INNER WORKINGS

Amassive star dies without a bang, revealing thesensitive nature of supernovaeKen Croswell, Science Writer

In 2008, a huge red star in another galaxy reached the endof its life. A star as heavy as this one, bornwith 25 times themass of the Sun, was supposed to go out in a fiery flash oflight known as a supernova, millions or billions of timesbrighter than our Sun. But this one refused to play the roleof drama queen. Instead, it brightened just a little, thenvanished, possibly leaving behind a black hole.

Noone had ever seen one of these huge red stars winkout of existence with so little fuss before. It was a sign thatthe lives and deaths of these stars are more complex thanour simplest theories had claimed. “As amazing and im-portant and fun and exciting as this is, it’s not a surprise,”says Stan Woosley at the University of California, SantaCruz. In fact, the discovery may help explain why themassive stars in computer models often fail to blow up.

Expand and CollapseConventional theory says that nearly all stars bornmore than eight times as massive as the Sun explode

as supernovae. When young, a massive star is brightand blue. Nuclear reactions in its core generate animmense amount of energy. This keeps the star hot sothat gas pressure pushes outward and partially coun-teracts the inward pull of the star’s gravity; so does thepressure of the many photons streaming out of thestar’s core. As long as it generates energy, the star canhold itself up.

In the end, though, gravity always wins. Later in life,as a massive star begins to run out of fuel, it expands.Stars born between eight and 25 or 30 solar massesexpand so much that their surfaces cool, and the starsbecome red supergiants. If the Sun were as large asthe largest red supergiant, it would engulf everyplanet from Mercury to Jupiter. Then, according tostandard lore, the star exhausts its fuel and its corecollapses. The collapse sparks a wave of neutrinos.These ghostly particles normally pass unimpededthrough matter, but the collapse of the core producesso many neutrinos that they blast off the star’s outerlayers, launching a titanic supernova explosion.

Indeed, astronomers see lots of supernova explo-sions in other galaxies, often in spiral arms, wheremassive stars reside. So the prevailing belief has beenthat nearly all stars born at more than eight solarmasses explode as supernovae.

Yet for decades, theorists such as Woosley havestruggled to make these massive stars explode incomputer models; instead, the model stars often col-lapse under their own weight. Researchers have fre-quently assumed that Shakespeare’s famous wordsrang 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 ProblemBut in recent years, observations have also begun tosuggest that some red supergiants don’t actually gosupernova. Starting in 1987, when observers saw asupernova in the Large Magellanic Cloud, a neigh-boring galaxy, astronomers have been able to exam-ine preexplosion images of galaxies and identifywhich star exploded.

By now, says Stephen Smartt of Queen’s UniversityBelfast, astronomers have performed 25 of these

The spiral galaxy NGC 6946 spawned the first, and so far the only, failed supernovaever seen: a red supergiant star that vanished from the heavens without exploding.Image credit: Science Source/Robert Gendler.

Published under the PNAS license.

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stellar autopsies. As expected, most of the doomedstars were red supergiants. But they didn’t span thefull range of mass from eight to 30 suns. “We havealmost no detections of stars above a [birth] mass of17 solar masses,” Smartt says, “and these should bethe 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—thoseborn at more than 17 solar masses—implode, theircores quietly collapsing into black holes.

That disappearing supergiant of 2008 is a likelyexample, Smartt says. The star’s home is a hyper-active spiral galaxy 25 million light-years from Earthnamed NGC 6946, which is infamous for its sun-dry supernovae. From 1917 to 2017 observers saw10 supernova explosions there, more than in anyother galaxy; but the supernova that didn’t happencould prove more significant than all of thosethat did.

No one noticed the star’s disappearance at thetime. In 2014, however, Christopher Kochanek andgraduate student Jill Gerke, both at Ohio State Uni-versity in Columbus, were examining images of galaxiesso near our own that we can detect their individualstars. These astronomers knew of the red supergiantproblem and the trouble theorists had in getting theirstars to explode. The galaxy images captured a millionred supergiants, each a potential future supernova. Bycomparing images from different years, the astrono-mers hoped to catch the exact opposite: a red super-giant dropping out of sight as it became a black hole.

“It was very nice and clean,” Gerke says of the2008 event. “You could see the star there, and thenyou could clearly see that, at least in our data, it was nolonger visible.” It is still the only time anyone had everseen a star vanish from the heavens without goingsupernova (3).

Woosley, who was not involved in the discovery,calls the claim credible. Although the star could con-ceivably still be shining behind a thick cloud of dust,starlight should heat that dust and make it glowstrongly at infrared wavelengths, which no one hasseen (4). Conclusive confirmation of the death of thestar awaits the James Webb Space Telescope, a largeinfrared-sensitive instrument that NASA plans tolaunch in 2021.

