Betelgeuse's Mysterious Dimming Solved. It was… Dust

At the beginning of 2020, the red giant star Betelgeuse started to dim significantly. Betelgeuse has been known to vary in brightness, but this one was unusual. It grew much dimmer than usual, and for a longer period. Since Betelgeuse is a star at the end of its life, it led some to speculate that perhaps it would go supernova. Astronomers didn’t think that was likely, and of course, Betelgeuse didn’t explode, and gradually its usual brightness returned. But astronomers were puzzled as to why Betelgeuse grew so dim.

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A New Study Says That Betelgeuse Won’t Be Exploding Any Time Soon

I have stood under Orion The Hunter on clear evenings willing its star Betelgeuse to explode. “C’mon, blow up!” In late 2019, Betelgeuse experienced an unprecedented dimming event dropping 1.6 magnitude to 1/3 its max brightness. Astronomers wondered – was this dimming precursor to supernova? How cosmically wonderful it would be to witness the moment Betelgeuse explodes. The star ripping apart in a blaze of light scattering the seeds of planets, moons, and possibly life throughout the Universe. Creative cataclysm.

Only about ten supernova have been seen with the naked eye in all recorded history. Now we can revisit ancient astronomical records with telescopes to discover supernova remnants like the brilliant SN 1006 (witnessed in 1006AD) whose explosion created one of the brightest objects ever seen in the sky. Unfortunately, latest research suggests we all might be waiting another 100,000 years for Betelgeuse to pop. However, studying this recent dimming event gleaned new information about Betelgeuse which may help us better understand stars in a pre-supernova state.

This comparison image shows Betelgeuse, before and after its unprecedented dimming
ESO / M. Montargès et al.
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Past Supernovae Could be Written Into Tree Rings

When stars reach the end of their lifespan, they undergo gravitational collapse at their cores. The type of explosion that results is one of the most awesome astronomical events imaginable and (on rare occasions) can even be seen with the naked eye. The last time this occurred was in 1604 when a Type Ia supernova took place over 20,000 light-years away – commonly-known as Kepler’s Supernova (aka. SN1604)

Given the massive amounts of radiation they release, past supernovae are believed to have played a role in the evolution of our planet and terrestrial life. According to new research by CU Boulder geoscientist Robert Brakenridge, these same supernovae may have left traces in our planet’s biology and geology. These findings could have implications given fears that Betelgeuse might be on the verge of going supernova.

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Wow, Betelgeuse Might Be 25% Closer than Previously Believed

In the last year, Betelgeuse has experienced two episodes of dimming. Normally, it’s one of the ten brightest stars in the sky, and astrophysicists and astronomers got busy trying to understand what was happening with the red supergiant. Different research came up with some possible answers: Enormous starspots, a build-up of dust, pre-supernova convulsions.

Now a new study is introducing another wrinkle into our understanding of Betelgeuse. The authors say that Betelgeuse is both smaller and closer than previously thought.

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Betelgeuse Is Brightening Again

The latest observations of Betelgeuse show that the star is now beginning to slowly brighten. No supernova today! Nothing to see, better luck next time.

Despite some of the hype, this behavior is exactly what astronomers expected. Betelgeuse is a very different star from our Sun. While our Sun is a main-sequence star in its prime of life, Betelgeuse is a red giant star on the verge of death. But the death of a star is not a simple process.

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A Mysterious Burst of Gravitational Waves Came From a Region Near Betelgeuse. But There’s Probably No Connection

Gravitational waves are caused by calamitous events in the Universe. Neutron stars that finally merge after circling each other for a long time can create them, and so can two black holes that collide with each other. But sometimes there’s a burst of gravitational waves that doesn’t have a clear cause.

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Waiting for Betelgeuse: What’s Up with the Tempestuous Star?

Betelgeuse

Have you noticed that Orion the Hunter—one of the most iconic and familiar of the wintertime constellations—is looking a little… different as of late? The culprit is its upper shoulder star Alpha Orionis, aka Betelgeuse, which is looking markedly faint, the faintest it has been for the 21st century.

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Amazing New Views of Betelgeuse Courtesy of ALMA

This orange blob is the nearby star Betelgeuse, as imaged recently by the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA/ESO/NRAO

Just. Wow.

An angry monster lurks in the shoulder of the Hunter. We’re talking about the red giant star Betelgeuse, also known as Alpha Orionis in the constellation Orion. Recently, the Atacama Large Millimeter Array (ALMA) gave us an amazing view of Betelgeuse, one of the very few stars that is large enough to be resolved as anything more than a point of light.

Located 650 light years distant, Betelgeuse is destined to live fast, and die young. The star is only eight million years old – young as stars go. Consider, for instance, our own Sun, which has been shining as a Main Sequence star for more than 500 times longer at 4.6 billion years – and already, the star is destined to go supernova at anytime in the next few thousand years or so, again, in a cosmic blink of an eye.

Still lumpy… Betelgeuse imaged by Hubble in 1996. Hubble/ESA/STScI

An estimated 12 times as massive as Sol, Betelgeuse is perhaps a staggering 6 AU or half a billion miles in diameter; plop it down in the center of our solar system, and the star might extend out past the orbit of Jupiter.

As with many astronomical images, the wow factor comes from knowing just what you’re seeing. The orange blob in the image is the hot roiling chromosphere of Betelgeuse, as viewed via ALMA at sub-millimeter wavelengths. Though massive, the star only appears 50 milliarcseconds across as seen from the Earth. To give you some idea just how small a milliarcsecond is, there’s a thousand of them in an arc second, and 60 arc seconds in an arc minute. The average Full Moon is 30 arc minutes across, or 1.8 million milliarcseconds in apparent diameter. Betelgeuse has one of the largest apparent diameters of any star in our night sky, exceeded only by R Doradus at 57 milliarcseconds.

The apparent diameter of Betelgeuse was first measured by Albert Michelson using the Mount Wilson 100-inch in 1920, who obtained an initial value of 240 million miles in diameter, about half the present accepted value, not a bad first attempt.

You can see hints of an asymmetrical bubble roiling across the surface of Betelgeuse in the ALMA image. Betelgeuse rotates once every 8.4 years. What’s going on under that uneasy surface? Infrared surveys show that the star is enveloped in an enormous bow-shock, a powder-keg of a star that will one day provide the Earth with an amazing light show.

The bowshock created by Betelgeuse as it plows through the local interstellar medium. JAXA/Akari

Thankfully, Betelgeuse is well out of the supernova “kill zone” of 25 to 100 light years (depending on the study). Along with Spica at 250 light years distant in the constellation Virgo, both are prime nearby supernovae candidates that will on day give astronomers a chance to study the anatomy of a supernova explosion up close. Riding high to the south in the northern hemisphere nighttime sky in the wintertime, +0.5 magnitude Betelgeuse would most likely flare up to negative magnitudes and would easily be visible in the daytime if it popped off in the Spring or Fall. This time of year in June would be the worst, as Alpha Orionis only lies 15 degrees from the Sun!

An early springtime supernova in the future? Stellarium

Of course, this cosmic spectacle could kick off tomorrow… or thousands of years from now. Maybe, the light of Betelgeuse gone supernova is already on its way now, traversing the 650 light years of open space. Ironically, the last naked eye supernova in our galaxy – Kepler’s Star in the constellation Ophiuchus in 1604 – kicked off just before Galileo first turned his crude telescope towards the heavens in 1610.

You could say we’re due.