If you are familiar with the night sky and have had some good weather recently, you may have noticed that something is wrong with the constellation of Orion. Normally, Betelgeuse, the bright, orange-red star, marking the left shoulder of the giant, is only a little fainter than Rigel, the blue-white star to the bottom-right of the constellation. Rigel is the seventh brightest star in the sky at magnitude +0.1, while Betelgeuse is around magnitude +0.4 or +0.5 and normally the ninth or tenth brightest star in the sky.

Betelgeuse, though, is a variable star and, right now, has faded so much that it is not even among the twenty brightest stars in the sky. This has got the Internet bubbling with excitement at the news of this “unprecedented event”, suggesting that Betelgeuse may be about to explode [Note from the author: it is not an “unprecedented” event, but we will come to that bit later]. I have even had colleagues asking excitedly over lunch “have you heard the news about Betelgeuse?”

Why is this fade of Betelgeuse causing so much excitement? The reason is that Betelgeuse is no ordinary star. It is very massive, very large, very old and very unstable. However, there is a lot of uncertainty about how large and how massive Betelgeuse is because its distance is quite uncertain and even more uncertainty about when its end-of-life crisis will arrive.

The variations in brightness come because Betelgeuse is pulsating erratically, shedding mass as it does, trying to attain a stability that it can never achieve. Strangely, when its diameter is largest, it is faintest. The reason for this is that, when it is smallest, its surface is hottest and thus emits most visible light. When Betelgeuse expands, its surface gets cooler and emits less visible light. Each time that it expands, it loses a little of its outer layer, which gets puffed out into interstellar space. Over its lifetime, Betelgeuse has become surrounded by a huge shell of expanding gas and dust – its shed outer layers – that was observed in the infrared by Herschel (right).

So, how far away is Betelgeuse?

For many years, a value of 520 light years was given for the distance, putting Betelgeuse just slightly out of range of the traditional method of calculating stellar distances by measuring the parallax by telescope from the surface of the Earth. It is true that, in the 1990s, ESA’s Hipparcos mission measured the parallax of Betelgeuse from space, giving a rather smaller distance of 430 light years, but with an error of around 20%, meaning that there was a one-in-three chance that the true distance was not even in the range between 350 and 510 light years. This, in turn, leads to an uncertainty of a factor of more than two in the calculated luminosity of the star, which causes uncertainty in its estimated mass, which causes uncertainty in its future evolution… and so on. A later re-working of the Hipparcos data gave a best value of 520 light years, with a range from 450-590.

Other attempts have been made to measure the parallax and thus the distance of Betelgeuse with big radio observatories. One such attempt was made with the Very Large Array (VLA), giving a best estimate of the distance of 640 light years and a likely range from 500-790 light years. While, another, combining the telescopes of ALMA, in the Atacama Desert of Chile with the e-Merlin array of telescopes in the United Kingdom, gives a best estimate of the distance of 720 light years and a likely range from 570-830 light years.

In other words, modern techniques of observation can tell us no more than the fact that Betelgeuse is at a distance somewhere between 450 and 830 light years[i], meaning that the light that we see from the star now, set out from Betelgeuse that many years ago. At one end of the range, we would be seeing Betelgeuse as it was at the end of the 12^th Century. At the other, we would be seeing it as it was somewhere in the second half sixteenth century.

Why are we so anxious to know the distance to Betelgeuse?

The reason is that, if we know that if we know the distance to Betelgeuse, we can calculate exactly how luminous it is and that tells us how massive the star is. We know that it is a supergiant star in both senses of the word: it is both much larger and much more massive than the Sun. In fact, Betelgeuse is one of the few stars that can (just about) be resolved by telescopes on Earth as a tiny disk. Of all the stars in the sky, there is just one (excluding the Sun), a much fainter star in the deep south of the sky, called R Doradus, which appears larger as seen from Earth. Not only can we see the star as a disk and not as a point of light, but we can even see some detail on that disk (left). Betelgeuse has a diameter about nine hundred times larger than our Sun. If Betelgeuse were placed in the centre of the Solar System, instead of the Sun, some estimates of its diameter imply that even Jupiter would be inside its glowing surface, known as the photosphere.

And the mass of Betelgeuse? Why is that so important?

