Does Betelgeuse Approaching a Crossroads?

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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?

Betelgeuse telescopic view
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.

Betelgeuse changes shape
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.

Dusty Betelgeuse
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.

Betelgeuse as a supernova
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.

Betelgeuse light curve
10 years of photometric data not only reveal the routine ups and downs of Betelgeuse but also the current remarkable minimum.
Edward Guinan
Betelgeuse slowdown
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.

The multiple pulses of Betelgeuse
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).

Stellar beast
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.

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!

Magnitude map
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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