Kepler watched two supernovae burst out of the surface of stars

(credit: NASA Ames, STScI/G. Bacon)

In general, astronomy is reactive—we spot something unusual by chance and point as many telescopes as we can manage to try to figure out what's going on. It's rare that we have something pointing in the right direction to catch an event right as it starts.

But the Kepler observatory was designed to point at the same spot and stare for years, capturing a constant stream of images. It's how the telescope was able to catch planets as they moved in front of their host stars. But interesting things went on behind the stars, and Kepler captured that data, too. Yesterday, astronomers announced that an analysis of the Kepler data captured the moments when a supernova burst through the surface of its host star—not once, but twice.

Most people think of supernovae as explosions that destroy a star. But type II supernovae are really the collapse and explosion of the core of the star. In cases where the star is a giant, this collapse and explosion happens so quickly that the outer layers of the star are unaffected by it and continue to look normal for nearly an hour even as catastrophic events are occurring behind the star. That seeming normalcy ends when the shockwave of the explosion reaches the surface and breaks out in a brilliant flash of light. This type of explosion is called a type II-P supernova.

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