Estimates of the mass of Betelgeuse range from about eight times the mass of the Sun to over twenty times. This is very exciting because it means that Betelgeuse is one of the most massive stars in the Galaxy and puts it in the mass range in which stellar death becomes a spectacular affair. Because it is so massive, Betelgeuse is unlikely to suffer a quiet death as a white dwarf star, as the companion of Sirius did. In 1930, the Indian astrophysicist, Subrahmanyan Chandrasekhar, established that a white dwarf star cannot be more than 1.4 times the mass of the Sun. If the remnant of a star is more than 1.4 times the mass of the Sun, the force that stops a white dwarf from collapsing further – known as electron degeneracy, due to the resistance of electrons to being crushed together – will be overwhelmed by gravity and the star will continue to shrink, either into a neutron star, or even, if it is massive enough, into a black hole.

In the case of Betelgeuse, our best guess is that it is sufficiently massive for the core of the star to collapse when it runs out of nuclear fuel and to produce a standard supernova explosion. However, because there is a really quite wide range of estimates of mass for the star, things are not quite as clear as has been suggested by most writers.

• If Betelgeuse is at the upper limit of the mass that is estimated, it could even become a black hole, although this seems unlikely.
• If Betelgeuse is at the lowest end of the estimated range of mass, it would not even be massive enough to become a fully-fledged, “core collapse supernova”, although still massive enough to produce a lower-intensity supernova.
• Given that the best estimate its mass is only just over ten times that of the Sun, we believe that Betelgeuse will become a supernova, but one that is towards the lower end of the supernova scale[1].

Here are the potential scenarios, according to the actual mass of a star:

Mass	Fate
≤7 solar masses	White dwarf. No supernova.
7-9 solar masses	Neutron star. Electron capture (low luminosity) supernova.
9-10.4 solar masses	Neutron star. Silicon flash/silicon deflagration supernova.
>10.4 solar masses	Normal Type II supernova.

We can see that Betelgeuse is around the point at which the fate of the star can change quite substantially for just a small change in mass.

What happens to Betelgeuse next is linked to what is going on in the heart of the star. Right now, the interior of the Betelgeuse contains a series of shells, rather like a very thick onion. As we dive beneath the surface towards the heart of Betelgeuse, we find, first, a very tenuous outer layer of hydrogen and, far below it, a layer in which the temperature (around 15 million degrees Centigrade) and pressure (the gas is compressed until it is 5 times as dense as water) are great enough for hydrogen to be combined to form helium. Hotter and deeper still, we find another layer in which helium is being combined into carbon.

Probably, Betelgeuse has gone no further than this so far, although we can only argue it statistically.

At some time, though, in the future, Betelgeuse will suffer the fate of all the most massive stars and develop, if only briefly, a full, seven-layer onion structure.

When this happens, as we continue down, each layer will be even hotter and denser than the previous one. After the carbon layer, a layer will form in which carbon combines to form neon, at 600 million degrees temperature and four million times the density of water. Then, inside that, there will be one where neon combines to form oxygen and, next, a shell in which oxygen combines to form silicon at 1.5 billion degrees. Finally, we will reach the innermost two: one in which silicon combines to form iron and, in the very heart of the star, will lie a nucleus of iron. There are no nuclear reactions that emit energy from combining iron into even heavier elements: every further reaction consumes the star’s energy rather than producing more. In other words, Betelgeuse is not only passing through an end-of-life crisis, it is suffering from heart problems that will soon become mortal.

For silicon to be able to “burn” in the heart of a star – in other words, to combine silicon into iron by nuclear fusion – the minimum temperature that the centre of the star must reach is 3.1 BILLION degrees (3100000000⁰C). How does this happen? The answer is that for the centre of the star to get so much hotter, it must get very much smaller and compress the silicon until the temperature and pressure have risen enough to overcome the resistance that tries to keep the silicon nuclei apart. Time and again through the process of burning successive elements, gravity compresses the nucleus of the star and the temperature rises until the next set of nuclear reactions can start in the core, providing it with stability and a temporary respite from final collapse and death. As the nucleus compresses, the outer layers of the star expand outwards, until the pressure exerted by the radiation counterbalances the inward force of gravity, creating the red supergiant that we see.

Each stage of nuclear reactions lasts less time than the previous one. For a star 25 times the mass of the Sun (i.e. a little larger than the largest estimate for the mass of Betelgeuse), the hydrogen burning continues for 7 million years before it is exhausted, but the carbon only lasts for some 600 years and the oxygen for some six months. By the time the star reaches its final resource – burning silicon – the fuel will sustain the 25 solar mass star for just one day; Betelgeuse is not this massive, so its silicon will probably last a few days but, even so, all seven “skins” of a star like Betelgeuse are only present in the final moments of its life.

