Wolf-Rayet stars are hot, very luminous and massive--typically three times the size and 25 times heavier than our sun and 100,000 times as bright. WR 104 is located 4,800 light years from Earth, in the direction of the constellation Sagittarius. The animation shown here was created by a team of researchers from the University of California at Berkeley, who pieced together a series of images made by the advanced imaging system on the Keck I telescope on Mauna Kea in Hawaii. The research team, headed by William C. Danchi of U.C. Berkeley's Space Sciences Laboratory Infrared Spatial Interferometer Group, offered an explanation for the star's odd shape in the April 8, 1999, issue of Nature. They believe that hidden in WR 104's glow is a smaller blue companion star of the OB type, forming a binary system consisting of two stars closely orbiting each other.
The Wolf-Rayet star is so hot and bright that its radiation drives off its outer atmosphere, creating a dense, high-velocity stellar wind surrounding the star. "Wolf-Rayet stars are so bright that they are literally flying apart," Danchi says. The plume is apparently dust in the stellar wind that glows in the infrared.
The trouble is that the radiation from the star should be so intense that it would incinerate any stellar dust almost as soon as it formed. So why is there a relatively cool area near WR 104 that permits dust to exist? This is where the second star comes in. When the stellar wind from the companion meets the wind from the Wolf-Rayet star, a shock front forms, which compresses and cools the material from the stellar winds. "It is in this 'cocoon,' shielded from the direct glare of the stars, that dust formation may flourish," explains Peter G. Tuthill, an assistant research physicist at the Berkeley laboratory.
Further evidence supporting the role of stellar shock waves in stardust formation comes from a team headed by Anthony Moffat of the department of physics at the University of Montreal. They trained NICMOS, the Near Infrared Camera and Multi-Object Spectrometer, of the Hubble Space Telescope on another Wolf-Rayet and O-type binary system known as HD 192641. In a forthcoming paper in the Astrophysical Journal they will report that dust is forming by compression as two stellar winds collide head-on at high speed (each wind flowing at some 2,000 kilometers per second, or seven million kilometers per hour).
The spiral of WR 104 results from the fact that binary stars, like planets, are in constant motion. The dust nursery at the collision front between the stellar winds is carried around with the orbital motion of the companion star, completing a rotation once every 220 days. The dust is spewing straight out, but like the jet of a rotating lawnsprinkler, it appears as a spiral when seen from above. The spiral is about 200 astronomical units across (one AU is the distance from the Earth to the sun), or 18 billion miles.
To obtain the detailed images of an object so distant, the researchers used a technique called aperture masking interferometry. Interference patterns from light admitted through 36 small circular regions of the Keck can be reconstructed into images that could only be obtained by a much larger telescope. The technique is much less sensitive to atmospheric twinkling than direct imaging and allows the astronomers to achieve a resolution of several tens of milliarcseconds.
The astronomers expect to turn up other star shapes that may be akin to the menagerie of spirals and other forms found in galaxies. "People have always assumed that stars were spherical or disk-shaped, because they couldn't image them," Danchi says. "With interferometric techniques like this, we are seeing stars with unexpected shapes and more complex shapes than we expected."