Astronomers studying Betelgeuse suggest it's much more evolved than previously
thought and will explode in a supernova any century or possibly any decade now.
That's the finding by a team led by Dr.
Hideyuki Saio and collaborators.
They determined
that Betelgeuse may be reaching the end of carbon and burning in its core.
And that's a remarkable considering that most astronomers think Betelgeuse
is still undergoing helium fusion in its core,
and therefore shouldn't supernova for at least another 100,000 years or so.
However, Saio's paper has not been peer reviewed.
Normally I wouldn't talk about an unpublished paper, but it was uploaded
to the arXiv (”archive”) preprint server and it went viral from there.
And that led another team of astronomers to publish a rebuttal, even though
Saio’s paper hasn't been published yet.
So there's a lot to unpack here from how Saio’s team arrived
at their controversial conclusion to the criticism
and to some other really strange things about Betelgeuse
that neither side appears to be addressing.
Let's start with a little bit of background.
Betelgeuse is a red supergiant star in the northeast shoulder of Orion.
It probably started off life as a very hot 19 or 20 solar mass star.
Massive stars like Betelgeuse evolve rapidly
because they burned through their fuel much hotter than the sun does.
As a result, it consumes their fuel much faster.
So while our sun might spend its first 10 billion years of its life fusing hydrogen
and helium, Betelgeuse fused its core hydrogen in less than 10 million years.
With each stage of the star's evolution, the core gets hotter and fuses faster.
So Betelgeuse should fuse its helium
core into carbon in just 100,000 years.
Carbon is fused into neon in a couple of centuries.
Neon fuses into oxygen in a few decades.
Oxygen is fused into silicon in a couple of months,
and silicon fuses into iron in about one day.
When the core becomes iron, it ceases to produce energy and collapses.
The star's interior implodes and rebounds in a supernova explosion.
It's never a question of if Betelgeuse will go supernova, but when.
The problem is we can't actually look inside to see what's going on in the core.
But Betelgeuse is a pulsating variable star,
and we have reliable light curves going back at least a century.
The most dramatic example of its variability came during the great
dimming of 2019, when it fell to about one third its normal brightness.
By the following spring, Betelgeuse had recovered.
But then in 2023, Betelgeuse shown about 50% brighter than normal.
The hotter of the core gets,
the more the star expands, and the longer it takes to pulsate.
So in principle, we should be able to use the star's pulsation period and work
backward to estimate its size and what must be going on in the core.
But stars are complex and usually have multiple pulsations going on at once.
Betelgeuse’s light curve, suggests it has four periods of 2200 days,
420 days, and two shorter periods of 230 and 185 days.
But those periods are approximate.
The star is so distended that some parts are expanding outward,
while other parts contract inward.
So the result is an undulating mess that looks less
like a star and more like an angry cloud.
We can even see its shape change when we image it.
Not only that, but Betelgeuse has shed anywhere from 8
to 10 solar masses worth of material as it pulsates.
Most of that ejected mass
has cooled into a giant cloud of dust surrounding the star.
All of these problems make it very hard to determine things like Betelgeuse’s
diameter, its distance, and even whether any given dip in
its brightness is really due to the star pulsating
or if maybe there a cloud of foreground dust or a sunspot that's forming.
Most measurements place Betelgeuse somewhere between 500 to 700 light years
away with a physical radius between 600 and 800 hundred times the sun's.
That means if it were placed where the sun is now,
Betelgeuse’s surface would extend well into the asteroid belt.
That's big,
but that's not thought to be large enough to suggest it's burning carbon.
And that's why most astronomers think Betelgeuse is still burning helium
and shouldn't supernova for another 100,000 years.
So why do Saio and his team think Betelgeuse is really fusing carbon
and therefore could supernova within our lifetimes?
And what exactly is so controversial about that?
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Saio’s team modeled Betelgeuse's Interior based off measurements
of the star's four pulsation cycles that range from 2200 down to 185 days.
Typically, the shorter periods key off the longest period,
which is called the fundamental.
So the short periods oscillate much like overtones over a base frequency.
