Space Sunday: a look at Betelgeuse

Astro photographer Alan Dyer captured this image of Orion on December 21st, 2019. Betelgeuse (top left) appears to be the same brightness as Bellatrix (top right). Normally, Betelgeuse is the 10th brightest star in our sky and Bellatrix the 27th. Credit: A Dyer

The constellation of Orion is one of the most familiar in the night skies. It is marked by a number of notable features, containing as it does three of the brighter stars in our night sky: Rigel: the 6th brightest star visible from Earth, and serving as Orion’s left foot; Betelgeuse, the 10th brightest, and serving as Orion’s right shoulder (so diagonally opposite Rigel); Bellatrix, the 26th brightest star in our sky, and sits at Orion’s left shoulder; and three  galaxies – the Orion Nebula, the Messier 43 nebula, the Running Man Nebula – all of which can be found in Orion’s “sword”.

Orion  – or more particularly – Betelgeuse – has been occupying a lot of the astronomy-related news cycles of late, with speculation that we might be witnessing the star’s potential move towards a cataclysmic supernova event.

Before I get down to the nitty-gritty of why Betelgeuse has astronomers all a-twitter (quite literally, given the amount of Twitter chat on the subject), some details about the star. Classified a M1-2 red supergiant, Betelgeuse has a very distinctive orange-red colouration that can again be seen with the naked eye. However, it’s exact size is hard to determine, because it is both a semiregular variable star, meaning the brightens and dims on a semi-regular basis as it physically pulses in size, and because it is surrounded by a light emitting circumstellar envelope composed of matter it has ejected.

This means calculations over the years have given many different estimates of the star’s size, suggesting it is roughly 2.7 to 8.9 AU in diameter (1 AU = the average distance between the Earth and the Sun). This means that were the centre of Betelgeuse to be placed at the exact centre of the Sun, then its “surface” would be at least out amidst the asteroid belt between Mars and Jupiter, or lie somewhere between Jupiter and Saturn!

A diagram showing the approximate size of Betelgeuse compared to our solar system. Credit: unknown

That Betelgeuse is pulsating and has a cloud of material around it, also makes it difficult to pin down its precise distance from us. However, the most recent estimates suggest it is most likely around 643 light years from Earth, with a possible variation of around +/- 146 light years.

Re giant stars are of a type that have a comparatively short life, generally no more than 10 million years at the most (compared to our Sun’s anticipated 9-10 billion years lifespan), with Betelgeuse estimated to be about 8.5 million years old. This is because these stars burn through their reserves of fuel at a high speed, although a temperature lower than typically found with Sun-type stars. Eventually, they reach a point where the temperatures generated by the nuclear process is insufficient to overcome the huge gravity created by their size, and they suddenly and violently collapse, compressing to a point where the pressure is so great, they explode outwards even more violently, tearing away most of their mass in an expanding cloud of hot gas called a nebula, leaving behind a tiny, dense core – or even a black hole.

However, while this final collapse and explosion takes place suddenly, the period leading up to it can be marked by observable changes in a star – and this is the reason for the excitement around Betelgeuse.

Comparison chart showing Jupiter, Wolf 359 – a red dwarf star (often featured in this column) – Rigel, , Betelgeuse and the biggest stars so far discovered. Credit: unknown

Over the last 20+ years Betelgeuse’s radius has shrunk by 15%. While this has not massively altered the star’s brightness over that time, it is still an astonishing amount of mass to lose over so short a period. More recently, however, there has been a further change in the star that has caused excitement: since mid-October 2019, Betelgeuse has gone through a stunning drop in its apparent magnitude – or brightness as seen from Earth’s location – dropping from being the 10th brightest object in our night sky to around the 27th, bringing with it a complete change in Orion’s appearance in our skies.

This sudden drop in brightness has been seen by some as a possible indicator that Betelgeuse may have gone supernova, and we’re now waiting for the light of the actual explosion to reach us. Such has been the interest, reference has been made to monitoring neutrino detectors for the first signs of an explosion. This is because whereas photons have to escape a star’s collapse, neutrinos don’t, and so will reach us ahead of any visible light; so a sudden increase in the number of them detected coming from the region of the sky occupied by Betelgeuse could be indicative of it having exploded.

