One of the most impressive deep-sky spectacles for professional and seasoned amateur astronomers alike are supernova explosions. Signifying the end stages in the lives of stars that are more massive than the Sun, supernovae are transient powerhouses of tremendous force that are integral in the perpetual cosmic cycle of life and death and the recycling of interstellar material that eventually gives rise to the next generation of stars and planetary systems. Even though astronomers have gained much understanding about the physical processes that drive these cosmic fireworks in the last couple of decades, many of the underlying details have remained sketchy to date. NASA’s Kepler space telescope recently added one important piece of knowledge to astronomers’ picture of supernovae, by directly observing its shockwave in visible wavelengths for the first time—the bright flash that immediately precedes the explosion itself before the progenitor star is completely torn apart.
All stars go through their lives by fusing hydrogen into helium deep inside their cores, a process that maintains them in hydrostatic equilibrium, balancing the inward pressure of the star’s mass itself with the outward pressure of the radiation and light that is produced from the nuclear fusion that takes place inside its core. When stars exhaust their hydrogen supplies during the end of their lives, they are no longer able to counteract the force of gravity and their cores start to collapse under their own weight, heating up the surrounding stellar layers and causing them to expand. At that point, the stars’ initial mass determines their eventual fate.
For low- and medium-mass ones up to approximately eight solar masses, when during core collapse temperatures and pressures are sufficiently high, the core begins to fuse helium into heavier elements like carbon and oxygen, leading to a huge inflation of the star which evolves to become a red giant. These types of stars are not massive enough for fusion to continue beyond that point and the red giant eventually blows off its outer layers into space, forming a planetary nebula. More massive stars, on the other hand, that have at least eight times the mass of the Sun go on to fuse even heavier elements when hydrogen is depleted. The nuclear fusion process of these stars slowly progresses up through the periodic table of elements, producing increasingly heavier atomic nuclei from carbon all the way to iron which are deposited on the star’s interior on successive layers above the core in an onion-like fashion, all the while the star inflates and becomes a red supergiant with a radius that is typically hundreds of times that of the Sun. When the star reaches the stage of iron fusion it can no longer sustain any further nuclear reactions and it begins its final collapse under its own weight. In a matter of seconds, the imploding stellar layers hit the core at tremendous speeds that can reach over 20 percent the speed of light and then rebound creating a ferocious shock wave that propagates outwards in an exploding manner, taking the star’s layers with it away from the core in what essentially constitutes the beginning of a supernova explosion.
Theoretical models had predicted that at this point the shock wave would traverse the star’s interior and reach the surface within a time span of less than an hour, before propagating farther out into the surrounding interstellar medium. In the exact instance that the shock wave would reach the photosphere, it would produce a sudden flash of light across multiple wavelengths. This flash, called a “shock breakout” would be detectable as a characteristic spike in brightness in the supernova’s overall light curve. Yet, since these shock waves travel through the dying star in such a short amount of time, detecting their photometric signature represented an overwhelming challenge for astronomers since they would have to be looking at the right place at the right time, just before a supernova would go off. Not to be deterred, their efforts paid off handsomely when NASA’s Kepler space telescope observed just such an event as it unfolded during one of its long observing campaigns.
Better known as a prolific planet-hunting mission, Kepler has been responsible for largely revolutionising the search for exoplanets around other stars since it was launched in March 2009, having already detected more than 4,700 exoplanet candidates and 1,040 confirmed discoveries to date. The space telescope was able to achieve these results by continuously staring at approximately 150,000 stars at a fixed field of view in the sky, searching for the characteristic dips in brightness that would signify the passage of an exoplanet across the stars’ disk. It turns out that the telescope’s pointing at a fixed place in the sky was exactly what was needed in the search for other transient astrophysical phenomena as well, like supernovae explosions. With that in mind, an international team of astronomers, led by Peter Garnavich, a professor of astrophysics at the University of Notre Dame in Indiana, set out in 2011 to monitor approximately 500 galaxies that were positioned in Kepler’s field of view, in the hunt for any brightness variations that would be indicative of a supernova. And find they did, when the space telescope observed two red supergiant stars in 30-minute intervals before and after they exploded.
The stars observed by Kepler were real behemoths, with a radii approximately 280 and 490 times that of the Sun respectively. For context, if both were placed at the center of the Solar System they would easily envelope all of the terrestrial planets. “To put their size into perspective, Earth’s orbit about our Sun would fit comfortably within these colossal stars,” says Garnavich. The study of their light curves showed that they were typical Type II-P supernovae, a subclass of Type II supernovae which are characterised by the presence of strong hydrogen emission lines in their spectra. These types of stellar explosions have been studied extensively by astronomers, and the progression of their light curves is well understood. Type II-P supernovae in particular reach peak brightness on a timescale of one to two weeks, which they can maintain for an extensive period of time, typically lasting for several months (a period that is represented as a characteristic “plateau” in the light curve) before slowly fading out.
