Over the last weekend, astronomy Twitter started noting that the red giant Betelgeuse, the
prominent shoulder of Orion was looking visibly dimmer in the sky, and I had a few people
reach out to me and ask me if it was really happening and if I knew what was going on.
This is exciting, of course, because Betelgeuse is living on borrowed time, and it could explode
as a supernova any day now.
Or, it might not detonate for another 100,000 years.
We just don’t know.
What’s Betelgeuse up to?
Is this a sign that it’s about to explode?
And what would it mean if it did?
When it comes to stars, you couldn’t have a star more different from our Sun than Betelgeuse.
It’s located about 640 light-years away from us in the constellation of Orion.
There are many stars much closer than Betelgeuse, but it’s still in the top 10 brightest stars
because it’s so large, and pumping out so much radiation.
Ethan Siegel blew my mind with the fact that if we could see all the radiation coming from
Betelgeuse, and not just the visible light, it would be the second brightest star in the
sky after our Sun.
If you dropped Betelgeuse into the Solar System, its size would engulf the asteroid belt, reaching
almost to Jupiter.
It’s one of the few stars that astronomers can actually reveal features on its surface.
Astronomers estimate that Betelgeuse has 12 to 20 times the mass of the Sun, and like
all stars with that much mass, they’re destined to live very short lives, just a few million
years before they explode as supernovae.
When Betelgeuse does explode, it’ll actually be the second brightest star in the sky, vastly
outshining everything else except for the Sun and the Moon.
In fact, some estimates say that it might even outshine even the full Moon.
Bright enough to see during the daytime, blazing away for an entire year until it finally fades
away.
Even the wreckage it leaves behind will be a stunning deep-sky object, which will continue
to enjoy for thousands of years as it fades away.
This is the Crab Nebula, the remnant from a supernova that exploded back in 1054 and
it’s roughly 6500 light-years away and has a diameter of about 11 light-years across.
The closest supernova in living memory is SN 1987a, which exploded over 30 years ago
in the Large Magellanic Cloud 170,000 light-years away.
It was visible to the unaided eye when it went off, and astronomers still watch the
expanding radiation shockwave as it expands through the cloud of debris shed by the star
as it was dying.
And don’t worry, even though Betelgeuse is closer than any supernova explosion on
record, it’s still far enough away that it doesn’t pose any risk at all to the Erth.
So what’s going on?
On December 8, 2019 the Astronomer’s Telegram reported that Betelgeuse was getting fainter.
According to astronomers from Villanova University and the American Association of Variable Star
Observers, Betelgeuse has been declining in brightness since October 2019, reaching its
modern all-time low of 1.12 magnitude of December 7th, 2019.
Betelgeuse was already known to be a variable star, changing in brightness every 420 days
+/- 15 days.
It also has a longer cycle of 5-6 years as well as a shorter cycle of 100-180 days.
But right now, it’s at its dimmest point in 25 years of continuous monitoring and 50
years of other observations.
So why is it changing in brightness?
Astronomers know a few of the mechanisms that make red supergiants variable stars.
Just like our Sun, they have granules on the surface, which are convection currents of
warmer material rising up from the interior and then releasing their heat.
Think of the bubbles you see on a pot of boiling water.
These bubbles are big.
They can be 60% the radius of the star.
Not 60% the radius of the Sun, but 60% the radius of Betelgeuse, which is almost 500x
bigger than the Sun.
Variable stars also pulsate in size and brightness.
Stars are always a balance between gravity pulling in and the light pressure pushing
out.
The interior of the star releases less radiation, gravity pulls the star inward.
Then the higher density and temperature creates more radiation, so it expands out again.
Betelgeuse is definitely dying.
It’s nearing the end of its life, and that means powerful solar winds are throwing its
material out into the Universe.
A red supergiant like Betelguese will blast out about half the mass of our Sun in powerful
solar winds.
This material is constantly hurled out into space for thousands of years, creating a bubble
of material that gets illuminated by the shockwave of radiation when it explodes.
The brightness of the star can be affected by the amount of dust that’s obscuring our
view.
There could even be new physics that astronomers don’t understand yet.
This is one of those situations where I’m probably the bearer of bad news.
There’s no reason to believe that Betelgeuse is dimming because it’s getting set to explode.
Betelgeuse is a variable star and it changes in brightness all the time.
The fact that it’s dimmed more than astronomers have seen in 50 years is exciting, and it’s
an opportunity to learn more about the star and its longer dimmer cycles.
There’ll probably be hundreds of stars that explode in the Milky Way before Betelgeuse
goes off.
I wanted to do this video to manage your expectations.
There are some pretty breathless articles out there speculating that it’s going to
explode, and you need to be very skeptical about that.
But is anyone working on a way to figure out when a supernova is about to explode?
Could we get at least a few hours warning?
I’ll talk about that in a second, but first I’d like to thank:
Noah Bagdonas Bart Segers
Julio Gonzales Cody Laeder
Colin Paterson
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Is there a way to know, with more precision when a supergiant star is going to explode?
Maybe, thanks to neutrinos.
When a star explodes as a supernova, it releases 99% of its energy in the form of neutrinos
in just 10 seconds.
These high-speed, low mass particles pass easily through almost all the mass they encounter,
but a tiny fraction are detectable in some of our largest detectors, like the 1-km Icecube
instrument in Antarctica.
As a star is in the process of exploding as a supernova, the neutrinos from the center
escape into space faster than the light does, which means that we should get a few hours
warning from the neutrinos before the light arrives.
A worldwide network of detectors called the Supernova Early Warning System is now online
to give scientists an alert when a supernova has gone off.
The supernova has got to be close.
A supernova that exploded in the galaxy M82 in 2014 didn’t result in any detections
- it was 11.4 million light-years away.
There’s a really cool idea that you could detect the neutrinos from a massive star that’s
about to go supernova by the amount of neutrinos coming from it.
In the last couple of hours, before it dies, the star is fusing heavier and heavier elements
on its way up to iron on the periodic table.
The releases a kind of neutrino that should be detectable above the background noise of
the Universe.
And in theory, detectors should be able to find stars about to go supernova to a distance
of about 100,000 light-years.
In other words, we’d get about two hours of notice of any star about to go supernova
across the Milky Way.
Betelgeuse, at only 650 light-years away would be very detectable.
Imagine a neutrino detector so powerful that it could observe stars in the sky by the neutrinos
they’re generating, and detect how they’re shifting over to new elements in their cores.
This is actually a tremendously difficult challenge because the Sun’s neutrinos overwhelm
our local environment.
But still, a man can dream.
So, is Betelguese about to explode?
Probably not.
This is an incredibly rare event, and it’s only happened a handful of times in recorded
history.
The last time a supernova went off close enough to be bright enough to see in the daytime
was Kepler’s Supernova, observed by the astronomer in 1604, and visible in the day
for a few weeks.
It’s unlikely that it’ll be us who see it go off.
And yet, it’s just a blink of the eye in the age of the Universe.
Hopefully, astronomers will still be here in the next few tens of thousands of years
when it does go off, capitalizing on the treasure trove of astronomical secrets it reveals.
But I really really want one of them to be me.
So come on Betelgeuse, explode already.
What do you think?
Let me know your thoughts in the comments.
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We talked briefly about neutrinos in this episode, but we did a whole video on them
and the enormous Icecube Observatory embedded in the ice in Antarctica.
And you can watch that here while you’re waiting for Betelgeuse to explode.