KSN 2011d had run out of nuclear fuel. Without it, the red supergiant star’s core began to accumulate mass into its core. When the core became too heavy to counteract its own gravitational force – boom. The star’s core collapses and became a supernova (Type II).
Seeing this happen in real-time is understandably difficult. But for the first time, an international science team led by Peter Garnavich, an astrophysics professor at the University of Notre Dame, did just that. Specifically, the team witnessed the ‘shock breakout’ – a short, blinding shockwave from the star.
Garnavich and his team poured over three years of data captured by NASA’s Kepler space telescope. While better known for finding exoplanets, Kepler’s data is handy because it’s always looking at stars. Light from 500 distant galaxies was in the data. That’s about 50 trillion stars.
What did Garnavich and his team end up finding? Five years ago, two gigantic stars exploded while Kepler was looking at them. KSN 2011a and KSN 2011d. And both of them are mind-blowingly huge. KSN 2011a comes in at almost 300 times the size of our sun. KSN 2011d is even bigger at 500 times.
“To put their size into perspective, Earth’s orbit about our sun would fit comfortably within these colossal stars,” said Garnavich.
I know space is big, but damn!
Two supernovas were seen, but what kind of timescales are we talking about? Many stellar events are notorious for moving at a glacial pace. Lucky for us, that’s not the case with a ‘shock breakout.’ These events last just 20 minutes.
Garnavich praised Kepler’s ability to constantly monitor the sky for making this landmark discovery possible. “You don’t know when a supernova is going to go off, and Kepler’s vigilance allowed us to be a witness as the explosion began.”
The Kepler space telescope witnessed both explosions, but a ‘shock breakout’ was only seen on the bigger star (KSN 2011d). What happened to the smaller star? Scientists don’t know for sure, but the most likely scenario is the smaller star was surrounded by gas and masked the shockwave.
“That is the puzzle of these results,” said Garnavich. “You look at two supernovae and see two different things. That’s maximum diversity.”
Here’s a video animation showing KSN 2011d. Watch how quickly the event unfolds. The visible data from Kepler can be seen in the lower left part of the video.
You can see the beginning of the ‘shock breakout’ at around 0:10 as plasma jets begin to protrude from the surface. Within 20 minutes, the full force of the shockwave hits the star’s surface and rips it apart.
Why study supernovae?
Hey, who isn’t a sucker for a big explosion? Supernovae are actually responsible for all the heavy elements in the universe. Metals like silver and copper here on Earth came from the stellar explosions seen by Kepler.
Steve Howell, project scientist for NASA’s Kepler missions, puts it another way. “Life exists because of supernovae.”
And Kepler is just warming up. Garnavich is part of a research team known as the Kepler Extragalactic Survey (KEGS). This team is just about done going over data from Kepler’s primary mission that wrapped up in 2013. That mission ended when reaction wheels aboard the spacecraft failed. But some NASA ingenuity is keeping Kepler going via the K2 mission.
The Kepler space telescope.
Once KEGS is done with the primary mission data, they can move over to K2 data and see what other supernovae Kepler has witnessed.
As for Kepler today? Scientists are about to try something new. The telescope had been looking behind itself to search for exoplanets. Soon, it will turn around and point towards where it’s going. There are some advantages and disadvantages of doing this.
First, Earth will move through its field of view. That means a lot of light is going into the telescope. On the flipside, Kepler will be looking towards the central bulge of the Milky Way Galaxy. And it won’t just be Kepler looking at the same area. Telescopes on the ground will also be observing the same area.
“This unique coordinated observing lets astronomers look for exoplanets and other dark bodies by taking advantage of their ability to act as a gravitational lens, focusing the light of a background star and temporarily causing that star to brighten. This approach has been successfully used to detect exoplanets from the ground, but doing it simultaneously from the ground and a spacecraft fifty million miles from Earth will allow astronomers to determine the distance to these bodies using parallax – the effect whereby the position or direction of an object appears to differ when viewed from a different position. Calculations around this process can reveal the mass of found exoplanets.”
Kepler’s mission team expects to start the new observing campaign in April and last through July.