A supernova is commonly thought of as the immense explosion that heralds the «death» of a massive star, in what is termed a Type II or core-collapse event. When a heavy star has finished consuming its necessary, nourishing supply of hydrogen fuel, it normally goes supernova–a blast so bright that it can briefly out-dazzle the doomed star’s entire host galaxy! Supernovae are the most powerful stellar blasts known–and they can be observed all the way out to the very edge of the observable Universe. However, a common supernova class termed Type Ia instead involves the detonations of white dwarfs–which are the small, dense stellar corpses of what were once less massive Sun-like stars. In June 2014, a team of astronomers using NASA’s Spitzer Space Telescope (SST) revealed a rare example of a Type Ia explosion, in which the dead Sun-like star feeds off of the gas of an elderly, still-living star, like a stellar vampire. This triggers a horrific blast that can prove to be very helpful to astronomers in their efforts to piece together the enduring puzzle of how these powerful and differing events occur.
«It’s kind of like being a detective. We look for clues in the remains to try to figure out what happened, even though we weren’t there to see it,» explained Dr. Brian Williams in a June 4, 2014 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Williams is of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and is lead author of this new study that has been submitted to the Astrophysical Journal. The JPL is in Pasadena, California.
The SST is an infrared space observatory, launched on August 25, 2003 from Florida’s Cape Canaveral Air Force Base. It is the fourth and final of NASA’s Great Observatories program. The other three Great Observatories are the Hubble Space Telescope, the Compton Gamma Ray Observatory, and the Chandra X-ray Observatory.
Supernovae are more than merely magnificent blasts of stellar rage. These horrific explosions are of critical importance because they churn out and hurl into Space heavy atomic elements. In astronomical jargon all atomic elements that are heavier than hydrogen and helium are called metals. All of the heavy metals were manufactured in the nuclear-fusing hearts of our Universe’s billions upon billions of stars (stellar nucleosynthesis), or else in their explosive supernovae grand finales. Without these heavy metals, we would not be here. The Inflationary Big Bang birth of our Universe, that occurred about 13.8 billion years ago, manufactured only hydrogen, helium, and traces of lithium. Everything else was created by the stars–the oxygen that we breathe, the iron in our blood, the sand on our beaches, the water that we drink, and the earth that we walk upon, were all made by the stars.
Type Ia supernovae tend to blow up in consistent ways. This makes them great «standard candles» for determining cosmological distances. As a result, they have been used for decades to enable astronomers to study the expansion and size of the Cosmos. Scientists believe that these events normally occur when white dwarfs blow themselves to pieces.
Evidence has been accumulating over the past decade that Type Ia events are set off when two orbiting white dwarfs undergo a collision. However, there is one notable, and very famous, exception to this general rule. Kepler’s supernova, named for the German mathematician Johannes Kepler (1571-1630), who was among those who observed the brilliant event in 1604, is generally thought to have been triggered by just one lone white dwarf and a badly victimized still-living, elderly, bloated sister star, called a red giant. When a star like our Sun begins to run out of hydrogen fuel, it first swells into a red giant of monstrous proportions, before it finally puffs its outer gaseous layers into Space to morph into a white dwarf--which is its dense relic core. Astronomers have known for a very long time that Kepler’s supernova remnant swims in a pool of dust and gas that has been cast off by the very unfortunate elderly star.
Stars of all masses–heavy and light–«live» out their stellar lives on the main-sequence, whereby they thrive by maintaining a crucial and delicate balance between two constantly warring forces–gravity and radiation pressure. The gravity of a star tends to squeeze everything towards the star–it is a crushing force that pulls the star’s material in. On the other hand, the star’s radiation pressure keeps it bouncy against gravity, by pushing its material outward and away from the star. A star’s radiation pressure is maintained by the process of nuclear fusion, which begins with the burning of hydrogen–the lightest and most abundant atomic element in the Universe–into helium. Helium is the second-lightest atomic element in the Universe. This process continually fuses heavier atomic elements out of the lighter ones. All stars are composed mostly of hydrogen.
When a massive main-sequence star, weighing at least eight times more than our Sun, has at long last succeeded in burning its entire necessary supply of hydrogen fuel, it has reached the end of the stellar road. The massive star, at this tragic stage, is doomed because it can no longer keep itself bouncy against the relentless crush of its own squeezing gravity. Gravity wins the war, because the star can no longer maintain radiation pressure by fusing lighter elements into heavier things.
