gamma radiation – an increase of over 70 octaves. As many celestial objects and processes are only accessible at wavelengths beyond the optical range, this has provided us with a much more complete view of the universe.
Most of this electromagnetic radiation – up to and including the X-ray regime – is emitted in black-body form by hot celestial objects such as stars. In other words, it is a shorter – or longer – wavelength version of the infrared "heat" radiation emitted by our bodies. Gamma rays, on the other hand, cannot be produced in this way because it would require implausibly high temperatures. Rather, such short-wave radiation is produced either by interactions of charged particles like electrons or nuclei that have been accelerated to high energies, or by the decay of very heavy particles. As such, gamma rays let us to probe the "non-thermal" universe that is associated with the most extreme astrophysical environments and exotic physical processes that are known.
The first source of very high-energy gamma rays, the Crab Nebula, was discovered in 1989 using the ground-based Whipple telescope on Mount Hopkins in southern Arizona. Since then, experiments such as CAT and HEGRA, operating in the French Pyrenees and on the island of La Palma, have increased the power of this technique. Now, the High Energy Stereoscopic System (HESS) telescope, which is sited in the mountains of Namibia in southern Africa, is taking gamma-ray astronomy to new heights.
In the March edition of Physics World, Werner Hoffman and Jim Hinton discuss how HESS's gamma-ray data have provided insights into various aspects of high-energy physics since the it became operational in 2004.
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