NE WORKINGS INNER INNER WORKINGS

A massive star dies without a bang, revealing the sensitive nature of supernovae Ken Croswell, Science Writer

In 2008, a huge red star in another galaxy reached the end as supernovae. When young, a massive star is bright of its life. A star as heavy as this one, born with 25 times the and blue. Nuclear reactions in its core generate an mass of the Sun, was supposed to go out in a fiery flash of immense amount of energy. This keeps the star hot so light known as a supernova, millions or billions of times that gas pressure pushes outward and partially coun- brighter than our Sun. But this one refused to play the role teracts the inward pull of the star’s gravity; so does the of drama queen. Instead, it brightened just a little, then pressure of the many photons streaming out of the vanished, possibly leaving behind a black hole. star’s core. As long as it generates energy, the star can No one had ever seen one of these huge red stars wink hold itself up. out of existence with so little fuss before. It was a sign that In the end, though, gravity always wins. Later in life, the lives and deaths of these stars are more complex than as a massive star begins to run out of fuel, it expands. our simplest theories had claimed. “As amazing and im- Stars born between eight and 25 or 30 solar masses portant and fun and exciting as this is, it’snotasurprise,” expand so much that their surfaces cool, and the stars says Stan Woosley at the University of California, Santa become red supergiants. If the Sun were as large as Cruz. In fact, the discovery may help explain why the the largest red supergiant, it would engulf every massive stars in computer models often fail to blow up. planet from Mercury to Jupiter. Then, according to standard lore, the star exhausts its fuel and its core Expand and Collapse collapses. The collapse sparks a wave of neutrinos. Conventional theory says that nearly all stars born These ghostly particles normally pass unimpeded more than eight times as massive as the Sun explode through matter, but the collapse of the core produces so many neutrinos that they blast off the star’s outer layers, launching a titanic supernova explosion. Indeed, astronomers see lots of supernova explo- sions in other galaxies, often in spiral arms, where massive stars reside. So the prevailing belief has been that nearly all stars born at more than eight solar masses explode as supernovae. Yet for decades, theorists such as Woosley have struggled to make these massive stars explode in computer models; instead, the model stars often col- lapse under their own weight. Researchers have fre- quently assumed that Shakespeare’s famous words rang true here: The fault is not in our stars, but in our- selves. The theoretical models may not mimic the ex- treme conditions in these extreme stars.

A Supergiant Problem But in recent years, observations have also begun to suggest that some red supergiants don’t actually go supernova. Starting in 1987, when observers saw a supernova in the Large Magellanic Cloud, a neigh- boring galaxy, astronomers have been able to exam- ine preexplosion images of galaxies and identify which star exploded. The spiral galaxy NGC 6946 spawned the first, and so far the only, failed supernova ’ ever seen: a red supergiant star that vanished from the heavens without exploding. By now, says Stephen Smartt of Queen s University Image credit: Science Source/Robert Gendler. Belfast, astronomers have performed 25 of these

Published under the PNAS license.

