An orchestra of hundreds of galaxies performs a symphony in the X-ray band, but until recently we heard only a general hum. The Coma Cluster is about a thousand galaxies immersed in a cloud of superheated gas at a temperature of a hundred million degrees. This gas glows in X-rays, and its spectrum tells us how it moves. The problem is that ordinary detectors turned fine emission lines into blurry blobs, and the individual "voices" of the streams merged into one indistinguishable chorus.

The new instrument, the Resolve microcalorimeter on the Japanese XRISM satellite, works like an ear capable of picking out every note in a chord. It measures the energy of each photon to within 4.5 electronvolts at an energy of 6000 electronvolts. That is like hearing a half-tone difference against the backdrop of an orchestra. When the instrument was pointed at the cluster's center, individual peaks emerged in the iron line: one main peak, a second with a slight shift toward the red edge, and a third with a strong shift toward the blue side. A blue shift means the gas is flying toward us at about 1274 kilometers per second.

Now that we have learned to distinguish individual parts in the X-ray orchestra, we can ask new questions. Exactly how does the energy of mergers turn into heat and turbulence? Why does the radio halo live so long? Future XRISM observations will add new voices to this score, and perhaps we will finally hear the Coma Cluster rehearsing its cosmic symphony.