A Star's Final Act: What Supernovae Teach Us About the Universe

The last supernova visible to the naked eye in our own galaxy occurred in 1604, observed by Johannes Kepler. That was over four hundred years ago. Yet today, professional and amateur astronomers together find about two thousand supernovae every year in distant galaxies. The gap between those two numbers captures something essential about how astronomy has changed: we cannot see the ones in our own backyard, but we have learned to watch the rest of the universe explode.

The Two Ways a Star Dies

A supernova is the catastrophic explosion of a star, and it happens in one of two ways. The first mechanism involves a white dwarf—the dense, dead remnant of a star like our Sun—that sits in a binary system with a companion star. If the white dwarf pulls enough material from its companion onto its surface, its temperature rises until it triggers runaway nuclear fusion. The entire star detonates and is completely destroyed, leaving nothing behind but an expanding cloud of debris. This is called a Type Ia supernova, and because these explosions happen at a fairly consistent brightness, astronomers use them as "standard candles" to measure cosmic distances.