In October 2019, the star Betelgeuse began dimming rapidly.
Even though Betelgeuse is a well-known variable star, the Great Dimming was an unprecedented
event.
By February 2020, the red supergiant lost more than two-thirds its normal brightness.
It got so faint it was easily noticeable to the unaided eye.
Then, Betelgeuse recovered, returning to its normal brightness before disappearing from
the night sky in late April.
Now, new results from the Hubble Space Telescope suggest that Betelgeuse may have ejected a
massive cloud of dust that obscured its view.
Meanwhile, NASA’s STEREO spacecraft imaged Betelgeuse over the summer of 2020, when it
wasn’t observable from Earth.
It turns out, Betelgeuse was dimming again.
Welcome back to Launch Pad, I’m Christian Ready, your friendly neighborhood astronomer.
There’s been quite a lot work done over the last few months to understand what caused
the “Great Dimming" of Betelgeuse last winter.
Betelgeuse is a red supergiant star that's near the end of its life.
It’s so large that if it were placed at the center of our solar system, its outer
layers would reach out to Jupiter’s orbit.
It’s also very massive.
We don’t know its precise mass, but it’s thought to have started out life at roughly
20 solar masses or so.
It’s been losing mass ever since and will likely end its life in a core-collapse supernova.
Exactly when and how that will happen is uncertain because we really don’t know much about
the kinds of stars that go supernova.
And that’s because such stars are rare; they make up less than 1% of the stars in
our Galaxy.
Lucky for us, Betelgeuse is only about 700 light-years away, so we’re fortunate to
have a supernova progenitor that’s close enough to be studied in detail, but still
far enough away just in case.
Because of its age and evolution, Betelgeuse is unstable.
It pulsates with a roughly 420 day period.
Meanwhile, its upper photosphere - that is, its surface - is strewn with gigantic convection
cells that bubble to several astronomical units across.
And that’s why astronomers got excited about the Great Dimming - not because we thought
Betelgeuse was about to go supernova right there and then, but because it gave us a chance
to understand something unique about supernova stars.
Fortunately, the Hubble Space Telescope just happened to be studying Betelgeuse when it
started dimming in 2019.
The Hubble observations were part of a study led by Dr. Andrea Dupree at the Harvard-Smithsonian
Center for Astrophysics.
The team’s plan was to revisit Betelgeuse multiple times over its 420 day pulsation
cycle.
Specifically, Dupree’s team used the Space Telescope Imaging Spectrograph, or STIS, to
measure the ultraviolet emission of singly-ionized magnesium in Betelgeuse’s atmosphere.
Because Betelgeuse is a relatively cool star, it doesn’t radiate very strongly in the
ultraviolet.
So, the Magnesium II emission lines really stand out.
That allowed the team to use the emission lines to take Betelgeuse’s temperature.
Each time Hubble revisited Betelgeuse, it took spectra at several different points across
the star's atmosphere, effectively measuring the temperature along the way.
As luck would have it, Hubble revisited Betelgeuse before, during, and after the Great Dimming,
and those Magnesium lines didn’t disappoint.
.... Who says that?
In October 2019, the lines brightened along with the background UV emission.
Shortly after these spectra were taken, the Great Dimming began.
When Hubble revisited Betelgeuse in the winter, the UV emission returned to its normal value.
The fact that the 2019 brightening preceded the Great Dimming is interesting, because
it wasn’t just the magnesium emission lines, but the overall UV continuum as well.
You see, the background UV radiation emerges close to Betelgeuse's “surface” (or whatever
counts as a surface for such a distended star).
The longer-wavelength Mg II h-lines form above that, and the shorter-wavelength Mg II k-lines
form highest of all in the atmosphere where it's hottest.
Now all of this is going on in the southeast region of Betelgeuse.
In 1995, Hubble measured a similar phenomenon in Betelgeuse's southwest region with its
first-generation spectrograph and Faint Object Camera.
They showed a noticeably brighter region in the southwest, right where the extra UV emission
was measured at the time.
So it’s likely there was a similar bright region in the southeast from September through
November 2019, though Dupree’s team only used Hubble’s spectrograph this time, so
there aren't any pictures.
In 1995, the bright area was at most 1.8 times brighter than the surrounding disk.
The 2019 brightening was 3.5 times brighter.
While Hubble was making its observations, the 1.8-meter STELLA-II telescope at the Izana
Observatory in the Canary Islands measured the doppler shift of Betelgeuse’s spectra.
It showed that Betelgeuse was expanding outward at the time Hubble observed the brightening
in the star’s atmosphere, and continued expanding as it was dimming.
So it looks the brightening in the southeast region occurred while the star was expanding,
and it's even possible that this brightening may have caused that expansion to accelerate.
There’s probably a cause and effect at work here between this brightening and the Great
Dimming.
Dupree and her team suspect that the brightening may have been due to an up flow in Betelgeuse’s
photosphere - perhaps a large convection cell - that coincided with the star’s normal
expansion.
Because Betelgeuse has these gigantic convection cells that are several AU across, it’s possible
a particularly energetic cell was amplified by star’s expansion as it bubbled upward,
heating up the southeastern region.
The plasma unleashed into the atmosphere would have produced the UV emission seen during
Hubble’s September and November visits.
Then the material would have cooled into a large cloud of dust in the atmosphere.
In this scenario, the appearance of the star in the December 26 image taken with the Very
Large Telescope would be explained by the escaping dust cloud obscuring the star's southern
hemisphere.
Over the next several weeks, the dust cloud grew more opaque, deepening the Great Dimming
in February 2020.
Meanwhile, the star and its atmosphere returned to its previous state, so as the dust cloud
dissipated, the star appeared to recover to its normal brightness.
Could this be what really happened to Betelgeuse?
It’s certainly plausible, but there are still some missing pieces.
For example, if dust were produced, it should be warmed by Betelgeuse and radiate in the
infrared and microwave.
But observations made with the SOFIA infrared telescope during the Great Dimming didn’t
show any excess radiation in the infrared.
Not only that but millimeter-wavelength observations made with the James Clerk Maxwell Telescope
in Hawaii showed that Betelgeuse was actually fainter in the microwave part of the spectrum
as well.
If anything, that’s the opposite of you’d expect to see if dust was being heated.
Neither of these observations necessarily disprove the dust model, but they leave the
door open for alternative models such as large star spots or rapidly cooling convection cells.
Then again, star spots would have to be really large and really cool to cause the Great Dimming.
That's not impossible, but observations by Emily Levesque showed that Betelgeuse didn’t
cool down by very much in February, which brings us back to dust.
If all of this sounds confusing to you, you’re in good company because Betelgeuse is bothering
a lot of us!
And it may not be done messing with our heads.
Betelgeuse is hidden in the daytime sky during the summer months, so we typically have to
wait until August when it starts to rise just before dawn.
But in the aftermath of the Great Dimming, waiting 3-4 months to observe Betelgeuse again
wasn’t an option this time.
So Dupree used the STEREO solar observatory to glimpse Betelgeuse in the summer of 2020.
Sure enough, it faded once again before leveling off at about 0.8 magnitude!
That’s interesting because Betelgeuse’s 420-day cycle isn’t expected to reach its
next minimum until April 2021.
If this marks the beginning of a new dimming event, we may be in for another show this
winter.
Now, if you’re still a little fuzzy on what happened to Betelgeuse last winter, make sure
to check out the videos I made earlier this year to get up to speed.
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Until next time, stay home, stay healthy, and stay curious, my friends.