Sky & Telescope - May 2019 - 38

Famous Stars, Part III

vide just the right amount of angular
momentum, the team discovered, to
spin up the star to 15 km/s.
But whether Betelgeuse ate a star
is speculative, and those giant eddies
regularly welling to the surface further
muddy the picture. Spectra reveal
those supergranules, as astronomers
call such massive convection cells,
have their own velocity associated with them. "You can see a
blueshift in the upwelling stuff and a redshift in the downwelling stuff," Wheeler says, indicating movement towards or
away from us, respectively. He wonders whether what Dupree
and colleagues detected was the turbulent overturn of these
eddies rather than the rotational velocity of the star itself.
Dupree wonders the same thing, and she plans to have
another look. Last year, she began assembling an international team of researchers interested in Alpha Ori. She
calls it "the MOB," for Months of Betelgeuse, denoting the
timespan over which she and about 20 colleagues around the
world hope to observe the star. Recently, she and the MOB
were awarded a three-year program to use Hubble four times
a year to obtain spatially resolved ultraviolet spectra across
Betelgeuse's extended atmosphere. The spectra will enable the

"Because it's so
close, we can ask
questions about
Betelgeuse that
we can't ask about
other stars."

FLAMETHROWER This infrared image taken by the ESO's
Very Large Telescope shows the nebula that surrounds
Betelgeuse, created by gaseous material the star continuously sheds. The tiny red circle in the center represents the
star's visible surface at about 4.5 times Earth's orbit. The
black disk corresponds to a mask used to reveal the smaller
nebula seen right around the star in an earlier observation.

M AY 2 0 1 9 * S K Y & T E L E S C O P E

team to detect the star's atmospheric
motions and possibly clarify the rotational uncertainty. That is, if she and
the MOB come up with the same 15
km/s ﬁgure, it would bolster the possibility that it is indeed the star's speed
of rotation. If the number differs
signiﬁcantly, it might be arising from
the upwellings, which astrophysicists
would expect to vary over time.
The MOB has other questions:
Where exactly is the star's pole? What
mix of hot and cold material does its
atmosphere possess, and is the hot
material coming from the supergranules? Does the mass the star loses on
a continual basis shoot out from the poles or via the convection cells? How does that mass outﬂow relate to the circumstellar material?
"It's just amazing when you think about it how many
techniques and technologies and instruments and everything have been pursuing this star," Dupree says. "We're
trying to pull it all together."

Variability
Over a season, Betelgeuse typically varies by three-toﬁve-tenths of a magnitude, but its variability itself varies.
This makes it challenging and often confusing to compare
observations secured at different epochs. Guinan has done
continuous photometry on the supergiant since 1980, working with former student Scott Wacker in the 1980s and ever

U LTR AV IO LE T IM AG E RY: A .K . D U PR E E & R .P. STE FA N IK / E AS
PUBLICATION S SERIES 2 013; N E B U L A : ESO / P. K E RV E LL A

u SPINNING STAR In this ultraviolet image
of Betelgeuse, researchers have added a wire
frame to give a sense of proposed rotation.
They think the bright spot possibly coincides
with the star's pole of rotation, though both
the pole's position and the speed of rotation
remain unconfirmed.

Sky & Telescope - May 2019

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