The recent dimming of the supergiant star Betelgeuse was most likely caused by an immense dust cloud of its own ejecta

Jan 27, 2020
Betelgeuse is an aging, red supergiant star that has swelled in size due to complex changes in its core as it ages. 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. This means that Betelgeuse would have a radius of about 484 million miles or some 778,547,200 km. I say about and some because our planets, including Jupiter, follow elliptical paths not concentric orbits.

Red supergiants look red because of their low surface temperatures. They range from about 3,500 - 4,500 Kelvin. According to Wien's law, the color at which a star radiates most strongly is directly related to its surface temperature. So, while their cores are extremely hot, the energy spreads out over the interior and surface of the star and the greater surface area there is, the faster it cools. A prime example of a red supergiant is the star Betelgeuse, in the constellation Orion, located in The Hunter's left shoulder.

Most stars of this type are between 200 and 800 times the radius of our Sun. The very largest stars in our galaxy, all red supergiants, are about 1,500 times the size of our home star. Because of their immense size and mass, these stars require an incredible amount of energy to sustain them and prevent gravitational collapse. As a result, they burn through their nuclear fuel very quickly and most live only a few tens of millions of years (their final age is inversely proportional to their actual mass).

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. 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 was covering a portion of the star.

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 (stellar)atmosphere, before the dust that formed 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," Dupree 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 it was a dark cloud resulting from the outflow that Hubble detected. Only Hubble gives us this evidence that led up to the dimming."

Massive supergiant stars like Betelgeuse expel heavy elements such as carbon into the interstellar medium which become the building blocks of new generations of stars. Carbon is a basic ingredient for life.

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. This hot, dense material continued to travel beyond Betelgeuse's visible surface, reaching millions of miles from the hot seething surface. At that distance, the material then 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 every star, is losing mass all the time, at a rate 30 million times higher than that of 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.

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.

Andrea K. Dupree, Klaus G. Strassmeier, Lynn D. Matthews, Han Uitenbroek, Thomas Calderwood, Thomas Granzer, Edward F. Guinan, Reimar Leike, Miguel Montargès, Anita M. S. Richards, Richard Wasatonic, Michael Weber. Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse. The Astrophysical Journal, 2020; 899 (1): 68 DOI: 10.3847/1538-4357/aba516

NASA/Goddard Space Flight Center. "Hubble finds that Betelgeuse's mysterious dimming is due to a traumatic outburst." ScienceDaily. ScienceDaily, 13 August 2020. <>.

I find it wonderful that we can see Betelgeuse and actually discern its own change in brightness from our backyard easy chairs with a pair of Zeiss binoculars. Rarely are cosmic changes and the possible coming calamity of a Supernova so easily visible.