The Kepler Space Telescope’s Supernova Surprise

Lisa R. Parker

Supernovae are stellar blasts that herald the deaths of stars, and they can be so brilliant that they may briefly out-dazzle their entire host galaxy. A particular class of supernovae, called Type Ia, proved to be a critical tool in the important discovery of the dark energy–a mysterious force that is causing the Universe to accelerate in its expansion, and constitutes the lion’s share of the mass-energy component of the Cosmos. Nevertheless, the process that triggers Type Ia supernovae conflagrations has remained a puzzle of Cosmic proportions. However, astronomers announced at the January 2014 winter meeting of the American Astronomical Society (AAS), held outside of Washington D.C. in National Harbor, Maryland, that NASA’s ill-fated, but nevertheless highly successful, planet-hunting Kepler Space Telescope had succeeded in the surprising discovery of two Type Ia supernovae explosions, that shed captivating light on their mysterious origins.

The Kepler mission was the first space telescope to be launched that was capable of detecting Earth-size exoplanets in our Galactic neighborhood situated in their stars’ habitable zones. Over 75% of the 3,500 exoplanet candidates spotted by Kepler sport sizes ranging from that of Earth to that of Neptune.

The habitable zone around a star is that “just right” Goldilocks region where water, in its life-loving liquid state, can exist on an orbiting world. Where liquid water exists, life as we know it can also evolve! This does not mean that life definitely exists on such a happy watery world–but it does mean that the possibility is there.

Kepler, launched on March 7, 2009, from Cape Canaveral, Florida had, as its primary mission, the task of staring at more than 100,000 stars, hunting for small dips in their brightnesses caused by transiting planets. Kepler, a special-purpose spacecraft, was designed to precisely measure these tiny alterations of the light of those distant stars, in search of alien planets causing subtle dips in their brilliant, fiery light.

For all four years of its mission, Kepler stared relentlessly at a single patch of sky, gathering brightness measurements every half hour. Sometimes the telescope fortuitously spotted tiny dips in a star’s brightness, indicating that planets had made a transit–that is, passed in front of–the glaring face of a parent-star. Unfortunately, the Kepler mission came to a premature end when a piece of its equipment failed in May 2013.

In late 2009, Dr. Robert Olling, an astronomer at the University of Maryland in College Park, began to think about what Kepler might be able to do if it also turned to stare at galaxies. Dr. Olling, who studies supernovae and black holes, realized that, like stars, galaxies sparkle with relatively consistent brightnesses. However, in the event of some unusual occurrence–such as the feeding frenzy of a voracious black hole, or the fatal explosion of a giant star–a galaxy’s brilliance could greatly intensify. After Dr. Olling and two of his colleagues, Dr. Richard Mushotsky and Dr. Edward Shaya, also of the University of Maryland, submitted a proposal to the Kepler team, the telescope began staring at 400 galaxies dancing around in its field of view.

What A Blast!

Most supernovae blast off when a solitary, lonely star explodes and “dies”. Frequently, the supernova progenitor is a heavy star, with a massive core weighing-in at about 1.4 solar-masses. This is what is called the Chandrasekhar limit. Smaller, less weighty stars–like our own Sun–usually do not perish in the brilliant violence of explosive supernovae blasts, like their more massive stellar kin. Small stars, like our Sun, go much more “gentle into that good night”, and perish in relative peace–and great beauty. Our Sun, at this point in time, is a very ordinary and rather petite (by stellar-standards), main-sequence (hydrogen-burning) star. It appears in our daytime sky as a large, enchanting, brilliantly sparkling golden sphere. There are eight major planets, a multitude of bewitching moons, and a rich assortment of other, smaller bodies in orbit around our Sun, which dwells happily in the far suburbs of a large, majestic, barred-spiral Galaxy, our Milky Way. Our Sun will not live forever. Like all stars, it is doomed to perish, at some point–but, in our Sun’s case, not for a very long time. A star, of our Sun’s relatively small mass, can “live” for about 10 billion years, blissfully fusing the hydrogen of its core into heavier atomic elements, in a process termed stellar nucleosynthesis.

However, our Sun is not currently a bouncing stellar baby. In fact, it is a middle-aged star. However, it is experiencing an active mid-life, and is still exuberant enough to go on merrily fusing hydrogen in its core for another 5 billion years, or so. Our Sun is currently about 4.56 billion years old–it is not young by star-standards, but it isn’t exactly old, either.

When stars like our Sun have at long last managed to fuse most of their supply of hydrogen, they begin to grow into glowering, swollen red giant stars. The now-elderly Sun-like star bears a heart of helium, surrounded by a shell in which hydrogen is still being fused into helium. The shell puffs itself up outward, and the star’s dying heart grows ever larger, as the star grows older. Then the helium heart itself begins to shrivel up under its own weight, and it becomes ever hotter and hotter until, at last, it has become so searing-hot at its center that the helium is now fused into the still-heavier atomic element, carbon. The Sun-like, small star ends up with a small, extremely hot heart that churns out more energy than it did, long ago, when it was a younger main-sequence star. The outer layers of the elderly, dying star have puffed up to hideous proportions. In our own Solar System, when our Sun has finally gone Red Giant, it will cannibalize some of its own planetary-children–first Mercury, then Venus–and then (perhaps), the Earth. The temperature at the flaming surface of this ghastly Red Giant will be considerably cooler than it was when our Sun was still an enchanting, young, vibrant main-sequence tiny, tiny Star!

