Where Are All the Supernovae?

Danish astronomer Tycho Brahe was lucky. In 1572, he saw a bright supernova explosion in the constellation of Cassiopeia. Just 32 years later, his German pupil Johannes Kepler observed a similar event in Ophiuchus. Ever since, the Milky Way has produced not a single visible supernova, although John Flamsteed may have observed a pretty faint one in 1680. Indeed, there have only been eight in the past 2,000 years. Yet astronomers believe there should be at least three per century. So where are they?

In all likelihood, a supernova probably does pop off in the Milky Way every few decades. But since they occur in the thin Galactic disc, their light is heavily obscured by interstellar dust. "The supernovae seen by Tycho and Kepler were relatively close to us in the Galaxy," says David Green of the Cavendish laboratory in Cambridge. "No doubt there have been others much further away, which would have been obscured more."

Supernova explosions leave expanding gaseous remnants, like the famous Crab Nebula. Also, the cores of the exploding stars collapse into dense neutron stars, some of which are detected as pulsars. But counting supernova remnants and pulsars doesn't help much in pinning down the true supernova rote. Ages of older remnants are poorly known; our inventory is far from complete, and most neutron stars go unnoticed.

Still, the predicted supernova rate is consistent with the figure derived from supernovae detected in other galaxies, so it seems likely that we just haven't seen them. But it's not all bad news: future telescopes will operate in a wide range of wavelengths, giving us a better chance of observing them, and therefore proving that they're really happening.