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Polar winds and excretion disks.

Polar winds and excretion disks

Red giants and white dwarfs are two of the more familiar types of stars. But stars come in a wide range of colors and sizes. In particular, one recently studied class of stars, known as B[e] supergiants, has a number of remarkable characteristics. These hot, bright, blue stars have surface temperatures higher than 10,000 kelvins. (The sun's surface temperature is 6,000 kelvins.) They are several hundred thousand times more luminous than the sun. However, unlike other known types of supergiant stars, B[e] supergiants have spectra that include unusual emission lines from hydrogen, iron and oxygen atoms. Other spectral features suggest that these stars are surrounded by cool dust and clouds containing molecules such as carbon monoxide.

To account for the unusual spectral features, FRanz-Josef Zickgraf and Roberta M. Humphreys of the University of Minnesota in Minneapolis propose that such stars have a two-component stellar wind. Whereas some gas escapes as fast, radiation-driven wind from the star's two polar regions, the bulk flows out slowly from the star's equatorial region to create a cool, dense "excretion disk" encircling the star. The outflows appear to be strong that such a star sheds a mass equal to the sun's mass in only 10,000 years.

B[e] supergiants apparently represent an advanced stage in the evolution of massive stars. This phase may last less than 100,000 years, during which time the star loses a large fraction of its mass. Such an evolutionary process could lead to the kind of star that exploded as supernova 1987A in the Large Magellanic Cloud, several astronomers suggest.
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Title Annotation:Astronomy
Author:Peterson, Ivars
Publication:Science News
Date:Jul 1, 1989
Words:270
Previous Article:Throwing tantrums in stellar nurseries.
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