e Space Telescope. The first direct image of the surface of a star other than the Sunshows an enormous object with an extended atmosphere and a mysterious, hot, bright spot -- whose diameter is more than 400 times the size of the Sun -- on its surface.The huge size of Betelgeuse revealedby this picture not only challenges standard stellar models, but the surprising bright structure in its atmosphere mayhave implications for understanding both the evolution of stars and the material surrounding them.
Andrea Dupree of the Harvard-Smithsonian Center forAstrophysics in Cambridge, MA, and Ronald Gilliland of the SpaceTelescope Science Institute in Baltimore, MD, announced theirdiscovery today at the American Astronomical Society meeting in SanAntonio, Texas.
The picture suggests that a totally new physical phenomenon may beaffecting the atmospheres of some stars, according to theresearchers. ``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 enormousbright area more than 200 K degrees hotter than the surrounding surface of thestar.'' Until follow-up observations ofBetelgeuse are completed, however, astronomers won't know the mystery spot's truenature--or how it formed.
For example, said Dupree, the spot might change position over time.Earlier observations of Betelgeuse by Dupree and colleagues using theInternational Ultraviolet Explorer satellite revealed that the starhas a 420-day period, during which it oscillates, or ``rings'' like abell. The oscillations, thought to be caused by turbulence below thesurface of the star, might cause changes in the bright spot. Futureoscillations might spur other bright spots on different regions ofthe star's surface, causing it to wink on and off like blinkinglights on a Christmas tree, said Dupree.
Alternatively, the spot might move systematically across the star'ssurface, which would imply that the star has magnetic fields strongenough to hold the bright spot's hot gas in position. Either scenariowould lead astronomers to re-think completely current ideas of howstars evolve. ``We hope this work will also pave the way for ageneration of space interferometers,'' said Dupree. Such instruments,which would use a cluster of widely-spaced telescopes whose separate observations are combined into a single image, could improve the resolution of structures on stellar surfaces.
A red supergiant star in the constellation Orion, Betelgeuse has longbeen a favorite target for star-imaging studies from theground because it is solarge, and hints of surface structure were noted earlier. Corrections made to HST's Faint Object Camera during the 1993 Hubblerepair mission made the new, high-resolution, ultraviolet imagepossible.
Additional measurements by Dupree and Gilliland revealed thatBetelgeuse's extended atmosphere is about two times larger in theultraviolet bands than in visible wavelengths. ``This star is like abig, puffy cloud--ten million times less dense than our Sun, butnearly 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 extendpast the orbit of Jupiter. The seventh brightest star visible in thenorthern hemisphere, Betelgeuse marks the shoulder of Orion, aconstellation now visible in the night sky, and appears reddishbecause it is so cool. Located over 500 light-yearsaway, the starlight we now see from Betelgeuse left its surfacearound the time that Columbus sailed to America.
A report on the new findings has been submitted to the AstrophysicalJournal Letters. This work is supported by NASA through theSpace Telescope Science Institute and the Smithsonian AstrophysicalObservatory.
Alpha Orionis Photomontage (3 Megabyte Postscript file)
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