1240–1242 | PNAS | January 21, 2020 | vol. 117 | no. 3 www.pnas.org/cgi/doi/10.1073/pnas.1920319116 Downloaded by guest on September 27, 2021 stellar autopsies. As expected, most of the doomed stars were red supergiants. But they didn’t span the full range of mass from eight to 30 suns. “We have almost no detections of stars above a [birth] mass of 17 solar masses,” Smartt says, “and these should be the brightest ones, the easiest ones to find on images.” He calls this failure the red supergiant problem (1, 2). Smartt suspects that only the lower-mass red supergi- ants blow up. The higher-mass red supergiants—those born at more than 17 solar masses—implode, their cores quietly collapsing into black holes. That disappearing supergiant of 2008 is a likely example, Smartt says. The star’shomeisahyper- active spiral galaxy 25 million light-years from Earth named NGC 6946, which is infamous for its sun- dry supernovae. From 1917 to 2017 observers saw 10 supernova explosions there, more than in any othergalaxy;butthesupernovathatdidn’t happen could prove more significant than all of those that did. No one noticed the star’s disappearance at the time. In 2014, however, Christopher Kochanek and graduate student Jill Gerke, both at Ohio State Uni- versity in Columbus, were examining images of galaxies Astronomers have long thought that Betelgeuse, the ruddy star (Top)inthe so near our own that we can detect their individual bright constellation Orion the Hunter, will someday explode in a brilliant stars. These astronomers knew of the red supergiant supernova. But new research raises the possibility that this expected explosion problem and the trouble theorists had in getting their may never happen. Image credit: Shutterstock/Genevieve de Messieres. stars to explode. The galaxy images captured a million red supergiants, each a potential future supernova. By the star, desperate to hold up its great weight, taps its comparing images from different years, the astrono- carbon, turning it into neon, sodium, and magnesium. mers hoped to catch the exact opposite: a red super- But carbon comes with a catch. It burns at such a giant dropping out of sight as it became a black hole. high temperature that the intense heat generates “It was very nice and clean,” Gerke says of the high-energy photons, which can turn into pairs of 2008 event. “You could see the star there, and then electrons and antielectrons. These usually annihilate you could clearly see that, at least in our data, it was no longer visible.” It is still the only time anyone had ever each other and can produce neutrinos and antineu- seen a star vanish from the heavens without going trinos, which zip out of the star, rob it of energy, and supernova (3). do nothing to hold it up against gravity. Because of Woosley, who was not involved in the discovery, neutrino losses, once carbon ignites, the star has no calls the claim credible. Although the star could con- more than a few thousand years to live. Then the star ceivably still be shining behind a thick cloud of dust, burns still heavier fuels until it runs out of options. The starlight should heat that dust and make it glow last reactions forge iron, which is a dead end, as the strongly at infrared wavelengths, which no one has star can wring no more nuclear fusion energy from this seen (4). Conclusive confirmation of the death of the most stable of all nuclei. With nothing to support it, star awaits the James Webb Space Telescope, a large the core collapses. infrared-sensitive instrument that NASA plans to But whether the star then explodes or implodes launch in 2021. depends primarily on how it burned its carbon at its center, Sukhbold proposes. “The way the burning Contrary Carbon takes place changes the star’s final core structure,” he In 2019, Tuguldur Sukhbold at Ohio State University says, “and that structure has a lot to say in what hap- proposed an explanation for why lower-mass red su- pens in the end—whether the star explodes or not.” In pergiants explode and higher-mass red supergiants lower-mass red supergiants, carbon burns con- don’t: “It’s ultimately a consequence of the way that vectively: The burning region bubbles and boils as carbon burns in a massive star,” he says (5). His work rising and falling pockets of gas ferry heat away from builds on the recognition a quarter century ago that the core. The convection also replenishes the central carbon burns differently depending on whether a region with fresh carbon fuel, thereby prolonging this massive star was born at more than or less than stage of the star’s evolution and causing great neu- a certain mass. trino losses; consequently, these lower-mass red su- For most of its life, a massive star converts hydro- pergiants wind up with compact cores. When the gen into helium at its center, as the Sun does. When cores collapse to form dense stellar objects called the hydrogen runs out, the helium ignites, creating neutron stars, they blast off the outer layers of the star carbon and oxygen. And when the helium runs out, in a supernova.

