Astronomers all over are waiting with bated breath to see
what Betelgeuse will do next. Is it going to start brightening again on
February 21st? Or will it continue to surprise?
What's next for this inconstant star? Michael J. Boyle
Astronomer Edward Guinan
of Villanova University has given Betelgeuse an ultimatum of sorts.
Guinan, who has closely tracked the star's brightness for the past 25
years, predicts that the supergiant will reach minimum brightness on February 21st,
plus or minus a week. In fact, Betelgeuse-watchers have noticed that
the rate of dimming has slowed in recent days which may be a sign that
an upturn is just around the corner.
This
comparison image shows the star Betelgeuse before and after its
unprecedented dimming. The observations, taken with the SPHERE
instrument on ESO’s Very Large Telescope in January and December 2019,
show how much the star has faded and how its apparent shape has changed.
New before and after photos taken by SPHERE (Spectro-Polarimetric
High-contrast Exoplanet REsearch Instrument) on the European Southern
Observatory’s Very Large Telescope (VLT) show not only how much the star has faded but also that its shape
has changed. A team led by astronomer Miguel Montargès, of KU Leuven in
Belgium, has been observing the star since December with the VLT and
released these stunning images just today (February 14th). Montargès
suspects that Betelgeuse's dramatic fading may be due either a cooling
of the surface or dust ejected by the star in our direction.
This
image, obtained with the VISIR instrument on ESO’s Very Large Telescope, shows the infrared light being emitted by the dust
surrounding Betelgeuse in December 2019. The clouds of dust, which
resemble flames in this dramatic image, are formed when the star sheds
its material back into space. The black disc obscures the star's centre
and much of its surroundings, which are very 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 has a size
close to that of Jupiter’s orbit. Credit: ESO/P. Kervella/M. Montargès et al., Acknowledgement: Eric Pantin
Dust is a great absorber of starlight, and Betelgeuse with its
powerful stellar winds produces oodles of the stuff. This dust fills an
enormous circumstellar shell that dwarfs the star itself. A massive red
supergiant like Betelgeuse possesses a relatively cool atmosphere in
which elements forged by the star combine to form the chemical compounds
that make up the dust. Astronomers have identified water, silicon monoxide, and aluminum oxide among other molecules in the star's effluent.
Amateur and professional astronomers around the planet have kept a
close watch on Betelgeuse during its dramatic "fainting" over the past
several months. For some it's more like a deathwatch. I've run into more
than a few people expecting or hoping that the famous supergiant will
explode as a supernova. Hold your horses! We'd all like to be dazzled by
a –11 magnitude supernova, I tell them, but we just don't know enough
to start circling dates on a calendar.
An
imaginary depiction of Betelgeuse should it one day explode as a
supernova. It will peak at around magnitude –11 — as bright as the
gibbous Moon! Edward Guinan
Betelgeuse has remained around magnitude 1.6 (or 1.7 by my visual
estimate) for the past couple weeks. Gazing at the star these February
nights, it's hard to believe that at peak brightness it can outshine its
fellow luminary Rigel. At the moment, Betelgeuse and its mate Bellatrix
(magnitude 1.6) are virtually equal in brightness, while Aldebaran
(0.9) in nearby Taurus overpowers the supergiant by three-quarters of a
magnitude. Guinan's photometric observations over the past week show
Betelgeuse at around 1.60 to 1.62 — the least luminous and coolest yet
measured during 25 years of photometry.
10 years of photometric data not only reveal the routine ups and downs of Betelgeuse but also the current remarkable minimum. Edward Guinan
Betelgeuse at first slowly and then steeply declined in brightness but now appears to be flattening out (right). Ed Guinan
Light variations on Betelgeuse arise in several ways: the aforementioned episodes of dust ejection; physical pulsations
that cause the star to expand and contract at regular and irregular
intervals and darkening caused by jumbo-sized starspots on the star's
surface. Guinan bases the February 21st date on the star's dominant
pulsation period of 430 days, which arrives on or about that date.
23
years of period analysis of Betelgeuse reveal that the star's
brightness varies with multiple periods but predominately over intervals
of 430 days and ~6 years. Imagine if you had five different pulse
rates! Courtesy of Edward Guinan
An analysis of Betelgeuse's light variations reveals evidence for
multiple periods of variation from as brief as around 242 days to as
long as 6.06 years. It's a splendid mess and the reason more and more
professional astronomers are scrutinizing it with every instrument they
can get their hands on.
Guinan and a team other other scientists were recently awarded time to observe the supergiant with NASA's Stratospheric Observatory for Infrared Astronomy
(SOFIA) in mid-infrared high-resolution spectroscopy. Many more efforts
are underway including but not limited to Hubble Space Telescope
near-infrared observations, 22-GHz and 15-GHz radio studies with
e-MERLIN (the enhanced Multi Element Remotely Linked
Interferometer Network), and the Arcminute Microkelvin Imager (AMI),
along with interferometric measurements (to determine the star's size
and shape) using VLTI-SPHERE and CHARA.
Your efforts count, too! Amateur astronomers like you have
contributed hundreds of recent visual, CCD, and photoelectric
observations of the star to the American Association of Variable Star Observers (AAVSO).
This
image, made with the Atacama Large Millimeter/submillimeter Array
(ALMA), shows the red supergiant Betelgeuse placed at the center of our
solar system. Some 1,400 times larger than the Sun, all the planets out
to Jupiter would orbit inside it. ESO
Betelgeuse remains in view until May, so there's lots of time for the
star to either resume its routine or confound us with more surprises.
We all have a front seat at this show. Walter Webb of the Red River Astronomy Club
in Texas wonders if NASA might be able to use the Mars Curiosity Rover
to extend observations of the star though solar conjunction from Gale
Crater. Great suggestion!
Use this photo to help you estimate the brightness of Betelgeuse. Magnitudes are show for Bellatrix and Aldebaran. Bob King
One thing is clear: Betelgeuse called out, and now we're listening
with every ounce of ingenuity we can muster. And if you're still hungry
for a supernova, have a look at SN 2020 ue in NGC 4636 in Virgo. It
still shines around magnitude 12, an easy catch in an 8-inch or larger
telescope. Click here for a finder chart and more information.
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