Digital sky survey shedding light on faint Milky Way stars

by Vince Stricherz of the University of Washington

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Glitzy tools such as the Hubble Space Telescope let modern astronomers peer deeper and deeper into space, billions of light years from Earth. But it's a small special-purpose telescope on a New Mexico mountaintop that is shedding new light on what lies in our celestial neighborhood.

Using that specialized telescope, the Sloan Digital Sky Survey is turning up dozens of dim binary star systems throughout the Milky Way galaxy, where Earth resides. It turns out there are many more of these systems, with two stars very close to each other, than previously thought.

Cataclysmic variables are binary stars that have occasional major outbursts of light as one star transfers mass to the other. In these close-binary, or two-star, systems, a red dwarf and a white dwarf are locked in a mutual orbit, with the red dwarf spewing its mass into an accretion ring around the white dwarf, which has stronger gravity. As the systems age, they cool and their light grows much fainter, making them a far-less-likely target for many telescopes, said Paula Szkody, a University of Washington astronomy professor. With our galaxy's age, most of its close-binary systems should be in that cool, faint state.

The Sloan project, aimed at mapping the universe out to 1 billion light years, has proven quite adept at picking out the oldest systems with the faintest light and the least transfer of matter from the red dwarf to the white dwarf. It is estimated there are some 1 million close-binary star systems in our galaxy in which there is transfer of mass from one star to the other. About 1,000 of these were previously known, but most of those were among the brightest systems with the most mass transfer. So far, Sloan is responsible for increasing the total by about 10 percent, but it has found the low-mass-transfer variety that models predict should be more common, Szkody said. Sloan also has located the coolest such system ever found, one in which the white dwarf is only 4,700 to 7,700 degrees C (typical systems range from 14,000 to 40,000 degrees C).

Szkody discussed the Sloan findings today at the American Astronomical Society's annual meeting in Seattle. She also is the lead author on a paper describing some of the Sloan work that will be published next month in The Astrophysical Journal.

"What this is saying is we are finding out what's really around us. Sloan allows us to look at the total population of stars in our galaxy to get a better picture of what's out there, instead of just the bright ones found in previous surveys," she said.

The 2.5-meter Sloan telescope near Sunspot, N.M., has been operating since mid-1998, making a three-dimensional map of the universe visible from the Northern Hemisphere out to 1 billion light years (a light year is 5.9 trillion miles). It is expected that, at the end of five years, the project will have found 500 million galaxies and a slightly larger number of stars, and will have accurately charted their positions and determined the brightness and color of each.

Among those stars are fading cataclysmic variables in our own galaxy. The stars are as old as 9 billion to 10 billion years old, about the age of the galaxy. But the rate at which mass accretes from the red dwarf to the white dwarf in close-binary systems has declined significantly over time, as has the systems' brightness. Szkody believes the Sloan telescope is uniquely capable of pinpointing them.

"I think the whole Sloan system is geared to finding systems that are old and cool," she said.

She also hopes to use the 3.5-meter Apache Point telescope to examine the newfound systems more closely. The Apache Point Observatory, also home to the Sloan telescope, is run by the seven-member Astrophysical Research Consortium, of which the UW is a member.

Szkody specifically would like to use the larger telescope to determine orbital periods for a number of these binary systems. The shortest orbital period measured in these systems so far is one in which the two stars orbit each other in about 70 minutes. Such a close orbital period usually indicates that system is very old, she said.

"Having a variety of tools is allowing us to explore zones within our own galaxy that we never could examine before," she said.

For more information, contact Szkody at 206-543-1988 or