Expand and Collapse

Conventional theory says that nearly all stars born more than eight times as massive as the Sun explode as supernovae. When young, a massive star is bright and blue. Nuclear reactions in its core generate an immense amount of energy. This keeps the star hot so that gas pressure pushes outward and partially counteracts the inward pull of the star’s gravity; so does the pressure of the many photons streaming out of the star’s core. As long as it generates energy, the star can hold 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 masses expand so much that their surfaces cool, and the stars become red supergiants. If the Sun were as large as the largest red supergiant, it would engulf every planet from Mercury to Jupiter. Then, according to standard lore, the star exhausts its fuel and its core collapses. The collapse sparks a wave of neutrinos. These ghostly particles normally pass unimpeded 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 explosions 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 collapse under their own weight. Researchers have frequently assumed that Shakespeare’s famous words rang true here: The fault is not in our stars, but in ourselves. The theoretical models may not mimic the extreme 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 neighboring galaxy, astronomers have been able to examine preexplosion images of galaxies and identify which star exploded.

By now, says Stephen Smartt of Queen’s University Belfast, astronomers have performed 25 of these 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 supergiants 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’s home is a hyperactive spiral galaxy 25 million light-years from Earth named NGC 6946, which is infamous for its sundry supernovae. From 1917 to 2017 observers saw 10 supernova explosions there, more than in any other galaxy; but the supernova that didn’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 University in Columbus, were examining images of galaxies so near our own that we can detect their individual stars. These astronomers knew of the red supergiant problem and the trouble theorists had in getting their stars to explode. The galaxy images captured a million red supergiants, each a potential future supernova. By comparing images from different years, the astronomers hoped to catch the exact opposite: a red supergiant dropping out of sight as it became a black hole.

“It was very nice and clean,” Gerke says of the 2008 event. “You could see the star there, and then you could clearly see that, at least in our data, it was no longer visible.” It is still the only time anyone had ever seen a star vanish from the heavens without going supernova (3).

Woosley, who was not involved in the discovery, calls the claim credible. Although the star could conceivably still be shining behind a thick cloud of dust, starlight should heat that dust and make it glow strongly at infrared wavelengths, which no one has seen (4). Conclusive confirmation of the death of the star awaits the James Webb Space Telescope, a large infrared-sensitive instrument that NASA plans to launch in 2021.

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

Contrary Carbon

In 2019, Tuguldur Sukhbold at Ohio State University proposed an explanation for why lower-mass red supergiants explode and higher-mass red supergiants don’t: “It’s ultimately a consequence of the way that carbon burns in a massive star,” he says (5). His work builds on the recognition a quarter century ago that carbon burns differently depending on whether a massive star was born at more than or less than a certain mass.

For most of its life, a massive star converts hydrogen into helium at its center, as the Sun does. When the hydrogen runs out, the helium ignites, creating carbon and oxygen. And when the helium runs out, the star, desperate to hold up its great weight, taps its carbon, turning it into neon, sodium, and magnesium.

But carbon comes with a catch. It burns at such a high temperature that the intense heat generates high-energy photons, which can turn into pairs of electrons and antielectrons. These usually annihilate each other and can produce neutrinos and antineutrinos, which zip out of the star, rob it of energy, and do nothing to hold it up against gravity. Because of neutrino losses, once carbon ignites, the star has no more than a few thousand years to live. Then the star burns still heavier fuels until it runs out of options. The last reactions forge iron, which is a dead end, as the star can wring no more nuclear fusion energy from this most stable of all nuclei. With nothing to support it, the core collapses.

But whether the star then explodes or implodes depends primarily on how it burned its carbon at its center, Sukhbold proposes. “The way the burning takes place changes the star’s final core structure,” he says, “and that structure has a lot to say in what happens in the end—whether the star explodes or not.” In lower-mass red supergiants, carbon burns convectively: The burning region bubbles and boils as rising and falling pockets of gas ferry heat away from the core. The convection also replenishes the central region with fresh carbon fuel, thereby prolonging this stage of the star’s evolution and causing great neutrino losses; consequently, these lower-mass red supergiants wind up with compact cores. When the cores collapse to form dense stellar objects called neutron stars, they blast off the outer layers of the star in a supernova.

In higher-mass red supergiants, however, carbon doesn’t burn convectively; this limits neutrino losses and leads to a more extended core with dense material around it. When the core collapses, the blast wave slams into the dense material above, which thwarts the explosion. Instead of creating a supernova, the star implodes, forming a black hole.

The dividing line between the two fates? A birth mass of about 19 solar masses, Sukhbold calculates—not far from Smartt’s observational determination of 17. Given uncertainties in both observation and theory, Sukhbold sees no conflict. In fact, he thinks that the true dividing line could be anywhere between 16 and 20 solar masses. Furthermore, theory says that there should be exceptions to the rule: A few stars below this mass can implode, and a few stars above this mass can explode.

This new thinking changes not only our view of the lives and deaths of massive stars but also calculations of how productive they have been in sprinkling their galaxies with new chemical elements. In massive stars, neutrons slowly convert the iron nuclei with which the star was born into heavier elements such as yttrium and zirconium. But if the stars never explode, these elements fall into the black hole, depriving the galaxies of the stars’ full chemical progeny.

With a Bang or a Whimper?

The brightest red supergiant in Earth’s sky is Betelgeuse, a stunning stellar ruby in Orion. All the other bright stars in Orion are blue. Only Betelgeuse has turned red, which means that by conventional wisdom it will be the next to explode.

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

The key determinant is the star’s birth mass. No one knows what that is for Betelgeuse, in part because the star’s distance is uncertain. That, in turn, means the star’s luminosity is uncertain, and astronomers need to know the luminosity to infer its mass. Astronomer Edward Guinan of Villanova University outside of Philadelphia, PA, who has long observed the star, puts its birth mass somewhere between eight and 18 solar masses. So Betelgeuse will probably explode as a supernova after all, in which case it will far outshine dazzling Venus in our skies. But if the star’s birth mass is near the upper end of Guinan’s estimate, around 18 suns, Betelgeuse could implode instead.

An implosion would be much less spectacular, and the failed supernova in NGC 6946 may foretell what we’d see. As that star died and became a black hole, it gently cast off its outer envelope and grew five times brighter. If Betelgeuse follows suit, its brightness will increase but never surpass that of Sirius, the brightest star in the night. Then Betelgeuse will disappear, leaving a literal hole in Orion.

Meanwhile, Kochanek’s team is seeking a second failed supernova. “This is a project best done with tenure,” he jokes. From 2008 to 2019, his team monitored 27 galaxies within 35 million light-years of Earth; in those galaxies, eight massive stars exploded as supernovae versus the one that failed.

It’s only a matter of time, he thinks, before he sees another big red star wink out and become a newborn black hole, illuminating the still mysterious lives of massive stars.