Astronomer Ed Shaya was in his office looking at data from NASA's Kepler space telescope in 2012 when he noticed something unusual: The light from a galaxy had quickly brightened by 10 percent. The sudden bump in light got Shaya instantly excited, but also nervous. The effect could be explained by the massive explosion of a star -- a supernova! -- or, more troublingly, a computer error.

"I just remember on that day, not knowing whether I should believe it or not," he remembers. Rather than celebrate, he thought, "Did I make a mistake? Am I doing this all wrong?"

This animation shows a kind of stellar explosion called a Fast-Evolving Luminous Transient. In this case, a giant star "burps" out a shell of gas and dust about a year before exploding. Most of the energy from the supernova turns into light when it hits this previously ejected material, resulting in a short, but brilliant burst of radiation. Credit: NASA/JPL-Caltech

Stellar explosions forge and distribute materials that make up the world in which we live, and also hold clues to how fast the universe is expanding. By understanding supernovae, scientists can unlock mysteries that are key to what we are made of and the fate of our universe. But to get the full picture, scientists must observe supernovae from a variety of perspectives, especially in the first moments of the explosion. That's really difficult -- there's no telling when or where a supernova might happen next.

A small group of astronomers, including Shaya, realized Kepler could offer a new technique for supernova-hunting. Launched in 2009, Kepler is best known for having discovered thousands of exoplanets. But as a telescope that stares at single patches of space for long periods of time, it can capture a vast trove of other cosmic treasures --especially the kind that change rapidly or pop in and out of view, like supernovae.

"Kepler opened up a new way of looking at the sky," said Jessie Dotson, Kepler's project scientist, based at NASA's Ames Research Center in California's Silicon Valley. "It was designed to do one thing really well, which was to find planets around other stars. In order to do that, it had to deliver high-precision, continuous data, which has been valuable for other areas of astronomy."

Originally, Shaya and colleagues were looking for active galactic nuclei in their Kepler data. An active galactic nucleus is an extremely bright area at the center of a galaxy where a voracious black hole is surrounded by a disk of hot gas. They had thought about searching for supernovae, but since supernovae are such rare events, they didn't mention it in their proposal. "It was too iffy," Shaya said.

Unsure if the supernova signal he found was real, Shaya and his University of Maryland colleague Robert Olling spent months developing software to better calibrate Kepler data, taking into account variations in temperature and pointing of the instrument. Still, the supernova signal persisted. In fact, they found five more supernovae in their Kepler sample of more than 400 galaxies. When Olling showed one of the signals to Armin Rest, who is now an astronomer at the Space Telescope Science Institute in Baltlimore, Rest's jaw dropped. "I started to drool," he said. The door had opened to a new way of tracking and understanding stellar explosions.

Today, these astronomers are part of the Kepler Extra-Galactic Survey, a collaboration between seven scientists in the United States, Australia and Chile looking for supernovae and active galactic nuclei to explore the physics of our universe. To date, they have found more than 20 supernovae using data from the Kepler spacecraft, including an exotic type reported by Rest in a new study in Nature Astronomy. NASA/Jet PROPULSION LABORATORY