Nearby supernova gives unique view of dying star's last days

For Scientific American

Every 10 seconds, somewhere in the universe, a star explodes. The light from a small fraction of these supernovae—roughly a few hundred per year—reaches us here on Earth to be pored over by astronomers. Studying supernovae is vital to gaining a deeper understanding of the cosmos because they spew forth radiation, dust and gas that help sculpt galaxies, form new stars and planets and enrich the universe with heavy elements. But most are so distant that we can do little more than guess at their exact stellar origins using a handful of hard-won photons to assemble an incomplete play-by-play of their epochal emergence. Earlier this year, however, astronomers spotted a supernova erupting just 21 million light-years away—a stone’s throw in the 94-billion-light-year width of the observable universe—making it the closest one to Earth seen in a decade. Thanks to the star’s proximity, astronomers are now piecing together its final days in lavish detail and yielding fresh insights into how these astrophysical cataclysms unfold and shape the cosmos at large.

Japanese amateur astronomer Koichi Itagaki was the first to see this supernova, known as SN 2023ixf, on May 19. Almost immediately, professional observers sprung to action. “The whole supernova community got on it as soon as they could,” says Griffin Hosseinzadeh of the University of Arizona, using such facilities as the Hubble Space Telescope, the International Gemini Observatory in Hawaii and the Lick Observatory in California. Soon they had pinpointed the supernova to somewhere within the Pinwheel Galaxy, also called M101. From there, one of the first tasks was to seek out the actual star that exploded, which is somewhat of a rarity to pinpoint for supernovae. Thankfully, Joanne Pledger of the University of Central Lancashire in England had previously spent time studying M101 as a postdoctoral researcher. “We’d got time on the Hubble Space Telescope,” she says. By zooming in on the location of the supernova in her early-2010s images of the galaxy, Pledger managed to identify the star that caused it. “It’s a huge step change,” she says. “There’s a wealth of data already there.”

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Pledger’s findings confirmed the stellar culprit to be a red supergiant. As a class, such stars are among the largest in the universe, with radii up to 1,500 times that of our sun and masses of up to 40 times larger than our home star. But 2023ixf’s star wasn’t quite so scale-tipping. It’s thought to have had only about 420 times the radius and 20 times the mass of our sun. That matched with astronomers’ initial identification of the event as a so-called type II supernova, in which a massive star exhausts its nuclear fuel, collapses in on itself and explosively ejects its outer layers after they bounce off its durable core, leaving behind a neutron star or a black hole. Such stars can grow puffy late in life and blow off lingering shells of gas and dust from their outer atmospheres well in advance of expiring as supernovae. Teams of astronomers were able to detect that circumstellar material for 2023ixf as the supernova expanded outward and crashed into it, producing a discernible shockwave. “It’s not the first time we’ve seen this happen,” Hosseinzadeh says. “But the detail has never been this good.”

In the two weeks after the supernova’s discovery, Wynn Jacobson-Galán of the University of California, Berkeley, and his colleagues saw clear evidence for “the supernova shockwave slamming into this dense shell,” he says. They estimate from those observations that the star lost less than 1 per cent of its mass in the years before the explosion. While seemingly small, that amount is “more than we would expect from a red supergiant star,” Jacobson-Galán says. “It points, maybe, to our ignorance about how red supergiants evolve and die in the last few years before explosion.”

Such follow-up work is revealing more about how these events enrich galaxies. “It’s telling us how stars lose mass, which has a big influence on how galaxies evolve,” says Azalee Bostroem of the University of Arizona, who has led Hubble observations of 2023ixf. “And it’s telling us a little bit about which stars explode as which type of supernovae.” In turn, it could reveal the very dynamics of supernovae themselves—whether the energy we see comes entirely from the explosion or partly from the impact of the supernova shockwave on the surrounding debris. “All of these things are linked with how much material is left on the star when it explodes,” Bostroem says.

There had previously been some debate, too, as to whether this ejected material would form a sphere around the star or some more asymmetrical shape. The results for 2023ixf suggest the latter, marking the earliest-ever detailed glimpse scientists have obtained of the rapidly evolving interaction between a supernova’s shockwave and the surrounding circumstellar material. “We are saying that the material is most likely in a disklike structure,” says Sergiy Vasylyev, also at U.C. Berkeley. The supernova’s ejecta expands in an “hourglass shape” as it impacts this disk. That could point to a surprising source of variety in type II supernovae evolution arising from the varied orientations of debris disks with respect to their exploding host star. “It tells you that these events are diverse,” Vasylyev says.

Another interesting feature of the star revealed by preexisting observations is that it had been pulsating—changing in size by a huge amount. Red supergiants are known to develop such pulsations in the denouement of their life. But until now this had never been seen in a progenitor star that subsequently exploded as a supernova. Monika Soraisam of the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) and her colleagues showed that the star experienced wild oscillations in brightness and repeatedly swelled and shrank its size by about 50 per cent over a period of about 1,000 days before exploding like an overfilled balloon during its last swing toward an especially swollen state.

Pulsations and supernovae are not thought to be directly linked. The former is caused by a “totally different” mechanism, Soraisam says, namely, instabilities in the flow of energy through a stellar atmosphere. Yet such instabilities remain poorly understood, leaving the possibility that there may indeed be some sort of link—which is the very sort of thing that could help researchers predict when other red supergiant stars will explode. (For instance, Betelgeuse—the red supergiant in the constellation of Orion—has been pulsating in recent years. Astrophysicists consider this a murky omen of an eventual supernova, but, at present, they estimate that such an event could still be up to 100,000 years in the future.) “That’s the intriguing thing about 2023ixf,” Soraisam says. “Very close to the explosion, we are still seeing very regular variability.”

Supernova 2023ixf’s exact “flavour” still needs to be constrained, too. Initially, it had been classed as a subcategory of hydrogen-rich type II supernovae called type II-P, in which the fading of the supernova’s afterglow pauses for a time (the P stands for “plateau”) before continuing its plunge into darkness. Astronomers now think it was instead a type II-L (or “linear”) explosion, which has a steadier decrease in brightness. “Normally, within about 40 days, you should see the plateau,” says Ian Sharp, an amateur astronomer in England and a co-author on the work proposing that 2023ixf is linear. “We don’t see any evidence of it plateauing. So we believe it’s an L.” The exact mechanism that produces these two distinct types of supernovae, however, is not clear. “‘We don’t know’ is the short answer,” Bostroem says. The difference between P and L, she says, may hinge on how much a dying star manages to hold on to its outer layers of hydrogen before its explosive demise. “The more mass was lost, the smaller the hydrogen envelope—and potentially the steeper or more linear the decline,” she says.

Supernova 2023ixf may give some much-needed answers on the matter, among the other particulars of how a red supergiant star collapses and ultimately explodes. “We can really test whether our picture holds up from end to end,” Bostroem says. Short of seeing a supernova in our own galaxy—every modern astronomer’s hopeful but as-yet-unrequited dream—this bright, brief spectacle in the Pinwheel Galaxy may be the best opportunity for many years to come to test contemporary models for type II supernovae and better see the creative destruction unleashed upon the cosmos. “This is being studied in such detail and with such precision,” Jacobson-Galán says. “It really is going to be one of the best-studied supernovae of the 21st century.”