This Will Happen When Betelgeuse Goes Supernova
Betelgeuse is our next door supergiant. It is almost 1000 times bigger than our sun and possibly it has already exploded in a giant supernova. How would we notice, how fast would the process be and what could we expect to see from Earth?
Something happened in 2019
Betelgeuse is currently in the final stages of its short life. So when the red supergiant abruptly darkened in late 2019, the behavior led many to speculate that it might be about to explode. The loss of brightness was far greater than anything previously recorded.
Analyzing data from Hubble Space Telescope and other observatories, astronomers have concluded that the red supergiant quite literally blew its top in 2019 (or more precisely around the middle ages when the Black Death had swept over Europe), losing a substantial part of its visible surface and producing a gigantic Surface Mass Ejection (SME). This is something never before seen in a normal star’s behavior.
The titanic outburst was possibly caused by a convective plume, more than a million miles across, bubbling up from deep inside the star. It produced shocks and pulsations that blasted off the chunk of the photosphere leaving the star with a large cool surface area under the dust cloud that was produced by the cooling piece of photosphere.
Now, the giant didn’t blow up, but it seems that there is a shift in both brightness and rhythm. It has become unstable.
The life of a red supergiant
Every star will one day run out of fuel in its core, and thus stop being a natural source of nuclear fusion in the universe. While stars like our sun fuse hydrogen into helium and then (as they swell into a red giant) helium into carbon, there are other, more massive stars that can reach high enough temperatures to further fuse carbon into even heavier elements.
Under these intense conditions, the star will swell into a red supergiant, destined for a possible supernova in about 100 000 years or so. And the brightest red supergiant in the entire night sky will be Betelgeuse, which can go supernova at any moment.
At its relatively close distance of 640 light years from us, it could have gone supernova at any time from the 14th century onwards, and we still wouldn't know. Betelgeuse is one of the ten brightest stars in the sky in visible light, but only 13 percent of its energy output can be detected by the human eye. If we could see the entire electromagnetic spectrum — including the infrared — Betelgeuse would, from our perspective, outshine every other star in the universe except our sun.
This is Betelgeuse
Betelgeuse was the first star ever to be resolved as more than a point source. It is approximately 900–1000 times larger than our Sun and would engulf Mercury, Venus, Earth, Mars and even the asteroid belt if it were to replace our parent star. It is a pulsating star, so its diameter changes with time. The overlaid annotation shows how large the star is compared to the Solar System.
Betelgeuse is constantly losing mass, as the intense fusion reactions begin to push out the outermost, weakly held layers. Direct radio observations can actually detect this nebula of blown matter, which resembles flames emanating from the star, and have found it to extend beyond the equivalent of Neptune’s orbit.
But when we look at the night sky, we see the past. And we know that Betelgeuse, with a mass between about 12 and 20 times that of our Sun, was never meant to live very long. The expected lifetime of such a star is perhaps only about 10 million years, one thousand times shorter than the sun.
What will happen when Betelgeuse goes boom?
The more massive a star is, the faster it burns through its fuel, and Betelgeuse burns with about 100 000 times the luminosity of our Sun. It is currently in the final stages of its life, which means that when the innermost core begins to melt silicon and sulfur into iron, nickel and cobalt, the star itself will only have a few minutes left.
In the final moments, the core will be incredibly hot, but the iron, nickel and cobalt will not be able to fuse into anything heavier. It is energetically unfavorable to do so, and therefore no new radiation will be produced in the innermost regions. However, gravity is still at work and tries to pull the core of the star into itself.
Without nuclear fusion to sustain it, the core has no other options and begins to implode. The contraction causes it to heat up, become denser and achieve a pressure never seen before. And when a critical limit has been passed, it happens: the atomic nuclei in the star’s core begin a galloping fusion reaction all at once.
This is what creates a type II supernova, which is distinguished from other types of supernovae by the presence of hydrogen in their spectra. The core collapse of a supermassive star takes only a second or two and it will generate a massive outward shockwave that propagates at a speed around 10% the speed of light.
The shockwave can take a couple of hours to reach the surface of the giant star. Along the way to the surface the shockwave creates new elements that the original star could never form in its core; gold, silver, platinum, uranium and anything heavier than iron.
After the brief initial flash, Betelgeuse will brighten enormously over several weeks, rising to a maximum brightness that is itself billions of times brighter than the Sun. It will remain at maximum brightness for months, as radioactive cobalt and expanding gases cause a continuous light emission.
At a distance of only 600 light years, Betelgeuse will be much closer than any supernova ever recorded by mankind. Fortunately, it is still far enough away to pose no danger to us. Earth’s magnetic field will deflect any energetic particles that happen to come our way, and it's far enough away that the high-energy radiation that reaches us will be of low density. We’re about 10 times too far away to feel any repercussions from the explosion.
But it will shine very bright! Not only will Betelgeuse be visible during the day, but it will compete with the Moon to be the second brightest object in the sky. It will possibly be the brightest object in the night sky (except for the moon) for more than a year until it finally fades away to a fainter state. It would be one of the most spectacular cosmological events of all time, as seen from Earth.
When will it explode?
We currently have no answer to the key question "when" and it may already have happended, hundreds of years ago. It could also be that thousands of other stars in the Milky Way may go supernova before Betelgeuse does.
Until we develop an extremely powerful neutrino telescope to measure the energy spectrum of neutrinos generated by a star like Betelgeuse, hundreds of light years away, we won't know how close it is to going supernova. But we could actually get a headstart just before the explosion is about to happen.
Neutrino detectors would likely pick up the signal hours or even days before the explosion itself becomes visible. When 1987A blew up, neutrino science was in its infancy — even so, two dozen neutrinos were recorded by three detectors working at the time.
If a supernova explodes within our galaxy now, the global network of detectors — known as Supernova Early Warning System (SNEWS) — will record hundreds or even thousands of neutrinos. This small headstart could allow us to prepare and point our instruments in the right direction.
What is the ultimate fate of Betelgeuse?
Betelgeuse will end up as a neutron star or a black hole. Which one depends upon how much material remains after the supernova. If the remains are less than 2–3 solar masses in the central region, then Betelgeuse will become a neutron star and will appear as a pulsar, as seen from the Earth. If the explosion remains are more than 3 solar masses in the central region, a black hole formation is possible.
Most astronomers believe that it is more likely that Betelgeuse ends up as a neutron star rather than a black hole, but there is currently no means of predicting exactly what would be the final fate of Betelgeuse until it explodes into a supernova.
