The next supernova?
We’re overdue a spectacular display of stellar fireworks, so where will it come from?
We're long overdue a spectacular display of stellar fireworks. But where will it come from?
In the near future, the full Moon will have a luminous rival as a cataclysmic event 650 light years away will momentarily shine with the brightness of 10 billion Suns. The aftereffects will hang in our sky for many months, and permanently alter our night sky in a way not witnessed in recent human history. Betelgeuse is one of our most recognisable stars. Forming part of the Orion constellation, its bright-red appearance, due to its vast size, has been noted by stargazers since antiquity.
Place Betelgeuse in the exact location of our Sun and the star’s equator would extend out to the orbit of Jupiter, with all the inner rocky planets moving along inside its fiery outer atmosphere. For the majority of people living under light-polluted city skies without a telescope, Betelgeuse is likely the largest object they will ever set their eyes on.
In recent months, however, it has been trickier to spot. The star described by the ancient Greek astronomer Ptolemy and named by medieval Islamic astronomers has started to fade from view. A notice posted in December by astronomer Edward Guinan at Villanova University described it as about half as bright as it was in September. Subsequent measurements by the same team have shown it has rapidly become the faintest, dimmest and reddest it has been in at least 40 years. It was off their charts.
“We changed the graph scale because I was plotting points in open space,” says Guinan. However, any reports of Betelgeuse’s demise are much exaggerated… or at least premature. One thing we know for certain about this supergiant star is that it won’t be going out with a whimper. In fact, our models of stellar evolution show it will end its life in a huge supernova that is due anytime in the next 100,000 years. And some have speculated Betelgeuse’s winter retreat might actually be preparation for an imminent grand finale.
When Betelgeuse does go supernova, whoever is around to see it will witness one of the greatest astronomical events of that decade, century and perhaps even millennium. If human civilisation is still going, the star will write its name in our astronomical history in a way that only a few others have. One example can be found in ancient Chinese manuscripts. These describe the appearance of a ‘guest star’ in the year 1054, an event since linked to the supernova that formed the famous Crab Nebula. Its appearance as a new light in the night sky, six-times brighter than Venus, was also noted in the Arab world, and more controversially has been linked to cave art in North America.
Fast forward 500 years and stargazers found themselves in a golden era. Tycho Brahe studied a supernova in 1572, and Johannes Kepler witnessed the last supernova seen within our own galaxy in 1604. “Two in 32 years and then none seen by humans for the next 400 years? It’s not fair,” says Saurabh Jha, a supernova physicist at Rutgers University, who also missed the closest thing we know of since, back in 1987.
Guinan was more fortunate. In the spring of that year he was on a cruise ship just off Argentina, giving astronomy talks. The light from an exploding star in the Large Magellanic Cloud, which orbits the Milky Way, began to reach Earth, and hit the headlines. While it was a challenge to see it with the naked eye from built-up urban areas, it was clearly visible in the dark skies over the southern Atlantic. Guinan was compelled to act. “I had to make up talks quickly so I could take people out on deck and look at it.”
To assess if Betelgeuse will add 2020 to the list of famous supernova years, we first need to understand their origin, and which stars produce them. While there are many types and subtypes, we can broadly divide these explosive events into two groups. Core collapse, or Type II supernovae, describe the final swansong of massive stars like Betelgeuse. These supergiants live fast and die young. They spend their 5 to 10 million years holding off the urge to collapse under their own massive gravity through rapid nuclear burning of their hydrogen fuel reserves. During their later life, once the hydrogen has been used up, increasingly larger elements are fused instead to balance the inward pressure. This act of resistance holds until the star’s core is composed of iron. The element’s refusal to fuse means gravity finally wins out.
The outer layers then rapidly collapse inwards, producing a rebounding shock wave. This blows the whole atmospheric envelope off the star in a spectacular supernova.
The other main category is Type Ia supernovae, also called thermonuclear supernovae. These occur when a white dwarf star, like the one our Sun will leave behind after it dies, is found in a binary pair. If the white dwarf is able to accrete matter from its companion, this extra mass will compress its core, triggering collapse and a supernova. While much rarer than core-collapse events, which go off roughly every 50 years in our galaxy, thermonuclear supernovae are brighter and can therefore be seen from further away.
Despite all we have observed and learned about these supergiant stars, scientists cannot say for sure when Betelgeuse will become the next supernova event, only that it will. And it is highly debatable whether the recent dimming makes 2020 any more likely than 3030, 9090 or the year 20020. One reason for doubt, says Elizabeth Stanway, an associate professor of astronomy at the University
of Warwick, is that this changeable behaviour is not out of the ordinary for supergiant stars with bubbling, cauldron-like surfaces. “Betelgeuse has huge convection cells which produce sunspots that could eat our own Sun whole. It’s fairly likely that the dimming just means it has more sunspots than normal.” Guinan generally agrees – especially as the decline in brightness appears to be bottoming out – though he also understands the renewed interest in the star. “It’s an unstable star. But it’s doing something that it has never done in the last 150 years.”
