Sky & Telescope - March 2021 - 19

Du
s

ll
he
ts

Chro
mo
sp
h

sphere of Betelgeuse is sandwiched between two layers the
Sun lacks: a molecular sphere (or MOLsphere) below, and a
dust shell above.
The MOLsphere is a recently discovered oddity of Betelgeuse, a region richer in gaseous molecules than the photosphere, which is how it gets its name. In our own Sun, it's
a puzzle how the chromosphere becomes hotter than the
photosphere below it. But in Betelgeuse the mystery is compounded, for the MOLsphere separating the photosphere and
chromosphere is cooler than both of them.
The dust shell is the end-product of mass loss in an RSG.
Astronomical dust is not the ﬂuff under your couch. It consists of tiny solids that have condensed from material that
has escaped from a star. Within the star, all material is gaseous, even metallic elements like aluminum and magnesium.
On the surface of a cool star like Betelgeuse, the molecules,
too, are in the form of gases. But if the atoms and molecules
can leap off and migrate to a cooler area, they then have
the chance to stick together and form grains such as silicon
monoxide (SiO) or aluminum oxide (Al2O3). These grains are
accelerated by unknown means and ﬂy out to enrich the composition of interstellar space (S&T: Dec. 2020, p. 34).

Molecular sphere

a
coron
nd
a
e
er

Photosphere

S EXTRA LAYERS Betelgeuse's atmosphere has two regions the Sun's
doesn't: a molecular layer and an outer dust shell. The perimeter of the
real dust shell is highly irregular, and the different layers flow into each
other as in a melting dessert. The diameter of the photosphere is about
the size of Jupiter's orbit around the Sun.

G REGG DINDER M A N / S&T

Witnesses for Dust
In the temperature versus dust debate over the cause of the
Great Dimming, Emily Levesque (University of Washington) and Philip Massey (Lowell Observatory) staked the ﬁrst
claim. Using a spectrograph on the 4.3-meter Lowell Discovery Telescope, they determined that the surface temperature
in February 2020 was not much lower than it had been in
March 2004, when the star was of normal brightness.
Earthbound astronomers cannot, of course, stick a thermometer into a distant star - the measurement was indirect.
Levesque and Massey examined absorption features caused by
titanium oxide (TiO) in Betelgeuse's atmosphere. TiO molecules start forming at about 3800K, and they become more
prevalent as the star gets cooler, absorbing ever more light.
The researchers didn't see big changes in the TiO lines compared to 2004, suggesting that the temperature had changed
little since then.
Professional astronomers also found evidence in support
of dust by studying polarized light coming from the envelope
around Betelgeuse. When sunlight reﬂects off the surface of
a lake or the hood of your car, it becomes partially polarized,
with the light waves vibrating at only certain angles. This is
why polarized sunglasses are so helpful when driving or boating on a sunny day. Starlight reﬂecting off dust also becomes
partially polarized. A team led by Boris Safonov (Lomosonov
Moscow State University, Russia) imaged Betelgeuse with an
instrument that picks out polarized light from an unpolarized background - sort of like sunglasses in reverse. Their
data showed a big increase in polarized light as Betelgeuse
recovered from its minimum. The interpretation was that,
ﬁrst, dense dust had formed, blocking light and causing the
star to fade. Then, as the grains spread out enough for light

to pass, a fresh wave of polarized reﬂections emerged from a
newly enriched dust zone.
Observations suggest an ejection of material happened
around the same time, which might be a source of dust.
Betelgeuse has been shedding mass for eons, as we can
see from the great cloud of dust that surrounds it. Dupree
and others discovered signs of a recent expulsion by using
Hubble's Space Telescope Imaging Spectrograph (STIS) to take
ultraviolet spectroscopy of the chromosphere. The Hubble
team scanned across Betelgeuse with the STIS, sampling narrow rectangles to build a kind of map of the star in UV. The
spectra indicate that the southeast chromosphere experienced
a shock throughout its levels during the fall of 2019. This is
consistent with a burst of material passing through the chromosphere, material that could have condensed upon emerging and cooling.
But the ultraviolet data cannot reveal fresh dust, if any
actually formed. And since we don't know how fast the material was moving through the chromosphere, we don't know
when it would have reached a zone cool enough for condensation to occur.
Research is now turning toward rogue cells: unusually
strong upwellings that could have the energy to push material
off the photosphere. Rather than create a uniform circumstellar envelope, the result would be a kind of Swiss cheese,
with pockets of new, hot gas injected into a generally cooler
environment. The chromosphere does, in fact, appear to have
regions of high temperature that occupy only a fraction of
that layer. Dupree speculates that Hubble caught an outﬂow
caused by a particularly powerful cell surfacing just as an
outward pulsation peaked.
sk yandtelescope.org * MARCH 2 021

http://www.skyandtelescope.org

Sky & Telescope - March 2021

Table of Contents for the Digital Edition of Sky & Telescope - March 2021