WASHINGTON, D.C. — About 5,000 light years away across our Milky Way galaxy, a highly brilliant star called VY Canis Majoris has long been thought to have smoke in its eyes because most of its light is blacked out by a cloud.

Now the mystery of this smoky shroud is partly unveiled. It turns out that the star appears to be blinded not by smoke but by sand and by whiskers, a form of iron.

The latest research on VY Canaris Majoris is being presented today (Jan. 10) at the American Astronomical Society meeting by Martin Harwit, Cornell professor emeritus of astronomy. His talk is titled “VY Canis Majoris: Observational Studies of the Outflow.”

Although the light output of VY Canis Majoris is a half million times greater than that of the sun, virtually none of this light is ever seen. Yet the pressure exerted by the outpouring of light is so great that the outer layers of the star are blown off. As this outßow recedes from the star, says Harwit, it cools, and fine dust particles condense to form a dense cloud billowing out like smoke from a giant stack. “The smoke absorbs the star’s light and re-emits this energy, which is heat detectable with infrared-sensing telescopes,” he says.

Using the European Space Agency’s Infrared Space Observatory (ISO), Harwit and his colleagues have found that this smoke consists of tiny particles of two dominant kinds: one made up of grains of iron in a highly elongated form called “whiskers” and the other made up of amorphous silicates — substances resembling common sand on a beach. Silicates are commonly found in nature, and they can be crystalline or amorphous. Crystals have their atoms arranged in a highly regular structure, while amorphous substances are more jumbled. Harwit says that the relative amount of iron, magnesium, silicon and oxygen in the dust particles surrounding VY Canis Majoris are consistent with the abundance in which these elements are found elsewhere in the Milky Way. Says Harwit: “We feel that we have come closer to understanding the nature of the outßow from this puzzling star.”

Harwit’s collaborators on the ISO studies were Koen Malfait, Leen Decin and Christoffel Waelkens of the University of Leuven, Belgium; Helmut Feuchtgruber of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany; and Gary Melnick of the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.

Astronomers have long sought to understand the massive outßow from VY Canis Majoris, which cannot be observed in detail because the star is too distant for telescopes to obtain a clear picture. Early computer models picturing the outßow imagined an outpouring of material into a disk surrounding the star. To best Þt the data, this disk had to be inclined to the line of sight.

These models, says Harwit, were incomplete in the sense that the light pressure driving the outßow from the star was not related to the heat radiated away. This still left the shape of the smoky shroud unresolved. “Was it disk-shaped or spherical?” asks Harwit.

Using ISO data, Harwit and his co-author of a forthcoming paper (Feb. 1, Astrophysical Journal Letters ), Edwin A. Bergin of the Harvard Smithsonian Center for Astrophysics, found that the infrared emission from water vapor in the VY Canis Majoris outflow conformed to the predictions of a previous, more complete spherical model. This had been developed by Moshe Elitzur, Victor Zubko and Zeljko Ivezic at the University of Kentucky. An even deeper analysis of water vapor data obtained with NASA’s Submillimeter Wave Astronomy Satellite also was consistent with this spherical model. “I am sure the star is still hiding a great deal from us,” says Bergin, “but we are getting a bit closer.”

The research was supported by NASA.

Contact: Blaine P. Friedlander, Jr.
Office: 607-255-3290
E-Mail: bpf2@cornell.edu