Croswell PNAS | January 21, 2020 | vol. 117 | no. 3 | 1241 Downloaded by guest on September 27, 2021 In higher-mass red supergiants, however, carbon Or will it? “We don’t know what Betelgeuse will do doesn’t burn convectively; this limits neutrino losses or when it will do it,” Woosley says. and leads to a more extended core with dense ma- The key determinant is the star’s birth mass. No terial around it. When the core collapses, the blast one knows what that is for Betelgeuse, in part because wave slams into the dense material above, which the star’s distance is uncertain. That, in turn, means the thwarts the explosion. Instead of creating a super- star’s luminosity is uncertain, and astronomers need to nova, the star implodes, forming a black hole. know the luminosity to infer its mass. Astronomer The dividing line between the two fates? A birth mass Edward Guinan of Villanova University outside of of about 19 solar masses, Sukhbold calculates—not Philadelphia, PA, who has long observed the star, puts far from Smartt’s observational determination of 17. its birth mass somewhere between eight and 18 solar Given uncertainties in both observation and theory, masses. So Betelgeuse will probably explode as a Sukhbold sees no conflict. In fact, he thinks that the supernova after all, in which case it will far outshine true dividing line could be anywhere between 16 and dazzling Venus in our skies. But if the star’s birth mass 20 solar masses. Furthermore, theory says that there is near the upper end of Guinan’s estimate, around should be exceptions to the rule: A few stars below 18 suns, Betelgeuse could implode instead. this mass can implode, and a few stars above this mass An implosion would be much less spectacular, and can explode. the failed supernova in NGC 6946 may foretell what This new thinking changes not only our view of the we’d see. As that star died and became a black hole, it lives and deaths of massive stars but also calculations gently cast off its outer envelope and grew five times of how productive they have been in sprinkling their brighter. If Betelgeuse follows suit, its brightness will galaxies with new chemical elements. In massive stars, increase but never surpass that of Sirius, the brightest neutrons slowly convert the iron nuclei with which the star in the night. Then Betelgeuse will disappear, star was born into heavier elements such as yttrium leaving a literal hole in Orion. and zirconium. But if the stars never explode, these Meanwhile, Kochanek’s team is seeking a second elements fall into the black hole, depriving the gal- failed supernova. “This is a project best done with axies of the stars’ full chemical progeny. tenure,” he jokes. From 2008 to 2019, his team monitored 27 galaxies within 35 million light-years of With a Bang or a Whimper? Earth; in those galaxies, eight massive stars exploded The brightest red supergiant in Earth’s sky is Betel- as supernovae versus the one that failed. geuse, a stunning stellar ruby in Orion. All the other It’s only a matter of time, he thinks, before he sees bright stars in Orion are blue. Only Betelgeuse has another big red star wink out and become a newborn turned red, which means that by conventional wisdom black hole, illuminating the still mysterious lives of it will be the next to explode. massive stars.

1 S. J. Smartt, Progenitors of core-collapse supernovae. Annu. Rev. Astron. Astrophys. 47, 63–106 (2009). ADS: https:// ui.adsabs.harvard.edu/#abs/2009ARA%26A..47...63S/abstract 2 S. J. Smartt, Observational constraints on the progenitors of core-collapse supernovae: The case for missing high-mass stars. Publ. Astron. Soc. Aust. 32, e016 (2015). ADS: https://ui.adsabs.harvard.edu/abs/2015PASA...32...16S/abstract. 3 J. R. Gerke, C. S. Kochanek, K. Z. Stanek, The search for failed supernovae with the Large Binocular Telescope: First candidates. Mon. Not. R. Astron. Soc. 450, 3289–3305 (2015). ADS: https://ui.adsabs.harvard.edu/abs/2015MNRAS.450.3289G/abstract. 4 S. M. Adams et al., The search for failed supernovae with the Large Binocular Telescope: Confirmation of a disappearing star. Mon. Not. R. Astron. Soc. 468, 4968–4981 (2017). ADS: https://ui.adsabs.harvard.edu/abs/2017MNRAS.468.4968A/abstract. 5 T. Sukhbold, S. Adams, (2019) Missing red supergiants and carbon burning. arXiv. https://arxiv.org/abs/1905.00474.

1242 | www.pnas.org/cgi/doi/10.1073/pnas.1920319116 Croswell Downloaded by guest on September 27, 2021