When Will Betelgeuse Explode?
Betelgeuse will not go quietly. It may not go soon, either.
One day, they say, Betelgeuse will explode. It promises to be spectacular: the star will detonate with the force of ten million suns; burning bright enough to be visible across half the known universe. From Earth it will appear to glow more intensely than the full moon, luminous enough to cast shadows at night and to shine through the midday sun.
Of course, such things have happened before. Roughly twice a century, astronomers calculate, a star explodes somewhere in our galaxy. But most do so far away; and so their light is obscured by clouds of dust and gas, and they are thus rendered invisible to the human eye. Historical records speak of just four visible supernovae in the past millennium, the last of which erupted in 1604.
Few people, then, are lucky enough to see one. Even fewer will see one as bright as that promised by Betelgeuse, of which only one in recorded history, that of the year 1006, comes close. Records speak of a new star appearing at the end of March that year; burning strongly enough to be seen during the day and at night casting shadows on the ground. For months it shone, appearing brighter and larger than even Venus.
In those days observers knew little of what caused such “guest” stars to suddenly appear in the sky. In China, astrologers interpreted it as a sign of prosperity, reporting it to the emperor as an auspicious omen. Others saw it as a portent of famine and plague, a warning of a coming disaster.
Nine centuries later, with the advantages offered by modern telescopes, a pair of America astronomers found the resting place of that long ago supernova. It lies seven thousand light years from Earth, marked by an expanding shell of glowing gas. At the heart of it should be a remnant - either a black hole or a neutron star - formed in the heat of the supernova, yet so far, at least, nothing has been spotted.
Betelgeuse, at a distance of some five hundred light years, is ten times closer than the 1006 supernova. It lies close enough, indeed, that some commentators worry its demise may have dramatic effects on our planet. Shockwaves from such a nearby supernova could slam into the solar system; radiation from it could strip the ozone layer and poison the atmosphere.
This nightmare scenario is unlikely. Betelgeuse is close, but not that close. A supernova would need to detonate within a hundred light years or so to be a real danger; and at present no known star presents such a risk. For the foreseeable future civilization is safe from the threat of an exploding star.
The supernova of Betelgeuse, then, promises to be a spectacular event, one that humanity can enjoy without fear of famine or plague. When it erupts, it will shine far more brightly than the event of 1006; more brightly, in all probability, than any other supernova since the dawn of civilization. The only remaining question is when, exactly, it will blow.
I. A Changing Star
Today Betelgeuse appears as a red star shining brightly in the corner of Orion. Ancient Chinese astronomers, however, described the star as yellow, contrasting it with the red of Antares. The Romans spoke of it likewise, comparing its hue to that of Saturn. Sometime around the first century its tone started to shift: Ptolemy, writing in the mid-second century, called it a tawny orange. By the Middle Ages, when observations became more systematic, its colour had become as firmly red as Antares.
This shift hints at a possible transformation in the nature of Betelgeuse. Many supergiant stars start out yellow, but gradually fade to red as they cool. This transition can happen quickly, taking place over a few thousand years at most. If this is what happened to Betelgeuse then it suggests it is a young supergiant star - and, in all likelihood, one that has a while yet to live.
Still, Betelgeuse has not only changed in colour, but also in brightness. Though it has almost always appeared as one of the brightest stars in the sky, its ranking among those stars has risen and fallen over time. Betelgeuse, it seems, fades and brightens in the sky; waxing and waning in strength over the years.
Over the past two centuries modern telescopes have uncovered a scientific reason for this. Betelgeuse is a pulsating star, one that shrinks and expands roughly once every four hundred days. That, in turn, creates a variation in its brightness; albeit one that appears to be unsteadily laid over various longer term cycles.
Recently, however, that cycle seems to have broken down. Early in 2019, Betelgeuse began to dim, just as researchers had expected. But instead of later beginning to brighten again, it kept getting darker and darker. By the end of the year, the star was fainter than it had been in at least a century, and so had lost its place among the twenty brightest stars.
This behaviour baffled scientists, and led to speculation that a supernova was imminent. Could it be, some wondered, the sign of an unexpected disturbance rippling through the star, an indicator of dramatic changes to come?
It was not to be. Within weeks the star was brightening again, and by August 2020 it had apparently returned to normal. No supernova followed - at least not yet - and the star has not dimmed so dramatically again. Whatever had happened to the star, it seemed to be a temporary hiccup; a blip that had soon passed.
