Celestial What Will a Betelgeuse Supernova Look Like From Earth?


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What Will a Betelgeuse Supernova Look Like From Earth?
Astronomers simulated what humans will see on Earth when the star Betelgeuse explodes as a supernova sometime in the next 100,000 years.

By Eric Betz
February 14, 2020 8:30 AM


A plume of gas nearly the size of our solar system erupts from Betelgeuse's surface in this artist's illustration of real observations gathered by astronomers using the Very Large Telescope in Chile. (Credit: European Southern Observatory/L. Calçada)

If you stargaze on a clear winter night, it’s hard to miss the constellation Orion the Hunter, with his shield in one arm and the other arm stretched high to the heavens. A bright red dot called Betelgeuse marks Orion’s shoulder, and this star's strange dimming has captivated skygazers for thousands of years. Aboriginal Australians may have even worked it into their oral histories.

Today, astronomers know that Betelgeuse varies in brightness because it’s a dying, red supergiant star with a diameter some 700 times larger than our sun. Someday, the star will explode as a supernova and give humanity a celestial show before disappearing from our night sky forever.

That eventual explosion explains why astronomers got excited when Betelgeuse started dimming dramatically in 2019. The 11th-brightest star dropped in magnitude two-and-a-half-fold. Could Betelgeuse have reached the end of its life? While unlikely, the idea of a supernova appearing in Earth’s skies caught the public’s attention.

And now new simulations are giving astronomers a more precise idea of what humans will see when Betelgeuse does eventually explode sometime in the next 100,000 years.

Betelgeuse supernova seen from earth

Astronomers used a software program called MESA+STELLA to simulate what humans might see when the star Betelgeuse explodes. They also included observations gathered during Supernova 1987A, which exploded in the Large Magellanic Cloud. (Credit: Jared Goldberg/University of California, Santa Barbara/MESA+STELLA)
Supernova Seen From Earth
With all the speculation about what a Betelgeuse supernova would look like from Earth, University of California, Santa Barbara, astronomer Andy Howell got tired of the back-of-the-envelope calculations. He put the problem to a pair of UCSB graduate students, Jared Goldberg and Evan Bauer, who created more precise simulations of the star’s dying days.

The astronomers say there’s still uncertainty over how the supernova would play out, but they were able to augment their accuracy using observations taken during Supernova 1987A, the closest known star to explode in centuries.

Life on Earth will be unharmed. But that doesn’t mean it will go unnoticed. Goldberg and Bauer found that when Betelgeuse explodes, it will shine as bright as the half-moon — nine times fainter than the full moon — for more than three months.

“All this brightness would be concentrated into one point,” Howell says. “So it would be this incredibly intense beacon in the sky that would cast shadows at night, and that you could see during the daytime. Everyone all over the world would be curious about it, because it would be unavoidable.”

Humans would be able to see the supernova in the daytime sky for roughly a year, he says. And it would be visible at night with the naked eye for several years, as the supernova aftermath dims.

“By the time it fades completely, Orion will be missing its left shoulder,” adds Sarafina Nance, a University of California, Berkeley, graduate student who’s published several studies of Betelgeuse.


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 2019 and December 2019, show how much the star has faded and how its apparent shape has changed. (Credit: ESO/M. Montargès et al.)

The Betelgeuse Show
There’s no need to worry about the stellar explosion. A supernova has to happen extremely close to Earth for the radiation to harm life — perhaps as little as several dozen light-years, according to some estimates. Betelgeuse is far outside that range, with recent studies suggesting it sits roughly 724 light-years away, well outside the danger zone.

But the supernova could still impact Earth in some surprising ways. For example, Howell points out that many animals use the moon for navigation and are confused by artificial lights. Adding a second object as bright as the moon could be disruptive. It’s not only wildlife that would be disturbed, either; ironically, astronomers themselves would have a hard time.

“Astronomical observations are already difficult when the moon is bright,” Howell says. “There would be no ‘dark time’ for a while.”

Even studying Betelgeuse would be a unique challenge. The bright light would overwhelm their instruments.

“We couldn't observe it with most ground-based telescopes, or most in space, either, like Swift or the Hubble Space Telescope,” he adds. Instead, they’d have to modify their telescopes to collect far less light.

And if Betelgeuse does defy the odds and blow up in our lifetimes, astronomers say there will be ample warning. Instruments on Earth would start detecting neutrinos or gravitational waves generated by the explosion as much as a day in advance.

