Stars are like people–they send their dazzling light through the Cosmos for a little while, but do not last forever in the universal tragicomedy of our existence. Supernovae herald the fatal explosions of massive stars that have come to the end of that long stellar road, after having burned their necessary supply of nuclear-fusing fuel–and have perished brilliantly and beautifully, as they scream explosively into oblivion. One of the ways that astronomers search for clues, hinting at how these massive stars blow themselves up, is to go on the hunt for what is termed the progenitor star of the supernova. In order to accomplish their quest, astronomers carefully sift through archival telescope images and try to determine the precise location and identity of the progenitor star before it blew itself to pieces. In November 2018, for the first time, a California Institute of Technology (Caltech) team of astronomers in Pasadena announced that they have likely discovered just such a stellar progenitor for a supernova class known as Type Ic (pronounced “one-C”). Of all the classes of supernovae, this is the only one that did not have a known stellar progenitor until their discovery. For this reason, its identification was considered by astronomers to be a kind of Holy Grail.
The Type Ic supernova, dubbed SN 2017, was first spotted in May 2017 by astronomers using the Tenagra Observatories in Arizona. It is situated in a spiral galaxy named NGC 3938, that is located about 65 million light-years from Earth. The Caltech astronomers were able to successfully track this supernova’s progenitor using archival images from NASA’s Hubble Space Telescope (HST), obtained back in 2007.
“An alert was sent out when the supernova was initially found. You can’t sleep once that happens and have to mobilize to try to find the progenitor to the explosion. Within a few weeks after the supernova was discovered, we found a candidate using both new and archival Hubble images. The new images were essential for pinpointing the candidate progenitor’s location,” noted Dr. Schuyler Van Dyk in a November 15, 2018 JPL Press Release. Dr. Van Dyk is a staff scientist at IPAC, which is a science and data center located at Caltech.
The progenitor turned out to be a luminous and very hot star, and it is thought to be either a singly massive star 48 to 49 times solar-mass or a massive binary system in which the star that went supernova weighed-in at a hefty 60 to 80 times the mass of our Sun.
Type Ic Supernovae
Type Ic supernovae, and their close cousins Type Ib supernovae, are classifications of supernovae that result from the explosive core collapse of massive stars. These doomed stars have flung off, or have been more gently stripped of, their outer envelope of hydrogen gas. When Type Ic and Type Ib supernovae are compared to Type Ia supernovae, they do not show the absorption line of silicon. When compared to Type Ib, Type Ic supernovae are believed to have lost more of their original gaseous envelope, including most of their helium. Astronomers usually refer to the two types as “stripped core-collapse supernovae.”
All stars, regardless of their mass, churn out energy by way of the process of the nuclear-fusion of atomic elements, which creates heavier elements out of lighter ones. Unlike our relatively small Sun, more massive stars contain suffient mass to fuse elements that have an atomic mass greater than hydrogen and helium–albeit at progressively greater and greater temperatures and pressures. This increase results in a shorter “life” for massive stars. Small stars, like our Sun, “live” on the hydrogen-burning branch of the Hertzsprung-Russell Diagram of Stellar Evolution for about 10 billion years. In dramatic contrast, massive stars “live” fast and “die” young. The more massive the star, the shorter its “life”. A hefty star fuses increasingly heavier atomic elements, commencing with hydrogen and helium, and then progressing through the familiar Periodic Table until a core of iron and nickel is formed. Because nuclear-fusion of iron or nickel manufactures no net energy output, no additional fusion can occur, leaving the nickel-iron core of the doomed massive star inert. Due to the lack of energy output creating the necessary outward thermal pressure to keep the heavy star bouncy against the relentless inward pull of its own gravity, the core shrivels. When the compacted mass of the inert iron and nickel core exceeds what is called the Chandrasekhar Limit of 1.4 solar-masses, radiation pressure cannot counter gravitational compression, and a cataclysmic implosion of the core occurs within seconds. At this point, lacking the support of the now-imploded inner core, the outer core of the erstwhile massive star collapses inward under the merciless force of gravity and attains a velocity of up to 23% the speed of light. The sudden, dramatic compression increases the temperature of the inner core to as much as 100 billion Kelvins. The collapse of the inner core is stopped by neutron degeneracy, resulting in the implosion to rebound and bounce outward. The energy of the expanding shock wave disrupts the overlying stellar material and accelerates it to escape velocity. A horrific, brilliant Type II supernova occurs, and where once there was a massive star there is a star no more. Depending on the hefty progenitor star’s mass, the souvenir that it leaves behind to remind the Universe of its former existence will be either a dense, city-sized neutron star or a stellar mass black hole.
Small stars go to their inevitable grand finale differently. Type Ia supernovae, unlike core-collapse Type II supernovae, do not originate from the funeral pyre of a massive progenitor star. Type Ia supernovae are the catastrophic leftovers of small stars, like our Sun, that have perished to become a type of dense stellar relic termed a white dwarf. Our Sun will never perish in the terrible beauty born from a Type Ia blast. This is because our Sun is a solitary Star. However, when small stars of our Sun’s mass dwell in a binary system with another still-living star, it’s a party ready to happen. If the dense, vampire-like white dwarf relentlessly gravitationally sips up its companion star’s material, it pays for its crime by “going critical.” That is, the murderous white dwarf steals enough mass from its companion to attain the critical mass to blow itself to pieces–just like its more massive stellar kin. Alternatively, a Type Ia supernova can also occur when a duo of white dwarfs, composing a binary system, blast into one another. When this happens, it also results in a horrific Type Ia supernova explosion.
Piecing together how each of these supernova types (Type II, Type Ib, Type Ia, and Type Ic) occur provides a greatly improved understanding of how the most massive stars in the Universe evolve.
Uncovering An Elusive, Doomed Stellar Progenitor
“Type Ic supernovae occur with the most massive stars. But we were surprised by how massive this one appears to be, and especially by the possibility of a massive double-star system as the progenitor. Although theories have existed for the last three decades that Type Ic supernovae could be the explosions of very massive single stars, alternative, more recent theories point toward stars of lower mass in binary systems as being the origins of these explosions,” Dr. Van Dyk explained in the November 15, 2018 Caltech Press Release.
Type Ib and Type Ic differ from Type II because their stellar progenitors lose their outer envelopes of material surrounding their central cores before going supernova. Type Ib and Ic also differ from each other slightly in chemical composition.
“The origins of such explosions are relevant to the entire astronomical community, not just supernova researchers. The results have implications on ideas from star formation to stellar evolution and feedback into the galaxy,” Dr. Ori Fox commented in the November 15, 2018 Caltech Press Release. Dr. Fox is a Support Scientist at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
Dr. van Dyk continued to note in the same Press Release that “Astronomers have been trying to find this progenitor for some 20 years. Humans wouldn’t be here without supernovae–they make the chemical elements from which we are made.”
The astronomers also commented that they should be able to confirm with certainty whether they have identified the correct progenitor to the Type Ic explosion within a few years, using Hubble or the upcoming James Webb Space Telescope, planned to launch in 2021. As the supernova dims as predicted, the astronomers will have a clearer view of the region surrounding it. If the luminous progenitor candidate was correctly identified in archival images, then it will have disappeared and should not be detected in the new images. If the scientists still see the candidate progenitor, that means it was misidentified and some other hidden star was the true culprit behind the cataclysmic blast.
In Memory of Mark.