Hubble finds that Betelgeuse’s mysterious fog is due to a traumatic eruption
Observations of NASA‘s Hubble Space Telescope shows that the unexpected attenuation of the supergiant star Betelgeuse was most likely caused by a huge amount of hot material that was thrown into space and formed a cloud of dust that blocked starlight from the surface of Betelgeuse.
Hubble researchers suggest that dust clouds formed when superheated plasma detached from a build-up 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 cloud of dust blocked light from about a quarter of the star’s surface, beginning in late 2019. In 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. The star is so huge now that if it replaced the sun in the center of our solar system, its outer surface would extend past the orbit Jupiter.
The unique phenomenon of Betelgeuse’s great fog, which was eventually felt even by the naked eye, began in October 2019. By mid-February 2020, the monster star had lost more than two-thirds of its luster.
This sudden attenuation has mystified astronomers, who crashed to develop several theories for the sudden change. One idea was that a huge, cool, dark “star spot” covered a wide spot of the visible surface. But the Hubble Observations, led by Andrea Dupree, Deputy Director of the Center for Astrophysics | Harvard & Smithsonian (CfA), Cambridge, Massachusetts, proposes a cloud of dust covering part of the star.
Several months of Hubble’s spectroscopic observations of ultraviolet light by Betelgeuse, which began in January 2019, provide a timeline leading to obscuration. These observations provide important new clues to the mechanism behind dimming.
Hubble captured signs of dense, heated material moving through the stellar atmosphere in September, October and November 2019. Since December, several ground-based telescopes have observed the star declining 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 made the star appear dimming,” Dupree said. “We could see the effect of a dense, hot region in the southeastern part of the star moving outwards.
“This material was two to four times brighter than the star’s normal brightness,” she continued. And then, about a month later, the southern part of Betelgeuse dimmed noticeably as the star weakened. We believe it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this proof that led to the fog. “
The team magazine will be shown online today (August 13, 2020) on The Astrophysical Journal.
Massive supergiant stars like Betelgeuse are important because they push heavy elements like coal into space that become the building blocks for new generations of stars. Carbon is also a basic ingredient for life as we know it.
Trace a traumatic outbreak
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 external atmosphere. Betelgeuse is a variable star that expands and contracts, brightens and dims, over a 420-day cycle.
Hubble’s ultraviolet light sensitivity enabled scientists to examine 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 partially heated by the star’s turbulent convection cells that bubble up to the surface.
Hubble spectra, taken in early and late 2019, and 2020, tested the star’s outer atmosphere by measuring magnesium II (magnesium-only ionized) lines. In September to November 2019, scientists measured materials moving about 200,000 miles per hour that passed from the star’s surface to its outer atmosphere.
This hot, dense material continued to travel beyond the visible surface of Betelgeuse, reaching millions of miles from the seventh star. At that distance, the material was cooled enough to form dust, the researchers said.
This interpretation is consistent with Hubble’s observations of ultraviolet light in February 2020, which showed that the behavior of the star’s outer atmosphere returned to normal, although visible light images showed that it was still fogging.
Although Dupree does not know the cause of the eruption, she believes it was helped by the star’s pulsation cycle, which continued normally through the event, which was recorded by visible light observations. The co-author of the article, Klaus Strassmeier, from the Leibniz Institute for Astrophysics Potsdam, used the institute’s automatic telescope called STELLar Activity (STELLA) to measure changes in gas velocity on the star’s surface as it rose and fell during the pulsation cycle. The star expanded in its cycle at the same time as the structure of the convective cell. The pulsation orbiting outwards from Betelgeuse may have contributed to driving outflowing plasma through the atmosphere.
Dupree estimates that about twice the normal amount of material from the southern hemisphere was lost during the three months of the eruption. 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 solve surface functions – making it the only such star, with the exception of our Sun, where surface details can be seen.
Hubble images taken by Dupree in 1995 first revealed a mottled surface that contains massive convection cells that shrink and swell, making them darker and lighter.
A Supernova predecessor?
The red supergiant is destined to end his life in a supernova explosion. Some astronomers believe that the sudden fog may be an event before a supernova. The star is relatively close, about 725 light-years away, which means that the fog would have happened around the year 1300. But its light is reaching the earth right now.
“No one knows what a star does right before it goes supernova, because it has never been observed,” Dupree explained. “Astronomers have tried stars maybe a year before they go supernova, but not within a few days or weeks before it happened. But the chance that the star will go supernova at any time soon is quite small. “
Dupree gets another chance to observe the star with Hubble in late August or early September. Right now, Betelgeuse is in the sky during the day, too close to the sun for Hubble observations. But NASA’s Solar Terrestrial Relations Observatory (STEREO) has taken pictures of the monster star from its place in space. These observations show that Betelgeuse fogged again from mid-May to mid-July, but not as dramatically as earlier in the year.
Dupree hopes to use STEREO for more follow-up observations to monitor the brightness of Betelgeuse. Her plan is to observe Betelgeuse again next year with STEREO when the star has expanded outwards again in its cycle to see if it releases another petulant eruption.
Reference: “Spatial Dissolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse” by Andrea K. Dupree, Klaus G. Strassmeier, Lynn D. Matthews, Han Uitenbroek, Thomas Calderwood, Thomas Granzer, Edward F. Guinan, Reimar Leike, Miguel Montargès, Anita MS Richards, Richard Wasatonic and Michael Weber, 13 August 2020, The Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / aba516