“Imagine a good fraction of the world staying up and staring at Betelgeuse, waiting for the light show to start, and a cheer going up around the planet when it does,” Howell says.

betelgeuse zoom in

This collage zooms in on the constellation Orion (left) to one of the sharpest images ever taken of Betelgeuse (far right). (Credit: ESO, P. Kervella, Digitized Sky Survey 2 and A. Fujii)
To Catch a Dying Star
But for scientists, Betelgeuse doesn’t have to explode to be interesting. It’s big and bright, making it relatively easy to study.

“It's fascinating from an astronomer’s perspective because we can study a star that is nearing the end of its life quite closely,” Nance says. “There's some fascinating physics going on in the internal structure of Betelgeuse.”

Their best guess as to what’s going on right now stems from what astronomers already know about the star and others like it. As Nance explains, that research shows Betelgeuse’s brightness could be changing for a number of reasons. Some astronomers even suspect that several different dimming mechanisms are playing out at once.

As their nuclear fuel runs out near the ends of their lives, red supergiant stars start to bloat and form growing envelopes of gas and dust. And as this envelope gets bigger, the star’s brightness grows. But that’s not the only way a star like Betelgeuse can dim and brighten. Red supergiant stars also have enormous convective cells on their surfaces — like much larger versions of those on our Sun — where turbulence makes hot material rise from inside the star. Once it reaches the surface, part of that material erupts violently into space like a giant, radioactive belch, which can temporarily change its brightness.

And Betelgeuse’s dimming could even be evidence that it is about to explode. As material erupts from a dying star’s surface, it typically collides, which makes it shine brighter. However, Nance says it’s possible that this material is shrouding the star instead, making it dimmer.

Whatever the root cause, the strange behavior should ultimately offer new insights into the dying days of red supergiant stars. And humanity will have a front-row seat.

“Betelgeuse provides a great setting for astronomers to study these last stages of nuclear burning before it explodes,” Nance says.


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Is Betelgeuse Approaching a Crossroads?

By: Bob King | February 14, 2020

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
Betelgeuse on January 6, 2020. 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. Credit: ESO/M. Montargès et al.
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 infrared image, obtained with the VISIR instrument on the VLT, shows the immensity of the patchy dust clouds surrounding Betelgeuse in December 2019. The clouds form when the star sheds its material back into space. The black disk masks the star and its immediate surroundings thus revealing the fainter dust plumes. The orange dot in the middle is the SPHERE image of Betelgeuse’s surface, which in size is close to that of Jupiter’s orbit.
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
Ten years of photometric data not only reveal Betelgeuse's routine ups and downs but also its current remarkable minimum.
Edward Guinan
Betelgeuse slowdown
Betelgeuse declined in brightness slowly beginning about four months ago, then quickened and now appears to be slowing down (right).
Edward 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
Twenty-three years of period analysis of Betelgeuse reveals that the star's brightness varies with multiple periods but dominated by those of 430 days and ~6 years. Imagine if you had five different pulse rates!
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
Beastly big! This image, made with the Atacama Large Millimeter/submillimeter Array (ALMA), depicts the red supergiant Betelgeuse placed at the center of our solar system. With a diameter some 1,400 times larger than the Sun, it would envelop all the inner planets as well as Jupiter.
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 shown 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.

Explore the Night with Bob King, Observing, Observing News & Current Celestial Events, Stellar Science, Variable Stars
Betelgeuse, red supergiant, supernova

The Latest on Betelgeuse, Plus a Bright Supernova and New Comet Iwamoto
Betelgeuse is Dimming . . . Why?

Bob King

About Bob King
About Bob King I've been a skywatcher and amateur astronomer since childhood. I'm also a long-time member of the American Association of Variable Star Observers (AAVSO), write the Astro Bob blog and am the author of three books: Night Sky with the Naked Eye (2016); Wonders of the Night Sky You Must See Before You Die (2018) and Urban Legends from Space(2019). The universe invites us on an adventure every single night. All we need to do is look up.
View all posts by Bob King →


Neither here nor there.
From 2013

Red Supergiant Betelgeuse braces for collision with mysterious "wall"


TB Fanatic
Yeah how about that no supper nova and if it did it would give people with telescopes something to observe for a few years. Really not much we can d about it if it did nova and being over 640 light years away it would have no effect on us. As Hfcomms posted above on Post N0#3 our seeing it happen it would have really happened long ago, so what we would be seeing is a event that happened 600+ years ago


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The Sciences
Why Did Betelgeuse Dim?
Ravinder Banyal

The Orion constellation, with Betelgeuse visible as a dull red star on the hunter’s right shoulder. Photo: alchemist_x/Flickr, CC BY 2.0.

