supernova explosions of the largest, short-lived stars generate synchrotron radiation that slowly diffuses through the galaxy.
While there are small knots of radio emission from M82 that could be attributed to current star formation, most of its radio light appears to be centrally concentrated, diffuse synchrotron radiation. From this observation astronomers have inferred that anywhere from 10 million to 100 million years ago there was a phenomenal burst of star formation in the center of the galaxy. The giant stars formed in this starburst probably had a total mass 100 million times that of the Sun and may have exploded as super-novae at the rate of one burst every three years. By way of contrast, our own Milky Way
