Quite a few distant galaxies turn out to be cosmic delivery rooms. Large numbers of massive stars are born in the hearts of these starburst galaxies, and later explode as supernovae. In the remnants they leave behind, particles are accelerated to very high energies.

Astrophysicists have now used the H.E.S.S. telescopes to make detailed measurements of the gamma rays from the NGC 253 galaxy. As predicted, these high-energy rays originate from the region of maximum supernova activity close to the centre.

At a distance of some twelve million light years away, NGC 253 is one our closest spiral galaxies outside the so-called local group of our Milky Way and its companions. Observations in the visible light as well as in the infrared and radio frequency ranges had already shown there was a small region at the centre of NGC 253 which gave birth to a very high number of stars. This region exhibits a very high density of interstellar dust and gas.

The high-mass stars born in this region use up their nuclear fuel relatively quickly and stagger into an energy crisis at the end of their life. The nucleus collapses while the star destroys itself in one final explosion. Such a supernova suddenly flares up a million or even a billion times brighter than before.

The charged particles accelerated to very high energies in the remnants of such explosions react with the surrounding medium or with electromagnetic fields to generate extremely high-energy gamma quanta.

Between 2005 and 2008, astrophysicists used the H.E.S.S. telescope system in Namibia over a total observation period of 119 hours to detect the expected gamma rays at energies exceeding 220 GeV (billion electronvolts). The source of these rays lies precisely at the optical centre of NGC 253 and appears as a point to H.E.S.S. This makes it the weakest source discovered to date in the very high-energy gamma radiation range.

The flux of radiation from the starburst region of NGC 253 measured by H.E.S.S. implies an enormous cosmic ray density - more than 1,000 times higher than at the centre of the Milky Way. Moreover, the high gas density makes the conversion of cosmic rays into gamma rays around one order of magnitude more efficient. Accordingly, the central region of NGC 253 shines around five times as brightly in the light of gamma rays as all the rest of the galaxy together.