Astronomers detect shockwave from supernova
Astronomers have captured evidence of 'shock breakouts,' or bright flashes that occur when red stars become supernovas, for the first time.
March 21, 2016
Caught For The First Time: The Early Flash Of An Exploding Star
The brilliant flash of an exploding star’s shockwave—what astronomers call the “shock breakout”—has been captured for the first time in the optical wavelength or visible light by NASA's planet-hunter, the Kepler space telescope.
An international science team led by Peter Garnavich, an astrophysics professor at the University of Notre Dame in Indiana, analyzed light captured by Kepler every 30 minutes over a three-year period from 500 distant galaxies, searching some 50 trillion stars. They were hunting for signs of massive stellar death explosions known as supernovae.
The brightness of a Type II supernova shock breakout
The diagram illustrates the brightness of a supernova event relative to the sun as it unfolds. For the first time, a supernova shockwave has been observed in the optical wavelength or visible light as it reaches the surface of the star. This early flash of light is called a shock breakout. The explosive death of this star, called KSN 2011d, as it reaches its maximum brightness takes 14 days. The shock breakout itself lasts only about 20 minutes, so catching the flash of energy is an investigative milestone for astronomers. The unceasing gaze of NASA's Kepler space telescope allowed astronomers to see, at last, this early moment as the star blows itself to bits. Supernovae like these — known as Type II — begin when the internal furnace of a star runs out of nuclear fuel causing its core to collapse as gravity takes over. This type of star is called a red supergiant star and it is 20,000 times brighter than our sun. As the supergiant star goes supernova, the energy traveling from the core reaches the surfaces with a burst of light that is 130,000,000 times brighter than the sun. The star continues to explode and grow reaching maximum brightness that is about 1,000,000,000 times brighter than the sun.
Credits: NASA Ames/W. Stenzel
In 2011, two of these massive stars, called red supergiants, exploded while in Kepler’s view. The first behemoth, KSN 2011a, is nearly 300 times the size of our sun and a mere 700 million light years from Earth. The second, KSN 2011d, is roughly 500 times the size of our sun and around 1.2 billion light years away.
“To put their size into perspective, Earth's orbit about our sun would fit comfortably within these colossal stars,” said Garnavich.
Whether it’s a plane crash, car wreck or supernova, capturing images of sudden, catastrophic events is extremely difficult but tremendously helpful in understanding root cause. Just as widespread deployment of mobile cameras has made forensic videos more common, the steady gaze of Kepler allowed astronomers to see, at last, a supernova shockwave as it reached the surface of a star. The shock breakout itself lasts only about 20 minutes, so catching the flash of energy is an investigative milestone for astronomers.
“In order to see something that happens on timescales of minutes, like a shock breakout, you want to have a camera continuously monitoring the sky,” said Garnavich. “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.”
Supernovae like these — known as Type II — begin when the internal furnace of a star runs out of nuclear fuel causing its core to collapse as gravity takes over.
The two supernovae matched up well with mathematical models of Type II explosions reinforcing existing theories. But they also revealed what could turn out to be an unexpected variety in the individual details of these cataclysmic stellar events.
While both explosions delivered a similar energetic punch, no shock breakout was seen in the smaller of the supergiants. Scientists think that is likely due to the smaller star being surrounded by gas, perhaps enough to mask the shockwave when it reached the star's surface.
“That is the puzzle of these results,” said Garnavich. “You look at two supernovae and see two different things. That’s maximum diversity.”
Understanding the physics of these violent events allows scientists to better understand how the seeds of chemical complexity and life itself have been scattered in space and time in our Milky Way galaxy
"All heavy elements in the universe come from supernova explosions. For example, all the silver, nickel, and copper in the earth and even in our bodies came from the explosive death throes of stars," said Steve Howell, project scientist for NASA's Kepler and K2 missions at NASA’s Ames Research Center in California's Silicon Valley. "Life exists because of supernovae."
Garnavich is part of a research team known as the Kepler Extragalactic Survey or KEGS. The team is nearly finished mining data from Kepler’s primary mission, which ended in 2013 with the failure of reaction wheels that helped keep the spacecraft steady. However, with the reboot of the Kepler spacecraft as NASA's K2 mission, the team is now combing through more data hunting for supernova events in even more galaxies far, far away.
