Since October 2019, the ageing red supergiant star Betelgeuse in the constellation of Orion has attracted the attention of astronomers because it’s become significantly fainter than usual. Unusually, this has attracted lots of media attention, because some scientists have suggested this phenomenon could mean the star is about to explode.
All stars are powered by fusion, initially the ‘burning’ of hydrogen into helium. Once the supply of hydrogen runs out a star continues to shine by burning helium into heavier, denser elements, such as carbon and oxygen.
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As this process continues, the energy generated by the fusion begins to lose a long-standing battle against the star’s own gravity, drawing in material to the core.
The final point of collapse happens in a matter of seconds, leading to a massive explosion as matter rebounds off the core. The result can outshine a galaxy, but generally don’t last for long.
What’s left behind largely depends on how large the star originally was: it can be a neutron star, which is extremely dense, or even a black hole—so dense that not even light can escape from its gravity.
Tycho is an example of a supernova remnant; what’s left after a star goes supernova. Credit: Chandra X-ray Observatory, Sloan Digital Sky Survey
Astronomers believe that supernovae not only help spread heavier elements throughout the Universe, but the pressure from the explosion can even encourage new star formation elsewhere.
“Probably everything below 20 solar masses makes a neutron star,” says Dr Matt Nicholl, a Royal Astronomical Society Research Fellow who is currently splitting his time between the Universities of Birmingham and Edinburgh.
Betelgeuse, which is estimated to be around 15 solar masses, is one of the most prominent stars in the night sky, clearly visible from both the northern and southern hemispheres.
“The radius of Betelgeuse is probably about 900 times the radius of the Sun,” says Matt Nicholl about the red supergiant star, “which means that, if you stuck it in the middle of our Solar System, its surface would extend out beyond Jupiter!
“We know the distance is around 650 lightyears – plus or minus 50 – but the big uncertainty in all of this is the distance, because it’s very hard to measure distance in space.
“If we knew exactly how far away it is, we’d know almost exactly how luminous it was and therefore how massive it was.
“We’d have a better idea of when it will explode too, because if we knew the mass and luminosity more accurately, we’d also know the age more accurately.”
A view of Betelgeuse, as seen by the Atacama Large Millimeter/submillimeter Array. Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella
Much of Matt Nicholl’s research focusses on a rare class of superluminous supernovae, but he’s pretty pessimistic about Betelgeuse exploding any time soon.
“Betelgeuse is a known variable star,” he says. “It has periodic variations; it has semi-random-looking non periodic variations, and it’s probably at least three different things that cause these different variations.
“These giant stars pulse; their envelope inflates and deflates in response to things that are happening at the core—when it gets bigger it looks brighter, so that’s one thing. There’s also a lot of convection in these big envelopes, so you get hotspots that come to the surface, so also look brighter.
“They also eject a lot of material—their stellar wind is much more powerful than our Solar wind. The gas that gets away from the star can cool down and make dust which can obscure our view for a period of time.
“So all these things, and probably other things we don’t know about, cause the brightness to go up or down. I think the preferred interpretation from astronomers is that all the different processes that make it go up and down are all in the down state at the minute.”
Not that he believes dimming is a prelude to a supernova explosion.
“As these massive stars get towards the end of their lives, they’re burning fuel in the core faster and faster, and producing more heat; I don’t think there’s any expectation that it should get fainter now.”
The variability of Betelgeuse is seen in this image captured by Amanda Cross: a composite showing the stars’ apparent movement across the sky. Credit: Amanda Cross.
While Betelgeuse is certainly expected to go supernova at some point “soon”, this is a relative term.
“Our best guess is that Betelgeuse – based on what we think the distance is, and what we think the luminosity is – will have finished hydrogen burning and is now burning helium into carbon and oxygen, which takes a few hundred thousand years.”
That said, Matt would love Betelgeuse to explode during his lifetime. “It would be a star about as bright as the full Moon, so it would be pretty fantastic. The Moon is spread out over a disc; a point that bright would be really impressive.”
For Betelgeuse to explode “now”, the actual supernova would have to occur some 650 years ago, given the time needed by light to reach us.
We would, however, at least get some warning: gravitational waves and neutrinos, unlike light, escape the star immediately.
Paul Cockburn is a science writer and journalist.