A cosmic egg wrapped in a gaseous shell.

A cosmic egg wrapped in a gaseous shell

Like Laurel and Hardy, the two whirling stars at the heart of a fuzzy patch of light known as Kohoutek 1-2 (K 1-2) are a mismatched pair. The smaller, hotter star tugs at its flabby, relatively cool companion, pulling it into a shape reminiscent of an egg. Its intense radiation heats the companion's facing side to a bluish-white color, while the opposite, cooler side stays reddish-orange. With its distinctive coloration, the companion star resembles a giant Easter egg.

"We believe this is the largest such [heating] effect yet found," says astronomer Howard E. Bond of the Space Telescope Science Institute in Baltimore. Bond's discovery came in the course of a systematic study of variations in the brightness of stars surrounded by expanding clouds of gas, or planetary nebulas. He reported his findings to an American Astronomical Society meeting this week in Kansas City.

K 1-2 is a very faint object about 7,000 light-years from earth in the direction of the southern constellation Pyxis. Bond's measurements show that the two stars making up this binary system are so close together -- just one or two stellar diameters apart -- that they appear as one. "They're almost but not quite touching," says Bond. The two stars orbit their center of mass every 16 hours. They lie enveloped in a glowing shell of gas made visible by the hotter star's intense radiation.

Out of roughly 40 planetary nebulas studied so far, Bond and his colleagues have discovered about half a dozen in which the central stars prove to be close pairs of stars. "This surprisingly high incidence of close binaries is starting to suggest that at least some planetary nebulae are actually ejected because of the interaction of binary stars," Bond says.

Until recently, astronomers thought red giant stars were the main source of planetary nebulas. Toward the end of its life, a red giant would become so large and its surface gravity so low that its outer layers could easily become detached. However, the exact mechanism by which a red giant could slough off its skin wasn't clear.

Bond proposes that the ejection of gaseous matter is a way in which a binary system can give off its excess energy as its two stars spiral closer together and as one draws matter from its companion in an act of stellar cannibalism. "It's always been a mystery not only where planetary nebulae come from but also where very close binary stars come from," Bond says. This mechanism allows widely separated binaries to evolve into close binaries.

The presence of a planetary nebula is a clue showing the violent interactions that can take place when two stars approach each other, says Bond. In a few billion years, the stars will get close enough to coalesce, setting the stage for a nova explosion.