This four-panel graph illustrates how the southern region of the fast-evolving, bright, red redundant star Betelgeuse in late 2019 and early 2020 suddenly became more beautiful in the first two panels, as seen in ultraviolet light. with the Hubble Space Telescope, a bright, warm blob of plasma is emitted from the formation of an enormous convection cell on the surface of the star. In panel three, the outflowing, expelled gas expands outward. It cools to form an enormous cloud of hidden dust grains. The final panel reveals the enormous cloud of dust that blocks the light (as seen from Earth) of a quarter of the star’s surface. Credit: NASA, ESA, and E. Wheatley (STScI)
Hubble finds Betelgeuse’s mysterious dimming due to a traumatic eruption
Observations by NASA‘s Hubble Space Telescope indicate that the unexpected dimming of the superfluous star Betelgeuse was probably caused by an enormous amount of hot material emitted into space, and formed a cloud of dust that blocked starlight from the surface of Betelgeuse.
Hubble researchers suggest that the dust cloud formed when it became super hot plasma detached from a coil of a large convection cell on the surface of the star, through the warm atmosphere went to the colder outer layers, where it formed cool and dust grains. The resulting dust cloud blocks 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 a graying, red redundant star that is in large swell due to complex, evolving changes in its nuclear fusion in the core. The star is now so enormous that if it replaced the sun at the center of our solar system, its outer surface would be past the orbit of Jupiter.
The unusual phenomenon of large dimming of Betelgeuse, eventually noticed for even 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 dimming has mystified astronomers, who shuddered to develop various theories for the abrupt change. One idea was that an enormous, cool, dark “tail 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 cloud of dust covering part of the star.
Several months of Hubble’s ultraviolet light spectroscopic observations from Betelgeuse, beginning in January 2019, provide a timeline leading to darkness. These observations provide important new indications for 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 telescopes saw the star decrease in brightness in its southern hemisphere.
“With Hubble, we see the material as it left the visible surface of the star and passed through the atmosphere before forming the dust that caused the star to appear,” Dupree said. “We could see the effect of a dense, hot region moving outward in the southeastern part of the star.
“This material was two to four times brighter than the normal brightness of the star,” she continued. ‘And then, about a month later, the southern part of Betelgeuse suddenly damned when the star got brighter. We think it is possible that a dark cloud resulted from the outflow that Hubble discovered. Only Hubble gives us this evidence that led to the dimming. ”
The team paper will appear online today (August 13, 2020) in The Astrophysical Journal.
Massive supergiant stars like Betelgeuse are important because they drive 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.
Detected 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 control variations in the star’s outer atmosphere. Betelgeuse is a variable star that expands and contracts, brightening and dimming, over a cycle of 420 days.
Hubble’s ultraviolet light sensitivity allowed researchers to study the layers above the star’s surface, which are so hot – more than 20,000 degrees Fahrenheit – they can not be detected at visible wavelengths. These layers are heated in part by the star’s turbulent convection cells bubbling to the surface.
Hubble spectra, taken in early and late 2019, and in 2020, examined the outer atmosphere of the star by measuring magnesium II (once ionized magnesium) lines. In September to November 2019, researchers measured material moving about 200,000 miles per hour from the star’s surface to its outer atmosphere.
This hot, dense material traveled far beyond the visible surface of Betelgeuse, reaching millions of miles from the boiling star. At that distance, the material cools enough to form dust, the researchers said.
This interpretation is consistent with Hubble observations of ultraviolet light 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 cause of the eruption, she thinks it was helped by the star’s pulsation cycle, which is normally continued by the event, as determined by observations with visible light. The co-author of the paper, Klaus Strassmeier, of the Leibniz Institute of Astrophysics Potsdam, used the institute’s automated telescope called STELLar Activity (STELLA), to detect changes in the velocity of the gas on the star’s surface. measured when it fell up and down during the pulsation cycle. The star expanded in its cycle at the same time as the convection of the convective cell. The pulsation that rippled out to Betelgeuse may have helped propel the 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, loses 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 managed to solve surface features – making it the only such star, except for our Sun, where surface detail can be seen.
Hubble imagery taken by Dupree in 1995 first revealed a bright surface with massive convection cells that shrink and swell, making them darker and brighter.
A Supernova forerunner?
The red supergiant is destined to end his life in a supernova explosion. Some astronomers think that the sudden dimming may be a pre-supernova event. The star is relatively close, about 725 light-years away, which means that dimming would have occurred around the year 1300. But the light is just now reaching the Earth.
“No one knows what a star does just before the supernova goes up, because it has never been observed,” Dupree explained. “Astronomers have been collecting stars, maybe a year before they become supernovae, but not within days or weeks before it happened. But the chance that the star will supernova at any moment is quite small. ”
Dupree will get another chance to observe the star with Hubble in late August or early September. Currently, 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 images of the monster star from its location in space. Those observations show that Betelgeuse dimmed again from mid-May to mid-July, though not as dramatically as earlier in the year.
Dupree hopes to use STEREO for more subsequent observations to check the brightness of Betelgeuse. Her plan is to re-observe Betelgeuse next year with STEREO, when the star in the cycle is expanded outward again to see if it releases another eruption of petulant.
Reference: “Spatially Solved 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
