The Mystery of Betelgeuse’s Dimming Has Been Solved

Will Betelgeuse explode soon? It could explode today, in one hundred years, in one thousand years, in one hundred thousand years; it’s difficult to predict. Stars in the supergiant phase of stellar evolution go though periods of variability in their brightness. Betelgeuse is a known variable star, with variations in brightness that occur over years to decades. The last dip in brightness was rapid and strong, with Betelgeuse appearing only about half its usual brightness in the sky. Does that mean it is about to explode? Probably not, but right now, skywatchers all over the world are hoping Betelgeuse will supernova soon! And by the way, we may have been able to explain the cause of the deep dimming two years ago…. You know our Sun? It’s your typical all-home-and-family star, with no whimsy to its head. A perfectly spherical, well-delineated ball of gas, with at most a few bumps rising from the photosphere every now and then. But very little. A never-excessive, medium-sized star, as there are so many others in the Milky Way. Punctual, neat, with a brightness that has remained constant since time immemorial. And long-lived… so much so that more than five billion years have passed since it was formed, and it can be considered a middle-aged star… No longer young, but not old either, able to heat its planetary system with the same intensity for a couple of billion years yet. To be clear: an atomic furnace 1.4 million kilometers in diameter. Well, you get the picture! Now compare our star to Betelgeuse. You know it, right? The second brightest star in the constellation Orion, marking the eastern shoulder of the hunter. Its name is derived in fact from the Arabic word “bat al-jawzāʾ”, which means “the giant’s shoulder.” It is easily discernible to even the casual observer, not only because of its brightness and position in the brilliant Orion but also because of its deep reddish color. Well, behind the apparent mildness of this star that shines in the winter nights of our northern hemisphere, lies a real monster. Not a well-behaved star like our sun, but a shapeless cluster of gas that boils on the surface, projecting up to light years away jets of incandescent plasma. A blob of matter grouped without rules or measures, from the photosphere that in comparison to the quiet one of our Sun looks like a stormy sea. But what shocks more are the dimensions. The “diameter” of the monster, if it is possible to call it a diameter in the absence of spherical symmetry, is about two billion kilometers! You got it right… more than 700 times that of the Sun. If this huge, flaming red jellyfish were placed at the center of our solar system, its boundaries would reach far beyond Jupiter, encompassing at least five planets… More than a hundred thousand times brighter than the Sun, twenty times more massive… about 600 light years away… Frightening numbers. And with all that, this monster is ultimately nothing more than a youngster, born perhaps 8.5 million years ago. Not billions, but millions… Very young if compared with the Sun, which has been shining for more than five billion years, but at the same time old and moribund, having arrived at the end of its very short life cycle. How can this be possible? Well… the fact is that, unlike small-mass stars, the lives of the most massive stars are much shorter and more violent. These stars, because of their larger amount of matter must produce nuclear fusion reactions much faster to resist the enormous gravitational pull that tends to compress them. Betelgeuse is therefore going through the final stages of its existence. In an astronomically “short” time that is not possible to quantify, it will encounter a fatal supernova explosion that will disintegrate it, leaving behind only a tiny and ultra-dense neutron star. The explosion will release immense energy. Betelgeuse, observed from Earth, will reach an approximate visual magnitude of -12.4, meaning that the supernova will brighten the sky with a light brighter than the full Moon. A shower of X-rays and gamma rays will hit the Earth, but perhaps, given the distance, will not be so powerful as to penetrate the atmosphere. Instead, the shock wave will spread much more slowly and will reach the solar system only 6 million years after the explosion. Plasma particles coming from Betelgeuse will compress then our heliosphere, but they will not be able to break through to Earth: calculations indicate that they will reach a point of stagnation with the solar wind at about 2.5 astronomical units from the Sun, well beyond the Earth’s orbit. In short, the violent end of Betelgeuse will not be a danger for life on our planet. Seen from here it will be just a grand celestial spectacle and an incredible source of knowledge for astronomers in the rather distant future. For now, the red supergiant is still in a state of turmoil. Betelgeuse’s annual mass loss is also impressive, around eight orders of magnitude greater than that of the Sun. The red supergiant disperses through its stellar wind about two millionths of a solar mass per year. This value, expressed in millionths, may give the impression that it is a negligible amount, but two millionths of solar mass corresponds to 66% of the mass of our planet. It means that in only three years Betelgeuse disperses in the circumstellar space an amount of matter equal to two planets like the Earth! “Hey, guys, just a moment before we continue… BE sure to join the Insanecuriosity Channel… Click on the bell, you will help us to make products of ever-higher quality!” In short, everything about Betelgeuse is oversize compared to the typical parameters with which astronomers measure stars, which are calibrated – obviously – on the characteristics of the Sun. The currency with which such power is paid is time: the red supergiant has had and has