Betelgeuse is an incredibly big star located about 700 light years away from Earth. It is about 800-times wider than the Sun and nearly 15-times as massive.. It is also the second brightest star (fig. 1) after Rigel in the Orion constellation, in the northern hemisphere.

Recently, Betelgeuse’s brightness began to dim in unprecedented ways, attracting the attention of astronomers and amateur stargazers around the world. The brightness of most stars, including our Sun, vary over time. But these variations are usually small, nothing beyond a few percentage points of the star’s total light output. Betelgeuse, however, has been up to something else.

It began fading sometime in October 2019 and had lost fully two-thirds of its shine by mid-February 2020. Normally ranked the tenth brightest star in the night sky, Betelgeuse has suddenly slipped to 25th. This is a stunning drop the likes of which we haven’t noticed with any other star before, and many astronomers and astrophysicists have been scrambling to make sense of the stellar drama.

Some astronomers suspect the star is nearing its death. They argue that Betelgeuse’s freakish decline could soon culminate in a sudden end triggered by a violent explosion known as a supernova. However, given how old and close we know Betelgeuse to be, a supernova event in our lifetime seems quite improbable.
Figure 1: (a) Image of Betelgeuse taken with Hubble Space Telescope’s faint object camera in 1996. (b) In the Orion constellation, Betelgeuse at the top left. Photo: NASA/ESAPIN IT
Betelgeuse belongs to a category of massive stars that are extremely rare. There are more low-mass stars in the Milky Way galaxy than there are high-mass stars. Astronomical surveys of the night sky have found that the star-count drops significantly as the mass increases. On average, for every 200 stars, there is only one Betelgeuse-type star.

So calling Betelgeuse a supergiant wouldn’t be an exaggeration. It is so big that 800 million Suns could fit inside it — and each Sun can pack in 1.3 million Earths.

The luminosity of a star is the amount of energy it releases from its surface every second. Betelgeuse’s luminosity is 100,000-times that of the Sun. However, its surface is also cooler – 3,600 K versus the Sun’s 5,800 K – so only about 13% of its radiant energy is emitted as visible light.

Traditionally, Betelgeuse is classified as a pulsating variable star. This means the star’s brightness changes as the star expands and contracts. In the past, Betelgeuse has displayed striking and unequivocal phases of pulsation. The English astronomer John Herschel first noticed the corresponding changes in brightness in 1836. In the last two centuries, the star is reported to have undergone several intermittent phases of brightening and dimming.

In 1920, Betelgeuse became the first star to have its angular diameter measured with a technique called interferometry, by Albert A. Michelson and Francis Pease.
Figure 2: The light curve of Betelgeuse for the past six months. The star’s magnitude had dropped to below 1.6 by early February 2020, amounting to a brightness reduction factor of 2.5. This is the largest observed change for any bright star. Since March 2020, the star has started recovering its brightness. Data source: AAVSOPIN IT
What happens inside a massive star?

Every star has to constantly grapple with two competing sets of forces throughout its life: the force of gravity that holds the star together and the forces driving the nuclear reactions that are the star’s source of energy. Stars are principally cosmic factories that fuse lighter elements into heavier ones. The star’s gravity pulls everything inwards while the heat and radiation from the reactions exert an outward pressure. The balance of these two opposing forces keeps the star together. Think of how a pressure cooker works. The hot steam inside the cooker is like energy from nuclear reactions. More heat creates more pressure and the steam tries to escape by forcing the lid open. The weight of the lid, or whistle, is like gravity: it keeps the pressure under control.

A star’s mass typically ranges from 0.1- to 150-times the solar mass. A ‘normal’ Sun-like star burns its fuel slowly and lives for several billion years while massive stars like Betelgeuse are short-lived – in the order of millions of years – because they consume their nuclear fuel faster.

