This composite image of M31 (a.k.a. the Andromeda galaxy) contains X-rays from Chandra (gold), optical emission (light blue), and infrared data (red). Seen in the inset box, the Chandra data suggest that the main trigger for so-called Type Ia supernovas in this and five other elliptical galaxies is the merger of two white dwarf stars. Type Ia supernovas are important objects used to measure vast cosmic distances.
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