60 AUSTRALIAN SKY & TELESCOPE July 2019
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 per year to obtain spatially resolved ultraviolet spectra
across Betelgeuse’s extended atmosphere. The spectra will
enable the 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-to-
five-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 since with Villanova colleague Rick Wasatonic. Over
the years they’ve noted many random changes. The star once
ceased varying altogether for several months, and Guinan
wondered whether it might be getting ready to blow. The
star’s pulsation period, which normally lies between about
370 and 425 days, also varies.
Guinan and Wasatonic recently conducted a study of
the supergiant’s period going back to the 1800s to see if it
has been increasing as the star gets bigger and redder. Rigel,
Betelgeuse’s blue-white Orion mate, usually outshines it, but
between 1837 and 1840, Betelgeuse was the brighter of the
two (hence its alpha designation). It might have experienced
an outburst at that time, Guinan says, perhaps one that
created the gaseous bow shock identified about 7 arcminutes
from Alpha Ori in the direction the star is heading.Evolutionary state
Most central to figuring how close to the end Betelgeuse
might be is knowing where exactly it lies in its lifetime. But
astronomers don’t have tight constraints on that either. For
years Wheeler, with a twinkle in his eye, has told his students
that Betelgeuse could go anytime and that they should keep a
watch on it and let him know if it starts to get bright. “After
doing it for a while, I realised, wait a minute, there’s a real
scientific issue here: We really are ignorant of when it’s going
to blow up. What can we possibly do about that?”
That realisation led him, again with his University of
Texas students, to explore whether asteroseismology could help
astronomers probe the interior state of Alpha Ori (and, by
extension, other red supergiants) and thereby give clues to its
evolutionary state. Asteroseismology involves analysing the
resonant frequencies of acoustic waves that arise deep within
stars. In theory, such studies can provide insight about stellar
interiors in the same way that seismic studies reveal details of
Earth’s subsurface.
Wheeler wondered whether any signal from such waves
in a red supergiant like Betelgeuse would propagate to the
surface and be observable as short-period oscillations in the
star’s light. Using the stellar evolution code MESA (Modules
for Experiments in Stellar Astrophysics), the team found
that such waves would likely dissipate before they reached
the far-off surface, especially the closer the star got to going
supernova. “I’m still trying to maintain a little optimism
that some kind of seismological signal will reach the surface,”
Wheeler says. But at the moment, he adds with a sigh, “it’s
not looking real good.”When will it blow?
Naturally, the timing of Betelgeuse’s annihilation is the
uncertainty most nagging to everyone, from researchers to
weekend astronomers. Everyone is dying to know when it will
detonate. Guinan, who admits Alpha Ori is a pet star of his,
even has a cap with pinholes ready to place over his telescope.
“That’s for when Betelgeuse is –11,” he laughs. “I don’t know
how scientifically valid it is, but it impresses our students.”
Levesque, for her part, likes to imagine how the world
would react if the star exploded today: “It would get a
hashtag. It would get photographed everywhere. People would
get really excited about it.”
We know it’s a fast-evolving star that, in astronomical
terms, is approaching the end of its life, and that it will likely
blast apart as a Type IIP supernova. (The ‘P’ refers to a plateau
seen in the light curve of this type of supernova.) But we
have no idea just when it might do so. “It’s difficult to know,
really,” says Guinan. “People have guessed that it’s within a
million years. There’s a paper that claims half a million years.
There’s no way of knowing exactly. It could be, like, tonight.”
Dupree, too, throws up her hands. “Maybe it’s already
exploded,” she chuckles. (At the distance reported by Harper
and colleagues in 2017, Betelgeuse’s light would take moreRED STAR