NASA’s Nuclear Spectroscopic Telescope Array – NuSTAR – has made the first map of the distribution of radioactive titanium-44 in supernova remnant Cassiopeia A. In the image, the distribution of 44Ti is shown in blue, where NuSTAR detected X-rays at energies between 68 and 78keV. The red, yellow and green regions are lower energy X-rays detected by the Chandra X-ray observatory, at energies between 1 and 7keV. The Chandra data arise from elements such as iron that were heated by shock waves away from the centre of the supernova. Radioactive elements such as 44Ti are formed during the supernova explosion, near the boundary between material falling back into the star and that being ejected; it is a direct tracer of the explosion process. The clumpy nature of the 44Ti distribution rules out symmetric explosions and asymmetric bipolar explosions resulting from a fast-rotating progenitor. The data provide strong evidence for the development of low-mode convective instabilities – material flowing asymmetrically within the supernova.