Type II supernovae explosions normally blast heavy, elderly, doomed stars to smithereens, casting their beautiful, glowing and glimmering sea of multicolored gases out into interstellar Space. A terrible beauty is born. The event occurs when the iron core of a heavy star fattens up to attain the heavy weight of 1.4 solar masses. This triggers the core-collapse Type II supernova event, which is extremely fiery, raging, brilliant, and beautiful. The most massive stars in the Cosmos collapse and blow themselves up to become stellar-mass black holes. Massive stars–that are not quite that massive–also blow themselves to pieces, but they leave behind a relic–an extremely dense core that tells its sad story. This relic core is called a neutron star, and it is a city-sized object, that is essentially one enormous atomic nucleus. One teaspoon full of neutron star stuff can weigh as much as a school of whales.
But small, less weighty Sun-like stars perish much more peacefully than their more massive kin. When a relatively petite star, like our own sparkling Sun, has at last burned up its necessary supply of nourishing hydrogen fuel, it first swells up to become a red giant, and then inevitably casts its outer, varicolored, shimmering gaseous layers into the Space between stars. The bloated red giant leaves behind a relic white dwarf. White dwarfs are usually surrounded by beautiful shimmering shells of gases (planetary nebulae), which are so lovely that astronomers frequently, and playfully, call them the «butterflies of the Cosmos». Our Sun will eventually become such a beautful, glittering «butterfly». It will first swell into a crimson monster of a hungry red giant that will incinerate first Mercury, then Venus–and possibly our own planet. But in the end, our Sun will morph into a tiny, dense white dwarf, encircled by a lovely shell of varicolored gases.
Although our Sun is a solitary star, many stars that are like it have more company. A large number of Sun-like stars inhabit binary systems where they are situated very close to a neighboring sister star. The sister star may remain among the stellar «living», long after the progenitor of its white dwarf companion has «died». The white dwarf, in this case, may take a sinister turn–as one did in the case of Kepler’s supernova–and relentlessly and mercilessly swallow star-stuff stolen from its still-«living» sister star. The white dwarf may continue to gulp down more and more of its victim’s gases until, at last, it can swallow no more. The wicked white dwarf finally devours all it can of its stellar sister’s nourishing gases–but the hideous feast backfires on the white dwarf. The white dwarf finally gulps down so much of its sister star’s material that it attains «critical mass» and blows itself up in a supernova blast–just like its more massive stellar kin. The white dwarf pays for its mischief, meeting its doom as one of that special class of supernova called a Type Ia.
Kepler’s Older Cousin
SST’s new observations, described in the study, have now revealed a second case of a supernova relic resembling Kepler’s. Dubbed N103B, the approximately 1,000 year-old relic of a stellar blast resides about 160,000 light-years away in the neighboring Large Magellanic Cloud, which is a small satellite galaxy of our large barred-spiral Milky Way.
«It’s like Kepler’s older cousin,» Dr. Williams commented in the June 4, 2014 JPL Press Release. He further explained that N103B, though somewhat older than Kepler’s supernova remnant, also swims in a pool of dust and gas that is believed to have been hurled out by an older sister star. «The region around the remnant is extraordinarily dense,» he added. However, unlike Kepler’s supernova remnant, there are no historical sightings of the stellar blast that gave birth to N103B that were ever recorded.
Both the Kepler and the N103B supernovae are believed to have occurred this way: an elderly star is in orbit around its sister–which is a white dwarf. As the elderly star sheds its gases into Space–in a way that is typical for aging stars–some of its ejected material somersaults onto the companion white dwarf. This results in the white dwarf gaining quite a bit of weight, and it eventually builds up enough mass to become unstable and blow itself up into oblivion.
According to the team of astronomers, this scenario may be an unusual occurrence. While the pairing of white dwarfs and red giant companions was believed to underlie literally all Type Ia supernovae as recently as about ten years ago, astronomers now think that collisions between two white dwarfs are the most frequent triggers. The new SST study reveals the complexity of these enormous and beautiful blasts–as well as the variety of triggers that light their fires. The case of what makes a stellar corpse blow up is still very much an unsolved mystery!