Imagine that you’re an astronomer in the early years of the 17th century. The telescope hasn’t yet been invented, so you scan the night sky only with the unaided eye. Then one day you see a remarkable sight: A bright new star appears, and for the next few weeks it outshines even the planet Venus. It’s so bright it can even be seen in broad daylight. It lingers in the sky for many months, gradually dimming over time.
That’s what the German astronomer Johannes Kepler saw in 1604; skywatchers elsewhere in Europe, the Middle East and Asia saw it too. We now know it wasn’t really a new star but rather a supernova explosion—an enormous blast that happens when certain stars reach the ends of their lives.
The 1604 event was the last time that a supernova appeared within our Milky Way galaxy. Or at least, the last one known to have been observed; it’s possible that there have been other nearby supernovas in the interim, likely obscured by intervening gas and dust. Astronomers can also view the remains of long-ago supernovas, such as the crab nebula, whose light first reached Earth in 1054. The next best thing to Kepler’s supernova in recent years was the supernova sighted in the Large Magellanic Cloud, a small companion galaxy of the Milky Way, in 1987 (and designated 1987A). Astronomers have also recorded many supernovas in other galaxies; these are visible telescopically but would have been entirely missed by skywatchers back in Kepler’s day.
In other words, it’s been a long wait—418 years since we’ve seen a star explode in our galaxy. So are we overdue for a bright, nearby supernova?
“That’s one of my favorite topics, over a beer,” says Brian Fields, an astronomer at the University of Illinois in Urbana-Champaign. Astronomers estimate that, on average, between one and three stars ought to explode in our galaxy every century. So a gap of four centuries is a bit more than one might expect. “Statistically, you can’t say that we’re overdue—but, informally, we all say that we’re overdue,” Fields says.
Today’s astronomers are much better prepared for the next supernova than Kepler would have been—or than anyone would have been just a few decades ago. Today’s scientists are equipped with telescopes that record visible light. These instruments will show what a supernova would look like if we could fly close to it and look at it with our own eyes. But we also have telescopes that can record infrared light—light whose colors lie beyond the red end of the visible spectrum. With its longer wavelengths, infrared light can pass more easily through gas and dust than visible light, revealing targets that may be impossible to see with traditional telescopes. The James Webb Space Telescope, for example, records primarily in the infrared. Both visible and infrared light are part of the “electromagnetic spectrum,” but supernovas also emit a different kind of radiation, in the form of subatomic particles called neutrinos—and today we have detectors to snare them, too. As well, astronomers now have detectors that can record subtle ripples in the fabric of spacetime known as gravitational waves, which are also believed to be unleashed by exploding stars.
“The real anticipation now is that we’ll have the trifecta—electromagnetic waves, gravitational waves and neutrinos—from a supernova explosion,” says Ray Jayawardhana, an astronomer at Cornell University. “That would be an incredibly rich source of information and insights.”
Scientists have described two distinct types of supernovas. In a Type I supernova, a white dwarf star pulls material off a companion star until a runaway nuclear reaction ignites; the white dwarf is blown apart, sending debris hurtling through space. Kepler’s was a Type I. In a Type II supernova, sometimes called a core-collapse supernova, a star exhausts its nuclear fuel supply and collapses under its own gravity; the collapse then “bounces,” triggering an explosion.
Either type of supernova can be so bright as to briefly outshine an entire galaxy. But Type II supernovas are particularly interesting because they release not only light but also enormous numbers of neutrinos. In fact, the emission of neutrinos can start a little bit ahead of the explosion itself, explains Kate Scholberg, an astronomer at Duke University.
“If the star is close enough, we actually might be able to observe some of these early pre-supernova neutrinos before the core-collapse actually happens,” says Scholberg. For example, if the red giant star Betelgeuse were to go supernova, neutrino detectors would likely pick up the signal hours or even days before the explosion itself became visible, she says. (Betelgeuse has been fluctuating in brightness in recent years, and some astronomers suggested it was on the verge of blowing up, but more recent studies suggest the dimming was caused either by clouds of dust or by sunspot activity on the star’s surface. Nonetheless, the giant star is expected to blow up sometime in the next 100,000 years.)
If neutrinos from a galactic supernova reach the Earth, astronomers will receive an automatic alert sent out by an array of neutrino detectors known as the Supernova Early Warning System, or SNEWS. Scholberg helped develop the first version of SNEWS in the early 2000s; today astronomers are ramping up “SNEWS 2.0” which will serve the same function as its predecessor but with improved triangulation ability, The network will use data from seven different detectors—located in six different countries plus Antarctica—to determine the supernova’s approximate direction in the sky, so that optical instruments can take a closer look.
When 1987A blew up, neutrino science was in its infancy—even so, two dozen neutrinos were recorded by three detectors working at the time. If a supernova explodes within our galaxy now, the global network of detectors will record hundreds or even thousands of neutrinos.
One particular case could produce an especially provocative signal: If a collapsing star is heavy enough, it could form a black hole—in which case “the whole explosion fizzles out,” says Scholberg. In that scenario, “the neutrino flux would turn off very rapidly. That would be really cool, because you would actually see this very sharp cutoff, which would indicate that a black hole had formed.” Astronomers could then look through catalogues of known stars to see which one had gone missing. “If you see a blank—a missing star—that could be the site of a newly-formed black hole,” Scholberg says.
Completing the trifecta would be the successful detection of gravitational waves from a galactic supernova. Predicted by Einstein more than a century ago, gravitational waves are distortions in spacetime that are created whenever a massive body is accelerated. They were first detected in 2015. The gravitational waves recorded so far were released by the mergers of massive objects such as black holes and neutron stars. But when a supernova eventually happens in our galaxy, that, too, should be detectable. Because gravitational waves would emanate from the core of a supernova, “they’ll give us information about how stars actually explode—which has so far eluded the astronomy community,” says David Radice, an astrophysicist at Penn State University. Although astronomers have been using computers simulations to model supernova explosions for decades, many of the details are still poorly understood. Data from gravitational waves could help illuminate the process, Radice says.
Could a nearby supernova pose a threat to life on Earth? Yes, in theory—but the blast would have to be very close, and at the moment no such nearby stars are at risk of exploding. Which is a good thing, because the blast of radiation from a nearby supernova would be devastating. Over a period of weeks, the supernova would emit ultraviolet rays, X-rays and gamma rays, which wouldn’t necessarily reach the ground, but would still wreak havoc on the Earth’s protective ozone layer, explains Fields. “So it wouldn’t turn us into the Hulk—but it would strip the ozone layer off the stratosphere,” he says. Without the ozone layer, the Earth would be awash in deadly ultraviolet radiation from the sun; this could wipe out phytoplankton in the oceans, with the effects working their way up the food chain, possibly leading to a mass extinction, Fields says.
Such an event may have happened over the course of our planet’s history. Fields and his colleagues have argued that a mass extinction at the end of the Devonian period, some 360 million years ago, may have been supernova-induced: They note that rocks from that period contain plant spores that appear sunburnt—as though blasted by ultraviolet radiation.
But supernovas don’t just destroy; they also create. Astronomers and physicists point out that many of the heavy elements that we depend on—the oxygen we breathe, the calcium in our bones, the iron on our blood—originated in the nuclear reactions that unfold deep within exploding stars, and which spread through space thanks to the blast waves they produce. As Carl Sagan famously put it, “we’re made of star stuff.” Which means that for astronomers like Fields, a supernova would the ultimate gift from the heavens. “I would love there to be a galactic Milky Way supernova in my lifetime,” he says.