HST OBSERVATIONS OF THE PULSATING STAR BETELGEUSE

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, adupree@cfa.harvard.edu
or
Megan Watzke, (617) 495-7463, mwatzke@cfa.harvard.edu