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


Aug. 13, 2020

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.

44 thoughts on “Hubble Finds That Betelgeuse’s Mysterious Dimming Is Due to a Traumatic Outburst

  1. That is pretty neat that we actually image Betelgeuse in the ultraviolet spectrum down to details on the star surface. If it is does go supernova in the years to come, we will probably be able to image it even better when the James Webb telescope makes it to space in the next few years. That is going to really open up new views on things, just like the quantum leap that the Hubble telescope did.

    If Betelgeuse is ejecting plasma at twice the previous rate that has been measured in years gone by, then it is possible that with it huffing and puffing, it just might go boom relatively soon. Of course, relatively soon could mean a few thousand years, which isn’t long in the scheme of things. But it would be really neat and ominous to see it explode real time for us. Would indeed be a sign in the Heavens and maybe not a very good one, considering what is going on around the good Earth these days. Comet Neowise would have had the ancients predicting doom, or in this case, maybe boom.

    • If we ever see it go nova, it already blew up many centuries ago, so it’s not real time, but tape delayed. If we meet an ET who has conquered space-time curvature and it’s already blown up in the last 700 years, they’ll be able to tell us when we can expect to see it arrive here. If we can conquer space-time curvature first, we can go take a look for ourselves and watch it explode over and over again, although at a further and further distance away each time.

      • Wouldn’t it really be a space-time continuum for us? We tend to think in only terms of time, but it is really space-time, all rolled into one, which is referred to as a fabric and relative. For us, it will be real time, just like Sun coming up in the morning, we see it real time, although it is 8 light minutes away.

        It will be interesting whether it is within the physical laws of nature if we can conquer space-time since it presents all kinds of paradoxes, such as could I go back in time and cancel my Grandfather, and then would I even exist? All kinds of good fun thinking these things through. After a few Gin and Tonics, it all becomes crystal clear.

        • There’s a big gap between tunneling through space-time for effective FTL travel and going backwards in time. You can go far away faster than light and look back at the EM history of where you came from, but it’s already happened and you can’t interact with it.

          I like to think of the fabric of space-time as a recording medium capturing the EM emitted by the Universe. The entire EM history of the Universe is then packed into this fabric.

  2. Betelgeuse let loose a big one. Must have been something it ate. Nearby stars immediately began to “social sistance” themselves.

    • Perhaps it ‘ate’ a nearby planet, and what we witnessed was that evaporation of that planet into the dust cloud that that was the by-product of such which dimmed the star. When the good Earth goes into its red giant phase, but not nova, it will consume Mercury and Venus and probably reach our planet. That must be quite an event. Would be interesting to know what planets that Betelgeuse has or had orbiting it.

  3. “Comet Neowise would have had the ancients predicting doom” Good thing we aren’t superstious anymore….oh, wait…

  4. Gotta love fake science. Maybe they know what happened, maybe not. But such stories are invariably accompanied by realistic-looking, artful illustrations/simulations, enabled by incredible advances in computer-aided graphics. The illustrations give the false idea that we really have any clue about how it actually appears(ed). They are just click-bait. NASA is great at this type of ruse. Show us the data, or spectra, or actual photos, or whatever is the basis for the interpretation so that we can appreciate how much (or little) is really known about a given subject. If it is boring and not mind-blowingly amazing, too bad.

    • Correct, they are quite vague about the “images”. Caption from the “images”, “Illustration credit: NASA, ESA, and E. Wheatley (STScI)”.

    • My problem with this explanation is the periodicity of the brigthening and dmimming we observe on Earth.
      Quoting this press release:
      “Betelgeuse is a variable star that expands and contracts, brightening and dimming, on a 420-day cycle.”

      Betelgeuse is also rotating (probably differentially with depth and latitude), but the proposed ejection-dimming ejection scenario as described here would mean the consistently star ejects that material with regularity and in the same direction. Although Schwarzchild proposed pulsing ejections from such stars that is corroborated by evidence, the regularity of the dimming we observe here on Earth would need to be explained.

      For those interested in Betelgeuse “shape”, a good place to start reading is here:
      “The close circumstellar environment of Betelgeuse – Adaptive optics spectro-imaging in the near-IR with VLT/NACO”

      where the authors conclude: “Our AO images show that the envelope surrounding Betelgeuse has a complex and irregular structure. We propose that the southwestern plume is linked either to the presence of a convective hot spot on the photosphere, or to the rotation of the star.”

      Note: AO is “adaptive optics” of the Very Large Telescope. The VLT is an array of 4 x 8.2 m telescopes in Paranal, Chile.

