Betelgeuse, the second-brightest star in Orion the Hunter, is a luminous red supergiant, and one of the nearest of such stars to Earth -- about 500 light-years. (A light-year is 5.88 trillion miles.) The star is also classified as a semi-regular variable, because it pulsates in brightness over a period of almost six years.

Betelgeuse has an enormous girth. If placed where the Sun is, its outer atmosphere, called the chromosphere, would extend out to Jupiter.

Because of its corpulent size and proximity to Earth, astronomers have been able to produce images of the star's surface by combining images made simultaneously with a number of telescopes a technique known as interferometry. That technique revealed that Betelgeuse has features that are not equally bright across its disk.

Between early 1998 and spring of 1999, the Space Telescope's Imaging Spectrograph was used to scan Betelgeuse's disk. The higher-resolution data reveal gas streams moving in opposite directions at about 10,000 m.p.h. (16,093 kilometers per hour). Moreover, Dr. Lobel's team discovered gas surging away from small regions in the star's chromosphere while splashing down at another.

"One side of the star doesn't know what the other side is doing because it is so big," said Dr. Lobel.

The astronomers hope their new line of evidence will lead to further insights into the mechanism that causes supergiant stars to pulsate so asymmetrically. To do this effectively, said Dr. Lobel, "sharper and more frequent images obtained during one pulsation cycle are needed."

Credit: Harvard-Smithsonian Center for Astrophysics

Like a human heart: Betelgeuse's chromosphere beats asymmetrically (clockwise from upper left-hand image).