Like a Human Heart: Betelgeuse’s Chromosphere Beats Asymmetrically

Contact Information:

Harvard-Smithsonian Center for Astrophysics

Alex Lobel, 617-497-7919, alobel@cfa.harvard.edu

Andrea Dupree, 617-495-7489, adupree@cfa.harvard.edu

Space Telescope Science Institute

Roland Gilliland, 301-470-4366, gillil@stsci.edu

For Release: 9:20 a.m. EST, January 13, 2000

LIKE A HUMAN HEART: BETELGEUSE’S CHROMOSPHERE BEATS ASYMMETRICALLY

ATLANTA, GA — Astronomers have long known that the atmospheres of pulsating stars either expand or
contract over time, but, thanks to new spectra and images of Betelgeuse taken with the Hubble Space
Telescope (HST), they have now discovered small regions on that star’s surface where gas is sometimes
being expelled at one side — while simultaneously splashing down at the other.

Alex Lobel and Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge
and Ronald Gilliland of the Space Telescope Institute in Baltimore, announced their results here today at
the 195th meeting of the American Astronomical Society.

Their observations provide the first direct evidence for such complex flows in the gas surrounding cool
oscillating stars and will help to solve the persistent question: “What physical mechanism drives these
amazing dynamics?”.

From early 1998 to spring 1999, the Space Telescope Imaging Spectrograph (STIS) was used to scan
Betelgeuse’s disk four times and to record spectra from small slices cut across its surface.

“We found the upper atmosphere or chromosphere warmer than the region below it and we observed
that it also contracted and expanded during this period”, says Lobel, an astrophysicist at the CfA. “But,
most surprisingly, a scan in fall 1998 suggests streams of gas that are heading in opposite directions —
with velocities of about 10,000 miles per hour.”

The astronomers can measure these differences in direction because moving material scatters light out
of bright spectral emission lines that emerge from the chromosphere, leaving two distinct peaks. By the
Doppler effect (like the change of pitch while a jet flies over or a train passes by), when the left peak is
higher than the right peak, gas is falling to the star, and the reverse when it is blown off into space.

Throughout the same period, the Faint Object Camera (FOC) was used to make new images of
Betelgeuse’s chromosphere in ultraviolet (UV) light. While the intensity of UV emitted by the
chromosphere varied with the star’s pulsation, brighter and rather subtle intensity patterns appeared at
different locations on the stellar disk. Although an FOC observation by Dupree and colleagues in 1995
revealed a bright spot-like area on Betelgeuse’s surface, the new images — with their improved resolution
— now also revealed that a bright ‘arc-like’ structure spanned a large portion of the disk in September
1998.

“As astonishing as these images may be, they also show us that the precise locations of the brighter
regions remain unresolved,” says Lobel. “Sharper and more frequent images obtained during one
pulsation cycle are needed to pinpoint their physical origin”.

The team is considering several explanations for the brighter structures seen in the images. For example,
huge convection cells produced deep in the photosphere during the oscillations could propel denser and
hotter gas into the chromosphere. Or, enormous shock waves generated by the pulsation could
fragment into smaller “shock trains” that climb into the chromophere and then migrate randomly across
the surface, leaving extra UV light in their hotter colliding wakes. Or, perhaps, unexpected strong
magnetic fields could form long hollow tubes that pierce the chromosphere, thus allowing hotter gas to
pour in from below.

“For all these scenarios, the new images show that the upper atmosphere changes in a rather unordered
manner just like the simultaneous up- and down-flows seen in the STIS spectra,” says Dupree.

One out of a million stars in our galaxy is a supergiant and fewer yet are cool supergiants. Not only is
Betelgeuse a cool supergiant, but it is also the seventh brightest star visible in the northern hemisphere,
appearing in the shoulder of the constellation Orion at a distance of 425 light-years from Earth. Because
its surface temperature can drop to below 3,000 K degrees, it shines reddish. Its atmosphere is like a
big puffy cloud, ten million times less dense than our Sun; and, under such conditions, slight perturbations
have dramatic effects on movements of its atmospheric gases.

Indeed, this star is so big that, if it replaced the Sun at the center of our Solar System, its pulsating
atmosphere would extend almost to the orbit of Jupiter. Additional measurements by Lobel, Dupree, and
Gilliland revealed that the star is wrapped in an even bigger and less dense envelope of warmer gas. Its
chromosphere extends up to 5,000 times the radius of our Sun, or out to Neptune’s orbit, where the
temperature can increase to about 5,000 K degrees.

This work is supported by NASA through the Space Telescope Science Institute and the Smithsonian
Astrophysical Observatory.

EDITORS: Images and caption of this result are available at:

http://cfa-www.harvard.edu/cfa/ep/alobelimg.html

IMAGE CAPTION:

The photo-montage made in ultraviolet light shows the pulsating atmosphere of Betelgeuse. The star is
scanned with the Space Telescope Imaging Spectrograph. Seven spectra are recorded through a small
field of view shown by the rectangle at different positions on the disk, marked by the crosses. The thick
yellow curves show the shape of a double-peaked emission line which emerges from Betelgeuse’s
chromosphere. The depression between the peaks is formed far out in this warmer envelope. The
spectra in the top images of January and March ’98 reveal that this outer region collapses since the
left-hand peak of the emission line is everywhere stronger than the right. But the STIS scan in the lower
left image of September ’98 unveils how atmospheric gas begins to move up in the fifth scan position,
where the right-hand peak exceeds the left. Here gas streams in opposite directions at the same time
through the star’s outer atmosphere. The spectral scan in the lower right image of March ’99 shows
that the expanding trend proceeds and extends further across the chromosphere. The arrows in the
images point North in the plane of the sky and should be aligned when comparing the location of surface
details. Note that the mean intensity levels of the four images have been equalized to bring out these
details. The top images are actually observed brighter than the lower ones. Note also that some
spectral scans are taken a week apart from the images.