Also read: WTF: The Story of the Strangest Star We’ve Known

Supergiant stars produce heavier elements like iron in their interiors in a series of nuclear burning cycles. The time scale of different burning stages is determined by the star’s initial mass. Every star spends about 90% of its lifetime fusing hydrogen into helium inside the core. Subsequently, helium fused into carbon, carbon into neon, neon into oxygen and so on. In high-mass stars, iron is the final product of this series of fusion reactions. And since iron’s atomic nuclei are very stable and tightly bound, they cannot be fused further. So nuclear reactions stop when the star’s core is full of iron.
Figure 3: The different evolutionary stages of a star. A massive star meets its end in a violent explosion called a supernova. Image: NASAPIN IT
Without nuclear fuel, the core begins to cool even as there’s nothing pushing back against the force of gravity, so gravity takes the upper hand. In less than a second, the iron core collapses catastrophically, forcing the material in the star’s outer parts to fall freely towards the shrinking core. The infalling matter strikes the heated core with tremendous force and rebounds violently in the form of a shockwave that travels outwards into space. This process produces heavier metals such as gold and platinum, as well as gravitational waves and fast neutrinos. The amount of energy released from such powerful explosions can momentarily exceed the combined energy of all stars in the host galaxy.

After this cataclysm, whatever is left of the core turns into a neutron star or, if it is dense enough, a black hole.
Figure 4: The inward gravitational pull of the gravity is balanced by outward pressure generated by heat and radiation produced by nuclear fusion in the core.PIN IT
Betelgeuse is already about 10 million years old, and it is the most promising star in the night sky to go supernova in future. We can only speculate the fate of Betelgeuse, and cross our fingers in hope. There is no exact way to predict the exact time of its demise. This said, when it does go supernova, instruments on Earth will register gravitational waves and fast neutrinos from the explosion several hours before the visual fireworks come on. This is because the gravitational waves are generated moments before the explosion, travel at the speed of light and aren’t disturbed by intervening matter. The neutrinos also travel at nearly the speed of light and don’t interact much with matter.
Figure 5: Images of Betelgeuse taken almost a year apart using ESO’s SPHERE/VLT facility at the Paranal Observatory in northern Chile. Recent dimming seems more prominent in the southern hemisphere of the star.PIN IT
Plausible explanations

Spot hypothesis

The energy produced at the star’s centre has to come out and reach the surface. In high-mass stars, the energy is transported by large blobs of hot and ionised material rising to the surface – much like bubbles rising from the bottom of a pot of water boiling over a stove. These superheated blobs of plasma are called convective cells. In a Sun-like star, convective cells are only a few hundred kilometres wide. On Betelgeuse, they are about 240 million km wide – the entire distance between Earth and Mars.

As it happens, the surface of most stars is laced occasionally by strong magnetic fields called star spots (just like sunspots). The magnetic field in these spots prevents energy in the star’s interior from being convected to the surface. Spot regions are therefore cooler and emit less energy. And yes, the larger the star, the bigger the spots.
Figure 6: Reconstructed images of Betelgeuse showing large convective cells responsible for transporting energy from the deeper layers of the star to its surface. Source: Asymmetric shocks in χ Cygni observed with linear spectropolarimetry | Astronomy & Astrophysics (A&A)PIN IT
It’s possible that a giant spot covering the surface of Betelgeuse has temporarily impeded convection over a large area, thus lowering the supergiant’s surface temperature. This would explain the current dimming.

However, astronomers Emily Levesque and Philip Massey have found in newer observations at the Lowell Observatory, Arizona, that Betelgeuse isn’t so cool after all. In a scientific paper that appeared in the March 2020 issue of the Astrophysical Journal, they reported a measured temperature not very different from what previous studies have found, meaning the star hasn’t undergone the sort of substantive cooling that could explain its brightness deficit.

The spot hypothesis is therefore unlikely to be the primary cause of dimming.
Figure 7: An image of Betelgeuse taken by ESA’s Herschel Space Observatory at infrared wavelengths, in 2012. The clumpy dust shells around the star suggest episodic and asymmetric mass loss.PIN IT
Dust hypothesis

In the last stage of its evolution, every star is known to lose mass. While Betelgeuse is huge, it is 117.5-million-times less dense than the Sun, which means it has a low surface gravity and a small escape velocity: 60 km/s versus the Sun’s 600 km/s. This in turn means gas and dust escape more easily from Betelgeuse’s surface into the circumstellar medium. And this way, Betelgeuse has been losing one Earth’s mass worth of material every year – material that condenses to form a nebula-like envelope of gas and dust that can be seen in images taken at infrared wavelengths.

