Radioactive decay of unstable isotopes leads to emission of gamma rays with a characteristic energy (in colour) being determined by properties of the atomic nucleus. ESA’s Integral satellite observatory has been measuring such gamma rays since October 2002. Radioactive isotopes are by-products of nuclear fusion reactions, which produce new atomic nuclei in stellar interiors and in supernovae. In the gamma-ray light of Al 26 isotopes, which decay after about a million years to magnesium, one sees the radioactive glow of regions of the galaxy with recent production of new nuclei. Visible light, on the other hand, often cannot reach us from stars in those regions, due to occulting interstellar gas clouds.

The background image shows our Milky Way galaxy, as it would appear in visible wavelengths, if projected into a galactic co-ordinate system where the plane of the galaxy is a horizontal straight line; superimposed is the Al 26 sky image from COMPTEL, which shows bright emission from regions with massive young stars. The insets at lower right show the effect of the rotating galaxy on the gamma rays reaching Integral.