Doubts over the immediacy of Betelgeuse’s big exit leave the door open for a wider community of so-called supergiant progenitor stars to step in and make their mark on our night skies. Antares, the red eye of the constellation of Scorpius, is a little bit closer to us than Betelgeuse, and at a similar stage in its life. Mu Cephei is further away, but at double the diameter of Betelgeuse may be closer to providing a ‘guest star’ akin to Venus in our dawn and dusk skies.
Further afield, Stanway points to Eta Carinae, a binary star, which observers have witnessed throwing out its outer layers in 1837, 1887 and 1998. Eta Carinae might be one of those rare stars which could produce a gamma-ray burst or superluminous supernova, though its distance poses no threat to Earth. “They are all on the launchpad. They are all unstable,” says Guinan, whose hedging is evidence of the limitations science faces making any bolder supernovae predictions.
“In some situations our simulations provide the only insight of what the inside of a massive star may look like in the moments before its core collapses,” says Carl E. Fields, an astronomy PhD
student developing these models at Michigan State University. This is because a lot of the important physics that happens in the final stages of a star’s life takes place deep within its core. And it takes days, weeks or even years for any evidence to affect the surface. “By contrast, the shock wave of a supernova can blast its way out of the star very rapidly, so will overtake most of the indications we’d be looking for,” says Stanway.
Uncertainty hasn’t stopped speculation. Guinan recently returned from the American Astronomical Society meeting in Hawaii. Many delegates wanted to speak to him about his notices on Betelgeuse and what they thought you might see if it was about to go supernova. “Some of them thought nothing. It would just happen. And then there were a few that thought there would be dimming, and then six months later it would take off.”
In the meantime, teams continue searching for ‘warning signs’, looking back through old observation plates to find stellar sources of laterobserved supernovae. However, because these events come out of the blue, there is never an opportunity to organise detailed surveys in the lead up. This leaves astronomers with at best a single image, which alone cannot reveal unusual variability or behaviour.
Others are looking outside optical light for clues. Type II supernovae are accompanied by huge amounts of ‘ghostly’ non-interacting neutrino particles that can pass seamlessly outwards through the star’s atmosphere. These are flung into space long before we expect anything to have changed on the star’s surface, as the light struggles to make its way out of these giant stars. When supernova 1987a exploded, Earth-based instruments detected a small spike of a dozen or so neutrinos. Betelgeuse is so near you could get 20,000 or 30,000 neutrino detections, says Guinan.
“We might start detecting neutrinos weeks or even months before the supernova explosion, and
the rate would keep increasing until the explosion,” says Jha. However, as the next generation of neutrino detectors are being built, research into the plausibility of a neutrino-based supernovae early warning system has thrown up some challenges, says Fields. “If an event is detected in a single detector, little to no information about its origin will be included in the signal.” While pre-supernovae physics might have to rely on simulations for a while longer, the observational data we will get from the next nearby supernova will keep scientists busy for years and decades to come. It will be a tantalising wait.
Wide-field arrays such as the All-Sky Automated Survey for Supernovae, featuring a global network of 20 robotic telescopes, could help pinpoint the source, and may even spot the first hints of surface change. In other wavelengths you have surveys capturing high-energy cosmic rays such as IceCube, while space telescopes like Fermi, Integral and Swift can record events in X-rays and gamma rays. All of these are already contributing significantly to a rapidly growing record of supernovae events further afield. In the first three weeks of 2020 the Transient Name Server, where the astronomy community logs their observations, added 983 new events, of which 68 have been confirmed as supernovae.
At Stanway’s University of Warwick, the Gravitational-wave Optical Transient Observer project is looking out for gravitational waves that accompany large supernovae events. In January Betelgeuse hit the headlines for a second time after the detection of exactly this type of wave, coming from the direction of Orion. The link has since been dismissed, but it’s exactly what’s expected if the red supergiant enters its final phase. “It got me scared,” says Guinan, who admits to immediately running outside to check on the star.
So where will the next supernovae take place? On a universal scale, several will have gone off across the cosmos by the time you finish this sentence. In terms of our immediate galactic neighbourhood, several new events are confirmed each day, though it’s very unlikely the next one, or even the next few dozen, will make ripples beyond the astronomy community. “The most likely scenario is that the next nearby event will come from an obscure star which doesn’t even have a name outside of scientific catalogues,“says Stanway.
If we are talking about the next supernova that will light up our night skies and stop people in the street, then Betelgeuse and the progenitors monitored by Guinan are certainly good bets. He, for one, is keeping fingers crossed. “It could be tonight. It could be a hundred thousand years from now. I would prefer it to be now. I’m getting old. It would be great to see it.”