    • I was thinking it ate too many Hush Puppy planets. I know that always does it for me. Room-clearer.

  5. 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.

    That is so strange to think about.

    Is Hubble really so powerful it can image this star’s disk? I thought at 725 light years, it would still be just a point source.

    • “On 13 December 1920, Betelgeuse became the first star outside the Solar System to have the angular size of its photosphere measured. Albert Michelson and Francis Pease mounted a 6-meter interferometer on the front of the 2.5-meter telescope at Mount Wilson Observatory. Helped by John Anderson, the trio measured the angular diameter of Betelgeuse at 0.047″, a figure which resulted in a diameter of 3.84×108 km (2.58 AU) based on the parallax value of 0.018″. However, limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements.”

    • It’s diameter is about the same as the orbit of Jupiter, so it’s a pretty big point. It would be equivalent to a pluto sized planet about 1 light hour away. Pluto is actually about 4.5 light hours away and Hubble can see differences across its surface.

    • The article made it sound like those were real pictures: “In the first two panels, as seen in ultraviolet light with the Hubble Space Telescope…” But no, those are really artist’s conception. The arrow in picture 3 pointing towards Earth would of course have to be a dot (the arrow as seen end-on) if this were really a photograph.

      A real Hubble photograph of Betelgeuse is seen in this article: That picture was taken in 1996, but Hubble hasn’t gotten a hundred times better resolution since then.

  6. I find it odd that covering a quarter of the star resulted in a two thirds reduction in brilliance.

  7. In my opinion NASA works best when it points its telescopes away from Earth, not inward through our atmosphere. If Betelgeuse does go super nova think of what it will do to the 100(?) Earth type planets within a 50 light years radius. Perhaps life is found on one of these planets, if so it make our pandemic a minor problem.

  8. They must have had some stellar warming. A spoiled brat like Greta Thunberg chucked tantrums and outbursts, muddying the water on what was really happening. Inhabitants of other planets all thought it was a bit dim, thinking it could be a prelude to a really massive explosion.

  9. Since the outer atmosphere of this star is almost completely hydrogen, what is the dust cloud made of? Any hydrogen expelled by the star could unite and become hydrogen molecules, but this is hardly dust. Any astronomers out there that can tell us what the dust cloud actually is?

    • Hi Loren,

      I’m not a professional astronomer but I can offer my best guess as an amateur. This event on Betelgeuse appears to be something like a Coronal Mass Ejection (CME), something the Sun experiences, but on a more massive scale. Spectral classes range from hot blue to cool red: OBAFGKMC. The Sun is a “G” star, Betelgeuse is a “M” star. “C” stars are known as Carbon stars. It is possible that a component of the ejected material was Carbon and perhaps other heavy elements.


    • The close circumstellar environment of Betelgeuse
      V. Rotation velocity and molecular envelope properties from ALMA
      Direct imaging of Carbon Monoxide by ALMA

      And this : Herschel Sees Betelgeuse on Collision Course with ‘Dusty Wall’ (images)
      So it is already dusty out there …. The star is ploughing through galactic dust with bow-waves, and if it ejects huge plumes into that stuff, hell, the chemistry must be worth a few PhD’s.

  10. The real question is how many layers has Betelgeuse burned off? If it’s down to its core, which is iron, then it won’t be long. Sanduleak 1998 took everyone by surprise because no one was watching.

    This sentence: 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. – article

    Sounds like an old man having a belly ache and trying to get rid of it to me. Betelgeuse has my sympathy, as I’ve had a gassy stomach ache myself on an off & on basis for about two weeks. Just burp it up and spook the cat out of a perfectly good nap. Yes, I have wondered , too, if Comet Neowise is a “harbinger of doom’, because it last appeared right around the end of the Bronze Age, when things literally began falling apart in the eastern Mediterranean: earthquakes, volcanoes erupting, tidal waves, locusts, drought, poor harvests, the Sea People (western Med) invading – stuff like that.

    We’re doomed.. Doomed, I tellya. Stock up on your favorite treats and make the most out of life.

  11. 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.

    What!! That is very politically incorrect! Carbon is very dangerous and should be banned — AOC has proposed a carbon-free universe.

  12. Very disappointing! I had great expectations to witness a Supernova close enough for the naked eye. As I recall 1054 and 1572 were the last two so we should be due one right about now?… Guess we’ll just have to be patient.

  13. Fake images. Look at the little cartoon looking face on the disc and note the latt and long and the direction arrow to Earth. Then look at the view from Earth. Same latt and long of the little face. Also, those two bright lights (mass ejected?) then appear to explode? Artists just making shit up.

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