By Judith E Braffman-Miller
Supernovae are the most powerful of all known stellar explosions, and they can be detected all the way to the very edge of the visible Universe. Indeed, for one brief shining moment, the fireworks display performed by these exploding stars can out-dazzle their entire host galaxy. Type II, or core-collapse supernovae, herald the final, fatal grand finale of a massive star that has finished burning its necessary supply of fuel by way of the process of nuclear fusion--thus creating heavier and heavier atomic elements out of lighter ones. However, the bitter, brilliant end must come at last when the star contains a core of iron that cannot be used as fuel. In April 2018, astronomers using NASA's Hubble Space Telescope (HST), announced that they had photographed within the fading afterglow of a supernova blast, the very first image of a surviving companion to a supernova. Their discovery is the most compelling evidence to date that some supernovae originate in binary (double-star) systems. But the stellar companion of the dead star was no innocent bystander to the blast--in fact, it was the culprit behind it.
The story began almost two decades ago when astronomers observed a supernova blast 40 million light-years away in a galaxy dubbed NGC 7424, situated in the southern constellation Grus (the Crane). Within the dim, lingering light of the fading afterglow of that once-brilliant blast, NASA's HST managed to obtain the very first image of the surviving companion to a star that has gone supernova. The image captured by the astronomers unveils the stellar survivor. This discovery suggests that some supernovae originate in binary star systems.
"We know that the majority of massive stars are in binary pairs. Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together," Dr. Stuart Ryder explained in an April 26, 2018 Space Telescope Science Institute (STSI) Press Release. Dr. Ryder is from the Australian Astronomical Observatory (AAO) in Sydney, Australia. The STSI is in Baltimore, Maryland.
The guilty stellar companion of the dead massive progenitor star had sucked up almost all of the hydrogen from its doomed companion's stellar envelope. A star's envelope is the region that transports energy from the stellar core to its atmosphere. Millions of years before the primary star perished in the fiery fury of a supernova conflagration, the companion star's vampire-like behavior had created an instability in the doomed primary star. This instabilty made the victimized star episodically blow off a cocoon and shells of hydrogen gas before it finally met its catastrophic fate.
The supernova blast, dubbed SN 2001ig, is a rare type of beast inhabiting the supernova zoo. SN 2001ig is categorized as a Type IIb stripped-envelope supernova. This type of stellar explosion, that heralds the final farewell performance of a doomed star, is considered to be somewhat unusual because most--if not all--of its hydrogen is already gone before the fatal fireworks have begun. This rare type of stellar explosion was first identified in 1987 by team member Dr. Alex Filippenko of the University of California, Berkeley.
Type IIb Supernovae
All of the various classes of Type II (core-collapse) supernovae result from the rapid collapse and horrific explosion of a massive star. However, the progenitor star must sport at least 8 times, but no more than 40 to 50 times, the mass of our Sun. Otherwise the star cannot experience this type of explosive end.
Type II blasts differ from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed inhabiting the spiral arms of galaxies and in H II regions. However, they are not found in elliptical galaxies, which do not possess spiral arms.
Stars manufacture energy by way of the process of nuclear fusion. Unlike our relatively small Sun, stars that are larger and heavier contain enough mass to fuse atomic elements all the way up to iron. Stars manage to perform this feat of atomic metamorphosis as the result of ever-increasing temperatures and pressures. The degeneracy pressure of electrons, along with the energy created by fusion reactions, wage war against the merciless pull of squeezing gravity. This continuous battle prevents the star from collapsing, and the still-"living" star is able to maintain stellar equilibrium. The star continues to fuse progressively heavier and heavier atomic elements out of lighter ones, commencing with the lightest atomic element--hydrogen--and then continuing on and on to fuse all of the atomic elements up to nickel and iron. Nuclear fusion of iron and nickel cannot manufacture net energy output. For this reason, no additional fusion of atomic elements can occur, leaving the nickel-iron core inert. Since there is no longer energy production to create outward pressure, equilibrium in broken, and the doomed star must meet its final violent fate. Within only seconds, the dying star's outer core reaches a breathtaking internal velocity of as much as 21% of the speed of light, while the temperature of the inner core screams upward to as much as 100 million Kelvin.
Type II supernovae usually completely wreck the massive progenitor star, blowing it to pieces and launching its brilliant rainbow of varicolored gaseous layers out into the space between stars. The most massive stars of all collapse and explode into stellar mass black holes. However, massive stars, that are not quite that massive, leave behind in their wreckage a dense, Chicago-sized object dubbed a neutron star. A baby neutron star is a rapidly spinning pulsar, that emits regular beams of light that are often likened to a lighthouse beacon. Neutron stars usually reside within the heart of a multicolored supernova remnant.
There are several categories of Type II supernova explosions, which are categorized according to their resulting light curves. A light curve is a graph of luminosity versus time in the aftermath of the explosion. Type IIb supernovae like SN 2001ig display only a weak hydrogen line in their initial spectrum--which is the reason why they are classified as Type II. However, as time passes, the hydrogen emission becomes undetectable, and there is also a second peak in the light curve that has a spectrum similar to a Type Ib supernova. The progenitor could have been a massive star that launched most of its outer layers into interstellar space. Alternatively, the progenitor could have been a star that lost most of its hydrogen envelope because of interactions with a vampire-like companion in a binary system--leaving behind, in its funeral pyre, a core that consists almost entirely of helium. As the ejecta of a Type IIb expands, the hydrogen layer rapidly becomes increasingly transparent and reveals the deeper gaseous layers. The IIb class of core-collapse supernovae was first introduced --as a theoretical concept--by Dr. Stanford E. Woosley et al. back in 1987. The class was soon applied to a duo of other supernovae: SN 1987 and SN 1993.