little time at its disposal. It is consuming its enormous reserve of nuclear fuel at a frantic pace, immensely faster than that of the Sun. And all this has consequences on its brightness as well. Yes… We didn’t tell you yet, but Betelgeuse is also a semiregular variable, a particular type of pulsating variable characterized by unpredictable and often high fluctuations in brightness with a quite chaotic cyclicity: the result of the superposition of two different cycles. The most credited explanation by astronomers provides that the diameter of the star goes through a slow expansion for some years, followed by a sudden contraction of the outer layers. Changes that cause a variation in the radiant surface, in the temperature, and therefore in the light emission. In this time frame, the star oscillates without warning around its average apparent magnitude of 0.5, with luminous excursions varying from cycle to cycle. That is why sometimes Betelgeuse seems as bright as Rigel, and sometimes as weak as Bellatrix, the other “shoulder” of Orion. We were therefore already accustomed to the small decreases in the brightness of Betelgeuse (however, the most consistent variations between stars of first magnitude), but what happened to start in the fall of 2019 surprised astronomers and even many occasional observers. The red supergiant was, in fact, losing part of its brightness, but at a rate so fast that the media, warned of the thing, amplified the event even starting to claim that the star was now close to exploding and that soon it would become a supernova of unprecedented proportions. Apart from exaggerations, that sudden decrease was too intense and prolonged to be considered a normal decrease in brightness due to the intrinsic variability of the star. And the thing had long since alarmed a team of researchers who under the guidance of astronomer Miguel Montargès were set to examine the disk of the star with the Very Large Telescope of ESO in interferometer mode. Being one of the nearest supergiants, Betelgeuse was in fact the first star of which it was possible to directly observe the “surface.” And this since 1996 thanks to the Hubble Space Telescope. Montargès and collaborators observed Betelgeuse in December 2019, and a comparison with images collected in January 2019, before the marked fading, showed a significantly darker stellar surface, especially in the southern hemisphere. The team then continued to observe the star until April, when it returned to normal brightness values. So that among the four models examined during the data analysis, the most plausible explanation turned out to be the one that predicted the darkening of a portion of the star by a newly formed dust bank. Dust that would be formed from one of the giant bubbles of gas that populate the turbulent surface of the star: the ejection of the bubble would have cooled the surface below, making the gas condense into solid material or dust. In practice, between September and November 2019, there was an immense release of material in the southern hemisphere of Betelgeuse, probably coming from one of these convective cells. The ultra-high temperature gas left the star at the insane speed of over 3-400,000 km per hour, and once it reached a distance of a few million kilometers from the star, it began to cool, giving rise to a dense dust cloud that, interposing itself prospectively between Betelgeuse and Earth, originated the observed decrease in brightness by blocking light from about a quarter of the star’s surface. In April 2020 the brightness of Betelgeuse has therefore returned to normal. We are talking about a cloud about 600 million to a billion kilometers across – big enough to stretch from the Sun to Saturn! – and roughly two billion kilometers from the star would do the trick. But not alone: It needed help. What they concluded is that the cloud already existed at that distance, but was mostly gas. When the star cooled, however, the cloud also dropped in temperature, enough that the silicates in it condensed, forming dusty clumps. These are opaque to visible light (the kind we see), and that is what caused the catastrophic dimming of the star. Then, eventually, the star heated up again, the dust molecules were destroyed, and Betelgeuse brightened once again. This does explain a lot, and fits what we know. For example, huge clouds of dust were previously found surrounding the star out to great distances, so we know something like this can exist. And other studies showed the star cooled a bit during this time as well. Putting the two together, dust condensation explains things pretty well. How massive was this cloud? They find the best fit to the data is a cloud with very roughly a billionth of the mass of the Sun in dust, and about 200 times that much gas. That may not sound like much, but it’s equivalent to roughly six times the mass of the Moon. That’s a big belch. It’s important to note that all these shenanigans Betelgeuse wrought probably have nothing to do with any imminent demise. It will explode, of that there’s no doubt, but it’s unlikely to go supernova on us for a hundred thousand years or so. Massive stars explode when their cores run out of nuclear fuel, but what happens buried a half billion kilometers under the surface doesn’t affect the outer layers strongly on such short timescales. Also, stars about to blow generally lose a lot more mass than Betelgeuse currently ejects, so again this indicates it’s holding its own for now. So is all this the last word on The Great Dimming? Somehow I doubt it. There have been a lot of arguments about what happened, and even if this general scenario is correct there are a lot of details to work out. For example: What caused the dust cloud to be erupted out? And, of course: Will this happen again? For that, we’ll have to let Betelgeuse let us know. And if it does, we’ll be watching