Some astronomers think an oddly shaped column of dust and gas produced this way has simply come in the way of our line of sight, and obstructed some of Betelgeuse’s starlight from reaching Earth.

This fortuitous conjunction seems to have lasted until about mid-February 2020. These days, the star appears to be regaining its lost shine.

The dust hypothesis seems to offer a satisfactory explanation of the dimming. However, we still need more observations to confirm this possibility beyond any reasonable doubt.


All stars die. The bigger ones just die more spectacularly.

Betelgeuse is too far from Earth to pose any major threat when it eventually explodes – but it’s close enough to offer a unique chance for astronomers and astrophysicists to study in great detail the rare cosmic event. Its supernova will be brighter than the full moon at night and will be visible even during the day.

Betelgeuse is also too short-lived (in stellar terms) for planets to form around them, leave alone harbour life. However, Betelgeuse and its supergiant peers are progenitors of life in a different way. The heavier elements formed in the core of a massive star are expelled into the interstellar medium after the supernova. This debris mixes with gas and dust to become the material for the subsequent generation of Sun-like stars, which then support planets.

Also read: The Story of Dust, Through Space and Time

In fact, we owe our existence to the death of a massive star. Our Solar System was formed from the remains of a similar explosion that predated the birth of the Sun. Many essential ingredients of the human body were first created in a faraway supernova. In the grand scheme of things, we are truly the children of stardust – and this is possibly the most profound and humbling thing modern science has helped us find.

Ravinder Banyal is a research scientist at the Indian Institute of Astrophysics, Bengaluru.


passin' thru
Or perhaps everything happens at once, and it's possible that it is happening NoW.

...purty pics tho, so I'm happy : )
More purty star pics, so I'm happy : ) No need for all the words, graphs & maff stuff, tho :lol:


passin' thru
Aug. 13, 2020

Hubble Finds That Betelgeuse's Mysterious Dimming Is Due to a Traumatic Outburst

Observations by NASA's Hubble Space Telescope are showing that the unexpected dimming of the supergiant star Betelgeuse was most likely caused by an immense amount of hot material ejected into space, forming a dust cloud that blocked starlight coming from Betelgeuse's surface.
Hubble researchers suggest that the dust cloud formed when superhot plasma unleashed from an upwelling of a large convection cell on the star's surface passed through the hot atmosphere to the colder outer layers, where it cooled and formed dust grains. The resulting dust cloud blocked light from about a quarter of the star's surface, beginning in late 2019. By April 2020, the star returned to normal brightness.

Betelgeuse is an aging, red supergiant star that has swelled in size due to complex, evolving changes in its nuclear fusion furnace at the core. The star is so huge now that if it replaced the Sun at the center of our solar system, its outer surface would extend past the orbit of Jupiter.
The unprecedented phenomenon for Betelgeuse's great dimming, eventually noticeable to even the naked eye, started in October 2019. By mid-February 2020, the monster star had lost more than two-thirds of its brilliance.

four illustrations of a red-hued star expelling gas, bringing the star into slight shadow
This four-panel graphic illustrates how the southern region of the rapidly evolving, bright, red supergiant star Betelgeuse may have suddenly become fainter for several months during late 2019 and early 2020. In the first two panels, as seen in ultraviolet light with the Hubble Space Telescope, a bright, hot blob of plasma is ejected from the emergence of a huge convection cell on the star's surface. In panel three, the outflowing, expelled gas rapidly expands outward. It cools to form an enormous cloud of obscuring dust grains. The final panel reveals the huge dust cloud blocking the light (as seen from Earth) from a quarter of the star's surface.
Illustration credit: NASA, ESA, and E. Wheatley (STScI)

This sudden dimming has mystified astronomers, who scrambled to develop several theories for the abrupt change. One idea was that a huge, cool, dark "star spot" covered a wide patch of the visible surface. But the Hubble observations, led by Andrea Dupree, associate director of the Center for Astrophysics | Harvard & Smithsonian (CfA), Cambridge, Massachusetts, suggest a dust cloud covering a portion of the star.
Several months of Hubble's ultraviolet-light spectroscopic observations of Betelgeuse, beginning in January 2019, yield a timeline leading up to the darkening. These observations provide important new clues to the mechanism behind the dimming.
Hubble captured signs of dense, heated material moving through the star's atmosphere in September, October, and November 2019. Then, in December, several ground-based telescopes observed the star decreasing in brightness in its southern hemisphere.

