An international group of astronomers, led by Prof. Peter Garnavich from the University of Notre Dame, has caught two Type II supernovae at the moment of explosion.
This false color image shows Cassiopeia A, a remnant of a Type IIb supernova. Image credit: NASA / JPL-Caltech.
Prof. Garnavich and his colleagues analyzed light captured by NASA’s Kepler Space Telescope every 30 minutes over a three-year period from 500 distant galaxies, searching some 50 trillion stars. They were hunting for signs of supernovae.
“Massive stars often puff up to supergiants before ending their lives as supernovae,” the astronomers said.
“When these stars run out of fuel in their center, their core collapses down to a neutron star and a supersonic shockwave is sent out to blow up the entire star.”
“When the shockwave reaches the surface of the star, a bright flash of light, called a shock breakout, is predicted.”
“The flash from a breakout should last about an hour, so you have to be very lucky or continuously stare at millions of stars just to catch one flash,” Prof. Garnavich added.
In 2011, two of these massive stars, called red supergiants, exploded while in Kepler’s view.
The first object, named KSN 2011a, is located approximately 700 million light-years away and is roughly 280 times the size of our Sun.
The second, KSN 2011d, is about 480 times the size of our Sun and approximately 1.2 billion light years away.
“To put their size into perspective, Earth’s orbit about the Sun would fit comfortably within these colossal stars,” Prof. Garnavich said.
Supernovae like these — Type II supernovae — begin when the internal furnace of a star runs out of nuclear fuel, causing its core to collapse as gravity takes over.
“Understanding the physics of these explosions allows scientists to better understand how the seeds of chemical complexity and life itself have been scattered in space and time in the Milky Way Galaxy,” the astronomers said.
The two supernovae matched up well with mathematical models of Type II explosions, thus reinforcing some existing theories. But they also revealed an unexpected variety in these cataclysmic stellar events.
While both events delivered a similar energetic punch, no shock breakout was seen in KSN 2011a.
Scientists think this is likely due to the smaller star being surrounded by gas — perhaps enough to mask the shock wave when it reached the star’s surface.
“That is the puzzle of these results. You look at two supernovae and see two different things. That’s maximum diversity,” said Prof. Garnavich, who is first author of a paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
“It is a thrill to be a part of theoretical predictions becoming an observed and tested phenomenon. We now have more than just theory to explain what happens when a supernova shock wave reaches the surface of a star as that star is totally torn apart,” said team member Dr. Edward Shaya, from the University of Maryland.
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P.M. Garnavich et al. 2016. Shock Breakout and Early Light Curves of Type II-P Supernovae Observed with Kepler. ApJ, accepted for publication; arXiv: 1603.05657
