HST OBSERVATIONS OF THE PULSATING STAR BETELGEUSE
Cambridge, MA--To the casual observer, stars appear as mere points of light in the night sky, but
thanks to new pictures and spectra of the star Betelgeuse's surface taken by the Hubble Space
Telescope (HST), astronomers are now learning what violent and fascinating objects stars can be.
These new HST images of the stellar behemoth Betelgeuse not only challenge standard stellar
models, but a surprising bright structure found in Betelgeuse's atmosphere may affect how
scientists understand the evolution of stars and the material surrounding them.
Andrea Dupree and Han Uitenbroek of the Harvard-Smithsonian Center for Astrophysics (CfA)
in Cambridge, MA, USA, and Ronald Gilliland of the Space Telescope Science Institute in
Baltimore, MD, USA, announced their results today at the General Assembly of the International
Astronomical Union in Kyoto, Japan.
The HST images suggest that a totally new physical phenomenon may be affecting the
atmospheres of some stars, according to the researchers. "What we see on Betelgeuse is
completely different from what occurs on the surface of the Sun," said Dupree, a senior scientist
at the CfA. "Instead of lots of little sunspots, we find an enormous bright area more than 200 K
degrees hotter than the surrounding surface of the star." The team saw such hot spots when they
observed Betelgeuse both in March 1995 and October 1996.
In addition, ultraviolet spectra reveal that the atmosphere is expanding, the star is rotating, and
its axis of rotation can be identified. The bright hot spots appear to occur near the pole of
Betelgeuse. Earlier observations of Betelgeuse by Dupree and colleagues using the International
Ultraviolet Explorer satellite revealed that the star has a 420-day period, during which it oscillates,
or "rings" like a bell. The oscillations might cause shocks in the atmosphere that break through
the surface near the rotation pole of Betelgeuse causing the bright spots in the chromosphere.
A red supergiant star, Betelgeuse has long been a favorite target for star-imaging studies from the
ground because it is so large, and hints of surface structure were noted earlier.
Additional measurements by Dupree and her colleagues revealed that Betelgeuse's extended
atmosphere is about two times larger in the ultraviolet bands than in visible wavelengths. "This
star is like a big, puffy cloud--ten million times less dense than our Sun, but nearly a thousand
times its diameter," said Dupree.
Indeed, Betelgeuse is so big that, if it replaced the Sun at the center of our Solar System, its outer
atmosphere would extend almost to the orbit of Jupiter. Its diameter, based on the new distances
recently reported by the European Space Agency satellite HIPPARCOS, is about 1500 times the
diameter of our Sun. Betelgeuse is the seventh brightest star visible in the northern hemisphere
and marks the shoulder of Orion, a constellation now visible in the night sky. Betelgeuse always
appears reddish, which indicates that it is a relatively cool star. Located about 425 light-years
away, the starlight we now see from Betelgeuse left its surface around 1572.
This work is supported by the United States National Aeronautics and Space Administration
through the Space Telescope Science Institute and the Smithsonian Astrophysical Observatory.
For more information, contact:
Andrea Dupree, (617) 495-7489, firstname.lastname@example.org
Megan Watzke, (617) 495-7463, email@example.com