As the silicon burns, the heart of the star fills with iron “ash” that chokes the nuclear flames and ensures that the star cannot generate the energy that it requires to overcome the force of gravity. There will be a crisis, like that of a tower block undergoing a controlled demolition: suddenly, the bottom layers are unable to sustain the weight on top of them and the whole structure collapses.

This collapse lasts just a few seconds. In that short time, suddenly, huge amounts of unburnt hydrogen, helium and carbon from the outer layers meet in the centre of the star in truly unbelievable conditions of temperature and density. The star starts an incredible frenzy of nuclear reactions, leading to a massive explosion of uncontrolled energy. This is a supernova explosion and results in about 90% of the mass of the star being blasted into space. The remaining 10% is the nucleus of the star, which is at the centre of the explosion and has been compressed to quite incredible density by the impact of the collapse. Normally, in a core collapse supernova, this core forms a neutron star, an object that may be up to three times the mass of the Sun, compressed into an object just a few kilometres across. However, if the surviving core of the star is more than three times the mass of the Sun, it is too massive even to form a neutron star: such a star continues to collapse for ever, into a black hole.

How bright would supernova Betelgeuse get?

Given that it is by far the nearest star to us that could explode as a supernova, “very” is the answer. The supernova that formed the Crab Nebula, in 1054, was about 6000 light years away and, yet, it was visible in daylight. Betelgeuse is about ten times closer and would become around one hundred times as bright. So, it would, most likely, get to be about as bright as a half Moon (remember that, in a half Moon, the light is spread over a significant area of sky – in supernova Betelgeuse, it would be concentrated into a tiny, quite blinding point of light).

Supernova Betelgeuse would provide a substantial problem for astronomers. Not only would its light be as dazzling as having another Moon in the sky, it would be far too bright to be studied by normal telescopes and instruments, meaning that a lof of ingenuity would be needed by astronomers to adapt their instrumentation.

Of course, what many people want to know is, is the current fade the precursor of the death of Betelgeuse?

Sadly, no. There is nothing that suggests that it is. Estimates of how close Betelgeuse is to becoming a supernova vary from the final explosion being anything from about twenty-five thousand to one hundred thousand years away. Given that the star is less than ten million years old, we can compare Betelgeuse to an octogenarian human but, as any octogenarian will tell you, just because he sneezes one morning, does not mean that he will die that day.

When Betelgeuse finally reaches the end of its life, things will move very fast indeed, but they will not necessarily be obvious to an outside observer. Only a few days will pass from the start of silicon burning, to the supernova explosion. The collapse from an apparently normal star, into a supernova, will take just a few seconds. All we can say is that, at this moment, probably, Betelgeuse is still burning helium in the centre. If it has moved already onto carbon-burning, it is actually quite possible that the supernova explosion has already happened and that news of it is already winging its way to us at the speed of light. However, it would be a quite incredible coincidence for the supernova to happen just at the moment in its several million years of lifetime when we are watching.

However, it is hard to know what to expect if a supergiant star like Betelgeuse were about to go supernova. Only in one case do we have any real information on the behaviour of a star before it exploded[2]. When, Sn1987a appeared, on February 23^rd 1987, it was realised that the precursor star had been observed “accidentally” many times over the years and was catalogued. Put another way, it was realised rapidly that Sanduleak -69 202, a very luminous 12^th magnitude, blue supergiant star, had gone missing and that it coincided exactly with the position of the supernova. Even better, quite a lot of people had captured the rise of the supernova to maximum brightness immediately after explosion. There were even some photographs of the Large Magellanic Cloud, the satellite galaxy of the Milky Way, in which the supernova appeared, that showed Sanduleak -69 202 in the weeks before explosion, demonstrating that it had not done anything particularly unusual in that time; even the night before, the star appeared completely normal. In fact, there is nothing to indicate that Sanduleak -69 202 had behaved in any way unusually in the decades before its collapse, as it was not obviously variable in any of the photographs of the Large Magellanic Cloud taken over the previous century.