SN 2001ig is located in the barred spiral galaxy dubbed NGC 7424 which is situated about 37.5 million light-years from Earth in the constellation Grus (the Crane). NGC 7424 is approximately 100,000 light-years in size, which makes it similar to our own barred spiral Milky Way Galaxy. It is classified as a grand design galaxy because of its well defined spiral arms. One supernova and a duo of ultraluminous X-ray sources have been detected in NGC 7424.
Supernova 2001ig was discovered by the Australian amateur astronomer Robert Evans on the outer edge of NGC 7424 on December 10, 2001. Initially, Type IIb supernovae show spectral lines of hydrogen--just like other Type IIs. However, these vanish after a short time, only to be replaced by calcium, magnesium, and oxygen--which makes them look like typical Ib and Ic supernovae.
On May 28, 2002, astrophysicist Dr. Stuart Ryder (University of Cambridge, UK) and his colleagues spotted what they believe to be the binary companion of SN 2001ig. The companion star is a massive, bright, and searing-hot O or B class star, that originally had an eccentric orbit around the supernova progenitor--a Wolf-Rayet star. Dr. Ryder and his team propose that the sinister companion star periodically devoured the hydrogen-rich envelope of the unlucky progenitor, which explains the observed spectral alterations. Dr. Alicia Soderberg (Princeton University), and her colleagues, also think that the progenitor was a Wolf-Rayet star. However, Dr. Soderberg and her team propose that the periodic shedding of mass was the result of a powerful stellar wind that this class of stars produce.
The Terrible Tale Of An Evil Star
How stripped-envelope supernovae, like SN 2001ig, lose their outer envelopes is not well understood. They were originally believed to be solitary stars with very fast and powerful winds that hurled off their outer envelopes, as Dr. Soderberg's team now proposes. However, there is a problem with this particular model. This is because when astronomers began to hunt for the primary stars from which supernovae are born, they couldn't find them for many stripped-envelope supernovae.
"That was especially bizarre, because astronomers expected that they would be the most massive and the brightest progenitor stars. Also, the sheer number of stripped-envelope supernovae is greater than predicted," explained team member Dr. Ori Fox in the April 26, 2018 STSI Press Release.
Hunting for a binary companion in the aftermath of a supernova blast is not easy. First, the companion must be at a sufficiently close distance from Earth in order for HST to detect such a faint star. SN 2001ig and its companion are located at about that limit. Within that distance range, not many supernovae explode. But--even more importantly--astronomers have to know the precise position through very exact measurements.
Back in 2002, soon after SN 2001ig had exploded, astronomers were able to determine the precise location of the supernova using the European Southern Observatory's (ESO's) Very Large Telescope (VLT) in Cerro Paranal, Chile. Two years later, they then followed up their earlier observations using the Gemini South Observatory in Cerro Pachon, Chile. This observation provided the first tantalizing hints that a surviving binary companion was present.
Dr. Ryder and his team, knowing the coordinates, were able to focus HST on that location 12 years later, as the supernova's brilliant glare faded. With HST's exquisite resolution and ultraviolet capability, the astronomers were able to detect and photograph the surviving companion. Currently, only the HST can do this.
Before the supernova blast, the orbit of the stellar duo around each other lasted for about a year.
When the primary star went supernova, it had less of a devastating impact on the surviving companion than expected. Imagine a plum pit--representing the dense core of the culprit companion star--trapped in a mold of blueberry jello. The blueberry jello represents the star's gaseous envelope. As a shock wave propagates, the jello might briefly both wobble and stretch. However, the plum pit would remain in one piece.
In 2014, Dr. Fox and his team used the HST to spot the companion of a different Type IIb supernova, known as SN 1993J. However, they obtained a spectrum--not an image. SN 2001ig is particularly important because it represents the first time a surviving companion star has been imaged. "We were finally able to catch the stellar thief, confirming our suspicions that one had to be there," Dr. Filippenko explained in the April 26, 2018 STSI Press Release.
It's possible that as many as 50% of all stripped-envelope supernovae are accompanied by a companion star--and the other 50% lose their outer envelopes as the result of fierce stellar winds. Dr. Ryder and his team plan to determine how many supernovae with stripped-envelopes possess companion stars.
Their next endeavor is to study completely stripped-envelope supernovae--as opposed to SN 2001ig and SN 1993J. In both of these cases, the supernovae were only approximately 50% stripped. These completely stripped-envelope supernovae don't have much in the way of shock interaction with the ambient stellar environment. This is because their outer envelopes were lost long before the final, fatal supernova blast. Without shock interaction, they fade much more rapidly. This means that the team will have to wait two or three years to look for surviving supernova companion stars. In the future, they hope to use the upcoming James Webb Space Telescope (JWST) to continue their hunt.
The paper on the team's research is published in the March 28, 2018 issue of The Astrophysical Journal.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