“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim,” Dupree said. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.
"This material was two to four times more luminous than the star's normal brightness," she continued. "And then, about a month later, the south part of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence that led up to the dimming."
The team's paper will appear online Aug. 13 in The Astrophysical Journal.
Massive supergiant stars like Betelgeuse are important because they expel heavy elements such as carbon into space that become the building blocks of new generations of stars. Carbon is also a basic ingredient for life as we know it.

Tracing a Traumatic Outburst
Dupree's team began using Hubble early last year to analyze the behemoth star. Their observations are part of a three-year Hubble study to monitor variations in the star's outer atmosphere. Betelgeuse is a variable star that expands and contracts, brightening and dimming, on a 420-day cycle.
Hubble's ultraviolet-light sensitivity allowed researchers to probe the layers above the star's surface, which are so hot — more than 20,000 degrees Fahrenheit — they cannot be detected at visible wavelengths. These layers are heated partly by the star's turbulent convection cells bubbling up to the surface.
Hubble spectra, taken in early and late 2019, and in 2020, probed the star's outer atmosphere by measuring magnesium II (singly ionized magnesium) lines. In September through November 2019, the researchers measured material moving about 200,000 miles per hour passing from the star's surface into its outer atmosphere.
Related: NASA Satellite’s Lone View of Betelgeuse Reveals More Strange Behavior
For several weeks in summer 2020, NASA’s STEREO had the solar system’s best view of the star Betelgeuse, revealing more unexpected dimming by the star.
This hot, dense material continued to travel beyond Betelgeuse's visible surface, reaching millions of miles from the seething star. At that distance, the material cooled down enough to form dust, the researchers said.
This interpretation is consistent with Hubble ultraviolet-light observations in February 2020, which showed that the behavior of the star's outer atmosphere returned to normal, even though visible-light images showed that it was still dimming.
Although Dupree does not know the outburst's cause, she thinks it was aided by the star's pulsation cycle, which continued normally though the event, as recorded by visible-light observations. The paper's co-author, Klaus Strassmeier, of the Leibniz Institute for Astrophysics Potsdam, used the institute's automated telescope called STELLar Activity (STELLA), to measure changes in the velocity of the gas on the star's surface as it rose and fell during the pulsation cycle. The star was expanding in its cycle at the same time as the upwelling of the convective cell. The pulsation rippling outward from Betelgeuse may have helped propel the outflowing plasma through the atmosphere.

Dupree estimates that about two times the normal amount of material from the southern hemisphere was lost over the three months of the outburst. Betelgeuse, like all stars, is losing mass all the time, in this case at a rate 30 million times higher than the Sun.
Betelgeuse is so close to Earth, and so large, that Hubble has been able to resolve surface features – making it the only such star, except for our Sun, where surface detail can be seen.
Hubble images taken by Dupree in 1995 first revealed a mottled surface containing massive convection cells that shrink and swell, which cause them to darken and brighten.

A Supernova Precursor?
The red supergiant is destined to end its life in a supernova blast. Some astronomers think the sudden dimming may be a pre-supernova event. The star is relatively nearby, about 725 light-years away, which means the dimming would have happened around the year 1300. But its light is just reaching Earth now.
"No one knows what a star does right before it goes supernova, because it's never been observed," Dupree explained. "Astronomers have sampled stars maybe a year ahead of them going supernova, but not within days or weeks before it happened. But the chance of the star going supernova anytime soon is pretty small."
Dupree will get another chance to observe the star with Hubble in late August or early September. Right now, Betelgeuse is in the daytime sky, too close to the Sun for Hubble observations. But NASA's Solar Terrestrial Relations Observatory (STEREO) has taken images of the monster star from its location in space. Those observations show that Betelgeuse dimmed again from mid-May to mid-July, although not as dramatically as earlier in the year.
Dupree hopes to use STEREO for more follow-up observations to monitor Betelgeuse's brightness. Her plan is to observe Betelgeuse again next year with STEREO when the star has expanded outward again in its cycle to see if it unleashes another petulant outburst.

Claire Andreoli
NASA's Goddard Space Flight Center, Greenbelt, Md.
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Andrea Dupree
CfA | Harvard & Smithsonian, Cambridge, Mass.

Last Updated: Aug. 13, 2020

Editor: Rob Garner