The relatively gentle deaths of small stars, like our Sun, are characterized by the tender puffing off of their outer layers of luminous, multi-colored gases, and these objects are so stunningly beautiful that they are frequently called the “butterflies of the Cosmos,” by enchanted astronomers.

Our Sun will die this way–with comparative peace, and great beauty. That is because our Sun is a loner. The Sun’s corpse will be a small, dense stellar remnant called a white dwarf, and its shroud will be a shimmering Cosmic “butterfly”.

However, something very different happens when a small solar-type star dwells in a binary system with another sister star. The sister star rudely interferes with its sibling’s precious, peaceful solitude, and in this case the dying small star goes supernova–just like its more massive starry kin, when they reach the end of the stellar road.

Supernova Surprise!

Kepler data revealed at least five–and possibly eight–supernovae over a two year period. At least two of them were identified as Type Ia, and their light was captured in greater temporal detail than ever before. This new information adds credibility to the theory that Type Ia supernovae result from the merger of two white dwarfs–the Earth-sized, extremely dense relics of Sun-like stars. This new discovery casts doubt on the older, longstanding model that Type Ia supernovae are the result of a solitary white dwarf sipping up material from a companion sister star–and victim. The companion star could be either a main-sequence Sun-like star, or an elderly, bloated red giant.

This new information was the surprising discovery of Kepler–whose main purpose was to hunt for alien planets by staring at stars in our Galactic neighborhood. Remote galaxies also danced around in the space telescope’s field of view, and its success in gathering data every half hour, along with its sensitivity to very small alterations in brightness, made it ideal for recording the rise and fall of light sent forth during supernovae blasts.

Dr. Olling was fortunate enough to spot the duo of Type Ia supernovae after a two-year study of some 400 galaxies in Kepler’s field. He reported his discovery on January 8, 2014, at the winter meeting of the AAS. “As a technical tour de force, it’s really cool to use Kepler for more than it was intended,” Dr. Robert P. Kirshner told the press at the AAS meeting. Dr. Kirshner is an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

In certain ways the data gathered are rudimentary. This is because they are composed only of the brightness measurements, so astronomers cannot calculate details like the two structures of the duo of Type Ia blasts, and the chemical composition of what they hurled violently into Space. Kepler also dispatched data back to Earth only once every three months. Because supernovae dim after several weeks of brilliance, astronomers were unable to point other telescopes at the supernovae that Kepler had spotted in order to gather more-perfect observations.

Type Ia explosions are the most commonly observed form of supernovae. Kepler’s data provided a precious clue as to what triggers these stellar blasts. The Kepler data helps astronomers to distinguish between the two competing supernovae scenarios. Both require that a white dwarf accumulates star-stuff from a companion, until the pressure sparks a runaway thermonuclear blast. However, in the companion model, the expanding shell of material from the white dwarf would crash into the sister star. This would churn out extra heat and light–that would show up as a bump in the first days of a supernova’s brightening. However, no such bump was seen in Dr. Olling’s data.

This essentially rules out red giant companions, Dr. Olling explained at the AAS meeting, because these large, bloated, elderly stars would cause a nice big bump. However, the data might still be compatible with the model of smaller, more Sun-like companions, noted Dr. Daniel Kassen to the press on January 14, 2014. Dr. Kassen is an astronomer at the University of California, Berkeley, and a collaborator with Dr. Olling on the survey. Not only would these relatively small stars cause a tinier bump, but the bump could well be overlooked completely depending on the observer’s viewpoint, Dr. Kassen continued to explain.

For a long time, the model of Type Ia supernovae being caused by merging white dwarfs was not particularly popular among astronomers because the end stages of the mergers were believed to occur very slowly–over the span of thousands of years. Such a gradual accretion of material would more likely lead to the creation of a neutron star. However, in 2010, simulations suggested that such mergers could occur much more rapidly–within seconds or minutes, and this would allow for the dramatic, sudden pressure alteration that triggers such a blast.

There may be some problems, however, with the merger scenario. Dr. Craig Wheeler noted in the January 14, 2014 issue of Nature News that simulations of the mergers frequently show highly asymmetric explosions–yet observations so far appear to be more spherical. Dr. Wheeler is a supernova theorist at the University of Texas at Austin.

Dr. Olling believes that it is important to make simultaneous observations using ground-based ‘scopes. This is because Kepler can only record brightness and cannot split light into spectra. However, in order to do this, Kepler needs to be pointed in the opposite direction. Dr. Olling hopes that the Kepler team will permit this when NASA reveals its future plans for the crippled spacecraft during the summer of 2014.

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