In the end it was a weather satellite that provided the answers. By chance the Japanese Himawari-8 satellite had caught images of Betelgeuse stretching over a period of several months. For astronomers this was a stroke of good luck; offering them a chance to see exactly how the star had changed in the weeks leading up to the Great Dimming of 2019.
Betelgeuse, they found, had blown out a vast cloud of stellar material. As this moved away from the star and cooled it formed a dust cloud, blocking much of the light coming from the star. From Earth, we saw this as an unexpected darkening of the star. Later, as the cloud dissipated, Betelgeuse seemed to brighten once more.
Exactly why this happened is still mysterious. Stars, even those close to death, do not normally belch out vast clouds of material. Indeed, the event seems to have upset the internal rhythms of Betelgeuse: ever since, the star has ceased following its four hundred day cycle, instead pulsating at a much faster rate.
II. The Bubbling Cauldron
Like all stars, Betelgeuse needs to pull off a balancing act to survive. On one side is gravity; which seeks to squash the star into an ever smaller volume. On the other is radiation pressure; an outwards force generated by the nuclear furnace burning at the heart of every star.
In stars like our sun these two forces stay in perfect balance for billions of years, resulting in a more or less calm star that outputs a steady flow of energy. In larger stars, however, the force of gravity grows stronger, and to stay in balance the star must burn its nuclear furnace hotter and faster.
That means big stars tend to burn through their fuel reserves quickly. Betelgeuse, which is at least sixteen times more massive than the Sun, probably exhausted its original supply of hydrogen after only ten million years. At that point it would have switched to burn helium, the next lightest element after hydrogen.
In doing so, however, the star would have changed. As helium needs high pressures and temperatures to fuse, Betelgeuse’s core would have contracted. At the same time its outer layers would have started to cool, expanding outwards and changing colour from yellow to red.
It is possible, indeed, that this transition happened fairly recently - and could explain why the ancients saw Betelgeuse as a yellow star. Other signs, including the balance of elements in its outer layers, also point to a recent transition. Most likely, in other words, Betelgeuse only became a red supergiant in the past few thousand years.
That is bad news for those hoping to see it explode. Stars like Betelgeuse can spend hundreds of thousands of years burning helium, a period that though not exactly stable, is also unlikely to trigger a supernova.
Indeed, none of Betelgeuse’s recent behaviour particularly points to an imminent supernova. Supergiant stars often fluctuate in brightness, a phenomenon caused by imbalances between gravity and radiation pressure. True, such imbalances do eventually result in a supernova - but they would have to get much stronger before that is a realistic prospect.
Even the cloud of dust it threw out in 2019 may not be that unusual. We know supergiant stars sometimes expel vast plumes of material. We know, too, that the surface of Betelgeuse is a boiling, bubbling cauldron of gas; one from which material may occasionally burst outwards. The subsequent shift in its fluctuations may just be a reaction to that disturbance. Over time, indeed, it will probably settle down into a regular cycle of brightening and darkening once more.
III. When the Reaper Knocks
Still, the star will eventually die. At some point it will run short of helium, switch to burning carbon and then - in no more than a few thousand years - it will finally be unable to resist the inexorable pull of gravity.
The end will be quick. As the internal radiation pressure falls away, the core of the star will implode at incredible speed, crushing its atoms into a sphere of pure neutrons. When the outer layers of the core crash against this sphere they will rebound outwards; triggering a shockwave that blows the star apart.
Many astronomers, based on the seeming age of Betelgeuse, reckon this fate is still some way off. Estimates vary, but most put it at least one hundred thousand years in the future. Yet one intriguing study, published earlier this year, thinks it could come a whole lot sooner - perhaps, they say, as early as a few decades from now.
They reached this conclusion by examining Betelgeuse’s cycles of brightening and darkening. The most prominent cycle is that which takes place over roughly four hundred days. But other cycles also seem to exist, together creating more complex fluctuations in the star’s brightness.
These cycles, they say, can be explained if Betelgeuse is already burning carbon. That would suggest the star is close to running out of fuel altogether - and, therefore, we should expect it to explode within the next three or four centuries.
Other astronomers have criticised their methods, pointing out possible flaws in their calculations. Most still think Betelgeuse is burning helium, not carbon, and that it will continue to do so for tens of thousands of years to come. That, based on the available evidence, is the most reasonable conclusion to reach.
Still, even if Betelgeuse continues to shine serenely on, sooner or later another nearby supernova will light up the sky. These things, after all, do happen from time to time - even if not all can be as spectacular as Betelgeuse promises to be.