Hubble finds that Betelgeuse's mysterious dimming is due to a traumatic outburst

Date:
August 13, 2020
Source:
NASA/Goddard Space Flight Center
Summary:
Observations 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.



FULL STORY ========================================================================== 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.

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.

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.


========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Andrea K. Dupree, Klaus G. Strassmeier, Lynn D. Matthews, Han
Uitenbroek,
Thomas Calderwood, Thomas Granzer, Edward F. Guinan, Reimar Leike,
Miguel Montarge`s, 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200813134558.htm

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