Kepler’s constant gaze on these two stars, named KSN 2011a and KSN 2011d, allowed astronomers to track their light curves in detail despite their great distances of 700 million and 1.2 billion light-years respectively, showing that they matched well with theoretical predictions of how Type II generally behave. Yet, most importantly, in the case of KSN 2011d the space telescope was able to observe a distinctive small spike in the star’s brightness just before the latter went supernova. Lasting no more than 20 minutes, this surge in brightness, which was recorded immediately before the star’s light curve began to rise toward maximum, was a clear sign of the long-theorised shock breakout which indicates the moment in time when the shockwave from the exploding supernova reaches the star’s surface.
“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,” says 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.”
“It is a thrill to be a part of theoretical predictions becoming an observed and tested phenomenon,” adds Ed Shaya, an associate research scientist at the University of Maryland, College Park and member of Garnavich’s team. “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.”
Despite the fact that both KSN 2011a and d displayed a similar energy output that was typical of Type II supernovae, the former’s light curve surprisingly lacked a similar signature of a shock breakout. Furthermore, KSN 2011a’s rise time to peak brightness was somewhat shorter (in the order of 10 days compared to 14 for KSN 2011a), which indicates that even though the overall driving mechanisms for Type II supernovae are the same, some of the details may differ. “That is the puzzle of these results,” comments Garnavich. “You look at two supernovae and see two different things. That’s maximum diversity.”
The most probable explanation for this discrepancy that fits theoretical predictions, according to the researchers, is that in the case of KSN 2011a the shock wave either didn’t travel all the way out to the photosphere due to the star’s much larger size, or if it did it was dispersed more evenly, possibly due to the presence of circumstellar material around the star, stealing away its luminosity that would otherwise have registered in the supernova’s light curve. “No fast shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar interaction suspected in the early light curve,” writes Garnavich’s team in their study, which was accepted for publication in the Astrophysical Journal. “The rapid rise in KSN2011a … suggests the supernova shock continued to propagate into circumstellar material allowing it to convert more kinetic energy into luminosity and diffuse the shock breakout over a longer time … KSN2011d does show excess emission at the time expected for shock breakout with a brightness of 12% that of supernova peak in the Kepler band. The time-scale and brightness observed for the breakout is consistent with model predictions.”
Kepler’s direct discovery of a supernova shock breakout showcases the fact that even though the space telescope was primarily designed as an exoplanet hunting mission, it is nevertheless a first-rate astrophysical observatory as well, which could play an equally important role in astrophysical research besides its planet-hunting duties in the years to come. Now well in its K2 “Second Light” mission, which followed the loss of its second reaction wheel in May 2013, the space telescope has been repurposed in pointing in a direction that is parallel to its orbital path around the Sun near the ecliptic plane, which has opened a host of new opportunities for research in many different fields of astrophysics. “We’re no longer an exoplanet mission,” said John Troeltzsch, program manager for Ball Aerospace, the prime contractor for Kepler which also devised the telescope’s K2 extended mission, during a presentation at the Laboratory for Atmospheric and Space Physics in Boulder, Colo. “We’re a general-purpose astrophysics observatory [for] astroseisomology, Solar System [studies], exoplanets, [star] clusters, stellar activity, binary stars, extragalactic [studies], etc. There’s a little something for everybody.”
The study of supernovae is expected to take center stage as Kepler’s K2 progresses, which will hopefully allow astronomers to gain important insights to the explosive processes that drive these cosmic fireworks, as well as the stellar alchemy that takes place in the cores of these stars, processes which maintain the recycling of stellar material and possibly even the continuation of life in other parts of the Cosmos. “All heavy elements in the Universe come from supernova explosions,” comments Steve Howell, project scientist for the Kepler and K2 missions at NASA’s Ames Research Center in California. “For example, all the silver, nickel, and copper in the earth and even in our bodies came from the explosive death throes of stars. Life exists because of supernovae.”
Kepler’s latest results underscore a fact that has been showcased time and time again in the history of space exploration: The study of the Universe is also a study of ourselves and no space mission is too small or insignificant in this regard. Important insights often come from paths that often seem inconsequential. In the end, that’s all the more reason to commit more strongly as a species to a vigorous program of space exploration.
A computer animation of a supernova explosion with its accompanying shockwave breakout, based on the observations of KSN 2011a and d, conducted with the Kepler space telescope. Video Credit: NASA Ames, STScI/G. Bacon