"While Kepler cracked the door open on observing the development of these spectacular events, K2 will push it wide open observing dozens more supernovae," said Tom Barclay, senior research scientist and director of the Kepler and K2 guest observer office at Ames. "These results are a tantalizing preamble to what's to come from K2!"
In addition to Notre Dame, the KEGS team also includes researchers from the University of Maryland in College Park; the Australian National University in Canberra, Australia; the Space Telescope Science Institute in Baltimore, Maryland; and the University of California, Berkeley.
The research paper reporting this discovery has been accepted for publication in the Astrophysical Journal.
Ames manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
Authored by H. Pat Brennan/JPL and Michele Johnson/Ames
The brilliant flash of an exploding star’s shockwave—what astronomers call the “shock breakout” -- is illustrated in this video animation. The cartoon video begins with a view of a red supergiant star that is 500 hundred times bigger and 20,000 brighter than our sun. When the star’s internal furnace can no longer sustain nuclear fusion its core to collapses under gravity. A shockwave from the implosion rushes upward through the star’s layers. The shockwave initially breaks through the star’s visible surface as a series of finger-like plasma jets. Only 20 minute later the full fury of the shockwave reaches the surface and the doomed star blasts apart as a supernova explosion. This animation is based on photometric observations made by NASA’s Kepler space telescope. By closely monitoring the star KSN 2011d, located 1.2 billion light-years away, Kepler caught the onset of the early flash and subsequent explosion.
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An international team of scientists recently discovered evidence of what they call “shock breakouts,” or the visible shockwave that occurs after the death of a star.
The team, led by Peter Garnavich of the University of Notre Dame, submitted their findings to Astrophysical Journal last week.
“This validates theoretical calculations of how supernovas work,” said research team member Ed Shaya in a phone interview with The Christian Science Monitor.
As stars run out of fuel at the end of their lives, the density of the core gets higher. Eventually the core becomes too heavy for the star to survive and it collapses, creating what is called a Type II supernova.
“Stars that are larger than eight times the mass of the sun will run out of mass in their center,” said Dr. Garnavich in a phone interview with the Monitor. “When that happens, their core collapses, and that collapse results in a shockwave that blows off the outer layers of the star.”
That shockwave is the “shock breakout” that astronomers have been hoping to spot for decades. According to NASA, the Kepler Space Telescope made this discovery possible.
Although shock breakouts were first proposed in a 1978 paper by a well-known astrophysicist named Roger Chevalier at the University of Virginia, and have been spotted using an X-ray telescope, says Garnavich, Kepler’s imaging capabilities allowed researchers to actually see the flash of a shock breakout for the first time in visible light.
“In order to see something that happens on timescales of minutes, like a shock breakout, you want to have a camera continuously monitoring the sky,” said Garnavich in a NASA press release. “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.”
Shock breakouts happen over the space of about 20 minutes, or the blink of an eye in astronomical terms. The team examined light data from 400 galaxies taken in by Kepler every half-hour over a three-year time span, beginning in 2011.
In all that time, they found six supernovas. Four occurred after the deaths of white dwarf stars – the stars were too small to create a shock breakout. Two, however, came after the deaths of red giant stars. The stars, KSN 2011a and KSN 2011d, are about a billion light years away from Earth, and are about 300 to 500 times the size of our sun, respectively.
Only one of the supernovas created a visible shock breakout. The other may have generated a shock breakout, but scientists were unable to see it due to a cloud of gas or dust obscuring their view.
“One had a rise to maximum light,” Garnavich told the Monitor, “and we suspect that it would have had a shock breakout but it had gas surrounding it.”
Why is it so important to understand this phenomenon when it only occurs for a specific sort of star, under specific conditions?
According to Steve Howell, Kepler’s project scientist, understanding supernovas is key to understanding the universe and its development.
“Supernovas teach us about the destiny of our universe, because they are incredibly large objects that we can see over great distances,” Dr. Howell told the Monitor by phone. “If we don’t understand how bright they are, and what happens to them, then we are using them, but we are using them incorrectly.”
In the NASA press release, Howell also reminded readers that supernovas create heavy elements that serve as some important building blocks for life, including life on Earth.
"Life exists because of supernovae,” he said.