Saio takes the 2200 day cycle as Betelgeuse’s fundamental period.
Now that might seem obvious, but many astronomers believe that the 2200
day cycle isn't an actual pulsation period of the star.
Rather, they think the 2200 day cycle must be due to some other cyclic phenomena
generally referred to as a Long Secondary Period.
The most common example of an LSP is when the pulsating star
is part of an eclipsing binary system.
Other examples are the periodic build up of large star spots
on the red giant surface or even orbiting clouds of dust.
But Saio’s team, took the 2200 day period as Betelgeuse’s fundamental cycle.
They modeled the star to find the best fit to all four pulsation periods,
as well as its observed temperature and luminosity, and from there
figured out what the core has to be doing.
We can visualize these models on the Hertzsprung-Russell diagram,
which plots stars’ increasing luminosity versus decreasing surface temperature.
Betelgeuse is here as a very cool yet luminous M-type red supergiant.
About 10 million years ago it was a smaller, hotter, O-type
main sequence star with an initial mass of around 20 solar masses.
But as the star evolved, it zigzagged across the diagram
as it exhausted one source of fuel and ignited another.
Exactly what path Beetlejuice actually took depends on a number of factors
like its rotation, the abundance of heavy elements in its core and so on.
But the most fundamental factor is the star's starting mass.
The greater the mass, the hotter the core and the faster it evolves.
So Saio’s team modeled evolutionary tracks for stars with different initial masses.
The green tips at the end of the tracks start at the end of helium
burning and continue through the end of carbon burning.
Stars rotate, but they can't rotate too fast or else they'd rip themselves
apart.
This fly-apart velocity is called the star's critical rotation.
The reason rotation is important
is because it mixes more of the star's interior into the core.
This increases the core’s
effective mass, which increases the star's overall luminosity.
You can see the effect of rotation here in these additional models of a 19 solar
mass star that started out rotating at 20% and 10% of its critical rotation.
All of the models under 20 solar masses fuse carbon within Betelgeuse’s
error bars.
The models also seem to fit the observed brightness variations
in all four of Betelgeuse's pulsation periods as well.
Saio’s team found that the overall best fits were that of a star
that started out with a mass of 19 solar masses, initially rotating at either 20%
or 40% of its critical velocity and is now fusing carbon in the core.
So if Betelgeuse is really fusing carbon, how long will it be before it goes boom?
Well, the team modeled this as well.
This is a plot of how the abundance of elements in the core change
as it gets closer to going supernova.
The vertical axis is the mass fraction of any given element in the core
and the horizontal axis is the logarithm of the number of years left.
So the five is ten to the five or 100,000 years out.
Zero is the one year mark and the negative five
is 0.00005 year
or about 5 minutes and 16 seconds before core collapse.
The star starts off with the core of almost entirely
hydrogen and helium, with trace amounts of heavier elements.
As it fuses hydrogen, the amount of helium increases.
Starting at around a million years
out, the amount of helium decreases as it begins fusing into carbon.
At 100,000 years out, the remaining helium rapidly depletes and carbon
produces more and more oxygen, which in turn fuses to create neon.
And notice that there's quite a bit of overlap here.
These elements
don't actually fuse sequentially, but as the lighter elements are depleted
and the heavier elements fuse, the timescales get shorter and shorter.
So let's say 100,000 years for helium, hundreds of years
for carbon, and so on, until about a day for silicon.
Iron implodes so quickly that it's not even plotted on this graph.
Anyway, notice how the abundance of carbon builds up and then drops off
in a series of steps until it gets down to about the one year mark.
If Saio’s models are correct and Betelgeuse is really in its carbon
burning phase, then the amount of carbon presently
in the course should give us an idea as to how much time the star has left.
The best fitting models are listed here.
They show that Betelgeuse could have as much as 17% of its core’s
mass in the form of carbon, but it could also have as little
as 0.67% carbon.
That could mean that the carbon is still just starting to build up in the core.
And if that's the case, then sure, it would be the better
part of 100,000 years before going supernova.
