A team of astronomers, led by Alex Lobel of the
Harvard-Smithsonian Center for Astrophysics (CfA), announced today at the 203rd
meeting of the American Astronomical Society in Atlanta that Hubble Space
Telescope observations of a nearby supergiant star directly show hot gas
escaping its boiling atmosphere at a larger distance than from any other star.
The expelled hot gas somehow survives the cold and harsh conditions in the
star’s bloated upper atmosphere.

New observations with the Space Telescope Imaging Spectrograph (STIS), Hubble’s
high-precision and ultra-sensitive spectrometer, show that the warm chromosphere
of Betelgeuse extends out to more than fifty times its radius in visible light,
a size five times larger than the orbit of Neptune. (The chromosphere is an
inner layer of a star’s atmosphere, between the photosphere and the corona. The
Sun’s chromosphere is visible as a thin reddish line during a total solar
eclipse, and extends outward for only a fraction of a solar radius.)

STIS detected the spectral signatures of tenuous hot gas in cold, remote, and
dusty places of Betelgeuse’s mammoth atmosphere. The observations help to
determine the mechanisms that form and sustain warm gaseous envelopes in many
other red and yellow stars, including the Sun.

The team investigated the atmosphere of Betelgeuse, the brightest star in the
constellation Orion, over the past five years with the STIS instrument aboard
Hubble. They found that the bubbling action of the chromosphere tosses gas out
one side of the star, while it falls inward at the other side, similar to the
slow-motion churning of a lava lamp.

"Betelgeuse’s upper chromosphere extends into the enormous cloud of cold dust
around this supergiant star. Our basic knowledge of how chromospheres form
should explain how it sends this warm gas so far into space. There is plenty of
gas below 2000 degrees Fahrenheit because of dust, but this gas is apparently
joined by much hotter ionized gas from the chromosphere near the star’s
surface," said Lobel.

Before the discovery, telescopes on Earth detected the warm gas in the star’s
weak chromosphere up to only about five times the star’s radius, a size bigger
than Saturn’s orbit. (The photospheric surface of Betelgeuse is about as large
as Jupiter’s orbit.)

When a match flame warms the air above it, the heat quickly disperses into the
cooler surrounding air. In Betelgeuse’s upper atmosphere, hot and cold gas mix
together, but the warm gas does not entirely dissipate away until far above the
heights where much colder gas is observed.

The new STIS spectra in ultraviolet light show that very remote parts of the
chromosphere contain hot gas above 4220 degrees Fahrenheit (2600 Kelvin). The
cold neighboring gas, however, is not warmer than 2240 degrees F (1500 K).
Higher temperatures would destroy the dust particles that glow in infrared light
at a large distance from the supergiant.

Running Shock Waves

The astronomers considered several explanations for the joint presence of hot
and cold gas in the upper chromosphere of this gargantuan star. One explanation
calls for long trains of shock waves which run through the chromosphere. The
front of a shock wave compresses the gas and heats it up. It chills in the
expanding wake of the passing wave. The shocks are strong enough to warm up a
large volume of gas far above the supergiant’s surface. The temperature in their
long wakes, however, decreases so rapidly that dust grains can form without
being completely destroyed by following waves.

The new observations also show that the outflow of warm gas accelerates with
larger distance in the upper chromosphere and dust shell. "This further supports
the shock wave model," said Lobel. "If the atmosphere were static, the observed
temperature differences would disappear with the natural exchange of heat."

Warm material moves far above the surface of Betelgeuse in a dynamic balance of
heat with colder gas inside its dust cloud. When large volumes of warm and cold
air collide in Earth’s atmosphere, devastating tropical storms can form with
wind powers that lift up cars. Similarly, the chromosphere of Betelgeuse is very
turbulent. The STIS spectra show that the speed of the turbulence is faster than
the local sound velocity. This supersonic turbulence could result from running
shocks or from the flow of energy between the newly discovered warm and cold gas.

Pulsations of the Chromosphere

Other models without shock waves consider oscillations of the chromosphere.
Parts of the star’s unstable surface sometimes vigorously bulge out in different
directions, piercing long warm plumes into the cold dust envelope. At large
distances from the surface, the density of the cold atmosphere strongly
decreases, which prevents it from absorbing the heat carried by the intruding
warmer plumes. These plumes cool off only far beyond the regions observed by
STIS, where the density decreases to levels similar to the cold gas, like a
plume of steam cools higher above the nozzle of a boiling kettle.

The team plans to request new high-precision observations to find out if the
chromospheres of other nearby supergiants also extend so far into space. "We
would like additional observations to confirm the presence of warm gas at
distances as great as fifty stellar radii," said team member Jason Aufdenberg, a
former research fellow at CfA, now at the National Optical Astronomy Observatory.

The search for the reasons why chromospheres are produced around stars started
decades ago with pioneering observations of the faint chromosphere of the Sun,
which extends only a few percent of the radius above the surface. One out of
million stars in our Galaxy is a supergiant like Betelgeuse, which even at a
distance of 425 light-years is the seventh brightest star visible in the
northern hemisphere. The new observations of its puffed-up chromosphere with the
Hubble Space Telescope bring scientists an important step closer to completing
that search.

The team presented parts of the research in this release at a meeting of the
International Astronomical Union of last July. See also
http://arXiv.org/abs/astro-ph/0312076 .

This research was supported by NASA and the Smithsonian Astrophysical
Observatory. The science team mentioned in this press release consists of Drs.
Alex Lobel, Andrea Dupree, Robert Kurucz, Robert Stefanik, Guillermo Torres (all
at the Harvard-Smithsonian Center for Astrophysics, Mass.), and Jason Aufdenberg
(National Optical Astronomy Observatory, Arizona).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists, organized into
six research divisions, study the origin, evolution and ultimate fate of the

Note to Editors:

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