The clearest image we have of Betelgeuse, captured by the European Southern Observatory’s Atacama Large Millimetre/sub-millimetre Array (ALMA). Credit: ESO / NAOJ/NRAO / E. O’Gorman / P. Kervella.

However, taking in the history of Betelgeuse’s variability, while the current dip to around 1.6 apparent magnitude may be greater than is usually experienced (the star usually fluctuates by 0.0 and 1.3), it’s not necessarily excessive or indicative of anything unusual.

This is because the star has two cycles in its variability. The first lasts around six years from brightest back to brightest, the second around 425 days. The current period dimming coincides with both cycles overlapping at their dimmest points – something that happens roughly every 25 years. As such, the likelihood is that Betelgeuse will continue to dim for another couple of weeks before it gradually returns to its more usual brightness. However, should it continue to dim beyond this, then that could be indicative that something exceptionally unusual is going on.

But what would it mean if Betelgeuse has gone supernova?

An artist’s impression of how Betelgeuse might look in our night sky when it goes supernova. Via wikimedia

It would start with a brief, initial flash. Then, over the course of the next few weeks it would brighten tremendously, shining bright enough to become the second-brightest object in the night sky after the Moon. Were it to reach the brightest some have estimated, then it could even rival the full Moon in brightness, cast shadows under the right night-time conditions, and be visible as a bright dot in the morning and evening sky, much like Venus appears as a morning or evening “star”. It would probably remain like this for a year or so before it fades from being visible to the naked eye. But what it will leave behind for telescopes to observe would remain spectacular.

The idea that we might be about to witness the star going supernova has brought with it – unsurprisingly – claims that it will be cataclysmic to Earth, thanks to the threat of things like gamma radiation. But while it is true gamma radiation released as a result of Betelgeuse exploding will reach Earth in sufficient quantities to be detectable, the explosion would be will beyond the considered “lethal” range for supernova to threaten Earth (50 light years), it will not ionise our atmosphere or put life on the planet at any risk.

Similarly, while some of the material expelled by any supernova event, it also will not pose a threat as some have imagined. By the time it reaches the solar system, the gases will have expanded and cooled to insignificance. So in terms of it being a threat to life here, any supernova Betelgeuse might experience really isn’t.

When seen through the Hubble Space Telescope, a Betelgeuse supernova might resemble the Crab Nebula, seen here in false colours. Credit: NASA / ESA

But even without the excitement of waiting to see if Betelgeuse did go supernova several hundred years ago, it remains a fascinating subject for astronomers. For one thing, this period of dimming offers a opportunity to gather new data on the star. Also, for such a massive star, Betelgeuse is spinning much faster than should be the case for a star of its size – by a factor of 150. One theory for this is that Betelgeuse many have once had a companion star, one which it “swallowed” about 100,000 years ago.

Whether or not Betelgeuse has any companions is also a matter for continued debate / study. It is generally considered to be a “runaway” star, meaning it is moving through the galaxy and is not currently associated with any cluster or star-forming region. This both leaves its birthplace unclear and makes the existence of any companions somewhat unlikely. However, analysis of polarisation data from 1968 through 1983 has suggested Betelgeuse might have two close companions, one potentially within its chromosphere. While more recent long-term observations of the star have failed to find any evidence for these companions, they also have not actively refuted their existence, either.

A composite colour image of the Herschel PACS 70, 100, 160 micron-wavelength images of Betelgeuse showing the star surrounding by rings of material previously ejected by the star. To the left is a “wall” of interstellar dust with which Betelgeuse will collide in some 17,500 years (although the “leading edge” of the shells surrounding the will hit the “wall” in some 5,000 years. Credit: ESA / Herschel / PACS / L. Decin et al

All of which adds up to Betelgeuse being one of the more fascinating stellar members of our galaxy, and one that is likely to continue to attract popular interest for as long as the current period of dimming continues.

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