Betelgeuse has been dramatically dimming since September of 2019.
At first, astronomers thought it would eventually return to normal brightness, but that hasn’t
happened yet.
Instead, it’s become even more faint.
As of February 2020, Betelgeuse is just 38% of its normal brightness.
And if that weren’t weird enough, new images from ESO’s Very Large Telescope show the
star as it has never appeared before.
Welcome back to Launch Pad, I’m Christian Ready, your friendly neighborhood astronomer.
And yeah, Betelgeuse is just getting more and more weird.
To recap, it’s been fading ever since it became visible once again in September 2019.
Initially, it wasn’t a huge surprise because Betelgeuse is semi-variable star that fluctuates
roughly every 420 days or so.
But starting in November, it became much fainter than in previous cycles and by the end of
December it had lost nearly half its normal brightness!
Much of the work involving Betelgeuse has been based on directly measuring its brightness
and seeing how it changes in comparison to nearby, non-variable stars.
This is a technique called photometry and nowadays it can be done by anyone with a telescope
and the right camera and software.
What’s a lot harder to do, however, is take a direct image of Betelgeuse.
Even though it’s relatively large on the sky, imaging the star requires ideal observing
conditions, ultra-sensitive detectors, and sophisticated image processing.
But in December, a team of astronomers led by Miguel Montargès of KU Leuven in Belgium
managed to image Betelgeuse, and the result is incredible.
I gotta admit, I’ve never seen anything like this!
Some news articles are claiming that Betelgeuse has physically changed its shape.
While that’s certainly possible, it's a little hard to conclude from the just the
press release image alone.
However, it is clear that one part of the star is very bright while the other is really,
really dim.
Even better, Montargès' team just happened to image Betelgeuse in January 2019.
The result is a stunning before and after comparison.
Both images were taken using the SPHERE instrument mounted on the 8-meter Very Large Telescope
in Chile.
It’s hard to overstate just how difficult these observations are.
It’s like being able to resolve a quarter from 100 km away!
I love how these images show how radically Betelgeuse has changed over the course of
just a year.
It’s clear that there’s much less light coming from it in the December image.
And that, of course, would explain the dimming.
And yet, it’s not clear why Betelgeuse appears so strange in the first place.
Like our Sun, Betelgeuse carries energy from its interior to its surface via convection.
But supergiant stars like Betelgeuse are so large, the convection is very uneven.
Remember, if Betelgeuse were placed at the center of our solar system, its outer edges
would reach Jupiter’s orbit.
So while our Sun is covered evenly with millions of small convection cells, Betelgeuse’s
distended envelope is just a handful of gigantic blobs.
It should look less like a star and more like an angry cloud.
The outer layers are so far from the star’s interior the gravitational pull on the surface
is very weak.
If enough energy gets convected up from the interior, a chunk of the surface can blow
away, forming Betelgeuse’s powerful stellar wind.
If the gas in this wind is enriched with carbon, it could condense into dust.
In fact, another team led by Pierre Kervella from the Observatory of Paris used the VISIR
instrument, also on the Very Large Telescope to reveal the dust environment surrounding
Betelgeuse.
The VISIR image was taken around the same time as the SPHERE image.
VISIR uses a disc to block out the star and its immediate surroundings, which are both
bright and must be masked to allow the fainter dust plumes to be seen.
The orange dot in the middle is the SPHERE image of Betelgeuse’s surface, which, remember,
reaches out to Jupiter’s orbit.
The dust cloud is formed by Betelgeuse’s stellar wind.
Is it possible that Betelgeuse is fading due to a clump obscuring dust?
Maybe, but that would imply it’s somehow only obscuring half the star’s surface.
Perhaps a large clump of dust was expelled from the star’s Southern Hemisphere or maybe
there's a clump of orbiting dust just happened to float past.
However, Ed Guinan of Villanova University and Rick Wasatonic have been monitoring Betelgeuse
for the last 40 years using photometry.
They were among the first to report on Betelgeuse’s fading in a pair of Astronomical telegrams
in December.
In a follow-up telegram on February 1st, Guinan reports that Betelgeuse has continued to fade
since the December image was taken.
Betelgeuse’s nominal magnitude is around +0.5 in middle of the visible part of spectrum,
called the V-band.
By the end of December it had fallen to about magnitude +1.5.
Now Guinan and Wasatonic report that the star may have faded further to magnitude +1.62.
That would make it as faint as nearby Bellatrix in the constellation Orion.
In other words, it appears that the star has lost at least 2.5 times its original brightness
since September.
However, the star doesn’t seem to be fading as rapidly as it did before, and it may in
fact be leveling off.
And, in a private communication, Guinan tells me it’s been holding relatively steady for
the last week or so, and if that's the case then it may - just may, mind you - be approaching
the end of its 420-day pulsation cycle and we may start to see it brighten again before
the end of winter.
Or it may just continue to fade and confound us even more [chuckles].
Still, despite the headlines, there isn’t any new evidence that Betelgeuse will go supernova
anytime soon.
I mean, sure, if its core becomes iron it could supernova tonight, but the reality is
that probably won’t happen for another 100,000 years...despite my wishes that it would happen
sooner...like tonight...while it's clear, and I'm outside looking at it...come on, Betelgeuse,
you owe me a solid...
I discuss why in my last video on Betelgeuse so make sure you check that out when we’re
done here.
In the meantime, make sure you out outside tonight and take a look at Betelgeuse in the
shoulder of Orion.
It’s remarkable how faint it’s become.
Now if we can just figure out why…
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Until next time, stay curious my friends.