Contrary CarbonIn 2019, Tuguldur Sukhbold at Ohio State Universityproposed an explanation for why lower-mass red su-pergiants explode and higher-mass red supergiantsdon’t: “It’s ultimately a consequence of the way thatcarbon burns in a massive star,” he says (5). His workbuilds on the recognition a quarter century ago thatcarbon burns differently depending on whether amassive star was born at more than or less thana certain mass.

For most of its life, a massive star converts hydro-gen into helium at its center, as the Sun does. Whenthe hydrogen runs out, the helium ignites, creatingcarbon and oxygen. And when the helium runs out,

the star, desperate to hold up its great weight, taps itscarbon, turning it into neon, sodium, and magnesium.

But carbon comes with a catch. It burns at such ahigh temperature that the intense heat generateshigh-energy photons, which can turn into pairs ofelectrons and antielectrons. These usually annihilateeach other and can produce neutrinos and antineu-trinos, which zip out of the star, rob it of energy, anddo nothing to hold it up against gravity. Because ofneutrino losses, once carbon ignites, the star has nomore than a few thousand years to live. Then the starburns still heavier fuels until it runs out of options. Thelast reactions forge iron, which is a dead end, as thestar can wring no more nuclear fusion energy from thismost stable of all nuclei. With nothing to support it,the core collapses.

But whether the star then explodes or implodesdepends primarily on how it burned its carbon at itscenter, Sukhbold proposes. “The way the burningtakes place changes the star’s final core structure,” hesays, “and that structure has a lot to say in what hap-pens in the end—whether the star explodes or not.” Inlower-mass red supergiants, carbon burns con-vectively: The burning region bubbles and boils asrising and falling pockets of gas ferry heat away fromthe core. The convection also replenishes the centralregion with fresh carbon fuel, thereby prolonging thisstage of the star’s evolution and causing great neu-trino losses; consequently, these lower-mass red su-pergiants wind up with compact cores. When thecores collapse to form dense stellar objects calledneutron stars, they blast off the outer layers of the starin a supernova.

Astronomers have long thought that Betelgeuse, the ruddy star (Top) in thebright constellation Orion the Hunter, will someday explode in a brilliantsupernova. But new research raises the possibility that this expected explosionmay never happen. Image credit: Shutterstock/Genevieve de Messieres.

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In higher-mass red supergiants, however, carbondoesn’t burn convectively; this limits neutrino lossesand leads to a more extended core with dense ma-terial around it. When the core collapses, the blastwave slams into the dense material above, whichthwarts the explosion. Instead of creating a super-nova, the star implodes, forming a black hole.

The dividing line between the two fates? A birthmassof about 19 solar masses, Sukhbold calculates—notfar from Smartt’s observational determination of 17.Given uncertainties in both observation and theory,Sukhbold sees no conflict. In fact, he thinks that thetrue dividing line could be anywhere between 16 and20 solar masses. Furthermore, theory says that thereshould be exceptions to the rule: A few stars belowthis mass can implode, and a few stars above this masscan explode.

This new thinking changes not only our view of thelives and deaths of massive stars but also calculationsof how productive they have been in sprinkling theirgalaxies with new chemical elements. In massive stars,neutrons slowly convert the iron nuclei with which thestar was born into heavier elements such as yttriumand zirconium. But if the stars never explode, theseelements fall into the black hole, depriving the gal-axies of the stars’ full chemical progeny.

With a Bang or a Whimper?The brightest red supergiant in Earth’s sky is Betel-geuse, a stunning stellar ruby in Orion. All the otherbright stars in Orion are blue. Only Betelgeuse hasturned red, which means that by conventional wisdomit will be the next to explode.

Or will it? “We don’t know what Betelgeuse will door when it will do it,” Woosley says.

The key determinant is the star’s birth mass. Noone knows what that is for Betelgeuse, in part becausethe star’s distance is uncertain. That, in turn, means thestar’s luminosity is uncertain, and astronomers need toknow the luminosity to infer its mass. AstronomerEdward Guinan of Villanova University outside ofPhiladelphia, PA, who has long observed the star, putsits birth mass somewhere between eight and 18 solarmasses. So Betelgeuse will probably explode as asupernova after all, in which case it will far outshinedazzling Venus in our skies. But if the star’s birth massis near the upper end of Guinan’s estimate, around18 suns, Betelgeuse could implode instead.

An implosion would be much less spectacular, andthe failed supernova in NGC 6946 may foretell whatwe’d see. As that star died and became a black hole, itgently cast off its outer envelope and grew five timesbrighter. If Betelgeuse follows suit, its brightness willincrease but never surpass that of Sirius, the brighteststar in the night. Then Betelgeuse will disappear,leaving a literal hole in Orion.

Meanwhile, Kochanek’s team is seeking a secondfailed supernova. “This is a project best done withtenure,” he jokes. From 2008 to 2019, his teammonitored 27 galaxies within 35 million light-years ofEarth; in those galaxies, eight massive stars explodedas supernovae versus the one that failed.

It’s only a matter of time, he thinks, before he seesanother big red star wink out and become a newbornblack hole, illuminating the still mysterious lives ofmassive 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.

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