However, there was one early warning to astronomers that Sanduleak -69 202 had exploded that, next time, we may be able to use to warn us. In fact, thanks to this astronomical tip-off, we know exactly when the core of the star collapsed, although no one realised it at the time. At 07:35:35UT on 23^rd February 1987, three special telescopes in different locations around the world, detected a sharp burst of neutrinos. The burst lasted a fraction over 12 seconds, during which time, 25 neutrinos were detected. For comparison, the famous solar neutrino experiment in Homestake Gold Mine in South Dakota, detected typically 1-2 neutrinos per day so, 25 in 12 seconds really did represent a deluge. The arrival of these neutrinos marked the exact moment of the sudden death of Sanduleak -69 202. The first optical detection of the supernova was at 05:40UT on February 24th, although made on a photographic exposure that had started three hours earlier, at a time when the brightness of the supernova was increasing rapidly.

Is what is happening to Betelgeuse, really as exceptional as has been stated?

This is where we started and is the $64000 question. Have a look at this light curve for Betelgeuse (right) for the 90 years from 1911 to 2001, taken from the archive of the American Association of Variable Star Observers (AAVSO), one of the most-respected bodies of observers in the world. As Betelgeuse is one of the most-observed stars in the sky, its variations are extremely well known since the year 1922, with sporadic observations going back a further couple of centuries.

The light curve charts how this fascinating star has varied in brightness over more than a century. Twice in the 1940s and, again, as many as four times in the 1970s and 1980s, we can see that Betelgeuse actually got about as faint, or even, rather fainter than it is now. So, while rather unusual, the current minimum of Betelgeuse is, by no stretch of the imagination, exceptional.

The biggest dimming event of all was in 1948, when Betelgeuse got almost as faint as magnitude 2, which is quite considerably fainter than it is now (magnitude 1.3). Below, is its light curve from 1992 to the present. We can see that the current fade is unique in the last thirty years but, for the current minimum to rival the 1948 event, it would have to fall right down out of the bottom of the graph. There are, though, at present, no signs that it is going to get any fainter. Of course, though, Betelgeuse may yet surprise us.

What is clear is that there is no good reason to believe that Betelgeuse is heading towards an imminent supernova explosion: it is not impossible, but it is unlikely that we are watching it at the critical moment. However, just as Sn1987a was a tremendous boost for stellar evolution studies as astronomers watched a nearby supernova explosion in real time, with modern instruments, supernova Betelgeuse would be an even more stunning development for astronomy… were it to happen some time in the near future.

Compared to other red giants, the activity of Betelgeuse is very modest. The record is probably held by the star Chi Cygni, which has been known to get as bright as magnitude 3 in the past, but can be as faint as magnitude 14 at minimum. Many stars like Chi Cygni are what are called Long Period Variables and, unlike Betelgeuse, have quite periodic behaviour – Chi Cygni’s period is a fairly reliable 407 days – even if the amplitude is quite variable from cycle to cycle. We can see that the variations are quite erratic. Only two and a half years ago, Betelgeuse rivalled Rigel in brightness for a time but, since then each annual minimum has been deeper than the previous one. There is a frequent suggestion found books and in the internet that there is a period of about 5 years in the light curve, but it is far from obvious in the data and other, much shorter periods, seem more evident.

What Betelgeuse will do in the next few weeks and months is uncertain. Most likely, it will start to brighten again in the next few weeks and will be back to its normal brightness by the end of the year. However, what makes a star like Betelgeuse so interesting is that you can never be quite sure what is going to happen next. The idea that it may be in the process of exploding is just wishful thinking, however much we might like it to be true but, like most astronomers, I would be love to be proved wrong on this.

[1] To become a core-collapse supernova, a star must, as the table shows, be more than ten times the mass of the Sun (https://iopscience.iop.org/article/10.1088/0004-637X/810/1/34). The current “best guess” for the mass of Betelgeuse puts it only just over this limit so, its exact fate is uncertain.

[2] Here, we ignore the unusual star, Eta Carinae, which may also be in a pre-supernova stage. It brightened in the 1830s and ‘40s until it was almost as brilliant as Sirius, until dropping back down to well below naked-eye visibility at the end of the century. Since the 1950s, Eta Carinae has started to brighten slowly again until, now, it is, again, reasonably easy to see with the naked eye.

[i] “What about Gaia?” you ask. Sadly, Betelgeuse is about ten times too bright to be measured with Gaia, even with the special techniques for measuring the brightness of stars that saturate this remarkable mission’s CCDs. I have checked (again) with the Gaia team and their answer remains that they have no plans to observe the star, because it is far too bright to give any kind of useful data. [Post data: It turns out that Gaia has measured Betelgeuse, but only in a special observing mode that cannot analysed as standard data – processing it would have to be done manually and implies a considerable amount of effort.]