But Saio seems to argue that based on Betelgeuse's observed
characteristics, it's probably closer
to the end of its carbon burning phase.
If it's at 17%, we're talking centuries.
If it's at 0.67%, we're talking any decade now.
So there you go, Betelgeuse is fusing carbon
and getting ready to check out in style, right?
Well, no, not according to the rebuttal published by Molnar, Joyce, and Leung.
Their main objection is that Saio’s model makes Betelgeuse too giant.
Saio’s models assume that Betelgeuse has 2200 day
period is a real physical pulsation of the star.
But if that's true, then Betelgeuse would have to be much larger;
somewhere in the order of 1300 solar radii.
At Betelgeuse’s distance, a star that big should have an angular diameter
of about 55 milliarcseconds on the sky.
To that end, Saio’s paper does quote several measurements that yield a similar
angular size and therefore radii between a thousand and 1500 suns.
The problem is that those measurements were made
in the mid-infrared at a wavelength of 11 microns.
Stars aren’t solid objects and different wavelengths penetrate
down to different depths.
Ideally, visible light measurements would reveal
the star's photosphere, and you could directly measure its size.
But Betelgeuse is surrounded by foreground dust.
That dust could make Betelgeuse appear too small
because it would dim the edges of the star that much more.
That's why Betelgeuse’s size is typically measured
in the near-infrared K-band at 2.19 microns.
That slightly longer wavelength is enough
to penetrate most of the dust and reveal the star.
K-band measurements place Betelgeuse has diameter at less than 45
milli arcseconds.
That's at least 10 million arcseconds smaller
than the measurements quoted by SILE.
That means that even at the largest distance estimate
of 880 light years, Beetlejuice
would still be much less than 1300 solar radii
too small to have the star pulsating every 2200 days.
Therefore, the 2200 day cycle must be a secondary period.
Maybe it's the cycle of dark spots forming in the photosphere,
or the formulation of dust, or maybe something else
that just isn't a true pulsation of the star.
The smaller Beetlejuice
still has a hot core, but just not hot enough to fuze carbon.
Hence it must be fuzing helium, in which case we're back 200,000 years to go.
But Sayo isn't the only one who thinks the 200 day period is real.
Others cite changes in Betelgeuse’s radial velocity over a similar period,
and that suggests it really is growing larger and expanding
and then smaller as it contracts over time.
As for Saio’s model, not taking dust into account, is that really a problem?
For instance, the critics argue that Betelgeuse’s Great
Dimming was caused by the star ejecting a large cloud of dust.
But others aren't so sure.
For example, in 2020 Thavisha Dharmawardena and collaborators showed
that Betelgeuse dimmed by a similar amount in the microwave spectrum as well.
Microwave penetrates dust, so any drop off in brightness
must have been due to the star really dimming, no dust required.
Now, to be clear, Betelgeuse does have a lot of dust surrounding it.
We can see it after all.
But maybe Saio’s models really just don't need dust in order to work.
And there's something else that I don't think either party fully addresses.
Betelgeuse rotates at six and a half kilometers per second at its equator.
That's really fast for a supergiant.
So where did all that extra angular momentum come from?
Well, no one knows for sure, but the best hypothesis is that Betelgeuse
may have once had a binary companion and swallowed it.
The companion's
angular momentum would have been added to Betelgeuse’s, making it spin up faster.
How would that affect Betelgeuse’s evolution?
I don't know, but as far as I can understand,
it hasn't really been fully addressed either.
All of this is to say that even though Betelgeuse
is the closest and best studied supernova progenitor, the star is a lot
stranger than we thought and we still have a lot more to learn about it.
As for sale, well, I don't know if he's right or not.
As of this filming, his paper hasn't been published yet.
But if Beetlejuice were to go Supernova, while I'm
looking at it now, I'd be inclined to give Saio the nod.
A huge thanks, as
always, to my patrons supporters for helping to keep this channel going.
Thank you so much.
And if you'd like to learn more about
Betelgeuse and its great dimming, I have some videos about that
that I invite you to check out when we're done here.
Until next time., stay curious, my friend.