Digital Sky Survey Detects New Stars in the Milky Way

Sharon Guynup
for National Geographic Today
January 31, 2003
Tools like the Hubble Space Telescope have given astronomers a new view of the cosmos, allowing them to gaze deep into the universe to observe far-off galaxies. But a new digital telescope here on Earth has opened a revelatory view of the universe as well as our own galactic neighborhood.

In 2001, an international consortium of about 200 astronomers began the Sloan Digital Sky Survey—the largest international astronomical survey ever undertaken--using a specialized telescope atop a peak in New Mexico's Sacramento Mountains.

Their mission: to map one-quarter of the entire universe out to 1 billion light years from Earth by 2005. It is expected that Sloan will find 500 million galaxies and a slightly larger number of stars—and will accurately chart their positions in the heavens.

"Sloan has changed the way that astronomers do astronomy," says Michael Turner, a cosmologist at the University of Chicago. "With Sloan, you can look at millions of objects: the structure of a galaxy or the universe."

The telescope was designed to see a much wider area of the sky than other telescopes—and uses new digital imaging technology and computer software to detect the dim, older stars that telescopes using photographic techniques missed. The universe contains many more of these faint stars than bright, hot stars.

Finding New Stars Close to Home

At last week's meeting of the American Astronomical Society meeting in Seattle, some of the excitement focused on discoveries much closer to home, including a previously unseen circle of stars surrounding the Milky Way.

What we uncovered was numerous stars that seemed to be where they didn't belong," says Brian Yanny, an astronomer at Fermi National Accelerator Laboratory in Batavia, Ill.

"The Milky Way resembles a big Frisbee," Yanny says. "We kept seeing these hot blue stars at a constant distance from the center of the galaxy that seemed to form a ring."

The ring was hidden from view behind stars and gases on the same visual plane as the Milky Way. But Sloan's digital cameras observe objects in the sky that appear in various colors according to their composition.

"These stars were bluer than stars of our sun's population, which are yellow or red," Yanny says. "This suggests that they were created at a different time or out of a different batch of gas."

How the Milky Way Formed

"The large area covered by Sloan and the accuracy of the multi-color observations allows us to revisit some of the classic questions in astronomy," said Yanny. "How did the Milky Way form? What does the galaxy look like as a whole?"

His team's hypothesis is that the ring may be the remains of a collision between the Milky Way and a smaller galaxy that happened billions of years ago. It's an indication that part of the Milky Way was formed by small galaxies joining together.

Astronomers believe that when small galaxies are pulled apart, the remains often form a stream of stars around larger galaxies.

Yanny and his colleagues estimate that this newly-discovered star belt is about 120,000 light years across. Gravity holds it in a circular orbit around the Milky Way.

"This ring is important because it lets us look at the motion of visible objects," Yanny says. "They give us clues to the nature of invisible dark matter pushing objects through space." Trying to understand the nature of dark matter—and what exactly it is—is the Holy Grail of astronomy.

Cataclysmic Binaries

Sloan is also revealing new space objects within the Milky Way itself. "We're finding a lot of stars in our own galaxy in the process," said Paula Szkody, an astronomer at the University of Washington in Seattle. Some of these are cataclysmic binary stars. These two-star systems are so named because they are no farther apart than the diameter of the sun, 864,000 miles (1.4 million kilometers).

"Many of them are close by—only 300 light-years away—but they are so faint we just couldn't see them before," says Szkody. (One light-year is 5.8 trillion miles or 9.5 trillion kilometers, the distance light travels in a vacuum in a year.) "Now we are finding out what's really around us. Sloan allows us to see the total population of stars in our galaxy."

These paired binary stars are made up of a cool, low-gravity red dwarf star that loses mass to a hotter, high-gravity white dwarf star.

"The theory was that cannibalism (among stars) was quite common," Turner says. "Now we're starting to find them."

Sixty percent of the stars in the sky are binaries, but most are light-years apart and don't interact. Szkody estimates that the Milky Way contains more than 1 million close binaries. But astronomers have detailed information on only a couple of hundred of them.

Using the Sloan telescope, astronomers have discovered a few hundred close binaries and expect to find at least 400 by the end of the survey.

Sloan is expanding the horizons of space. "All of a sudden, questions you couldn't ask can be addressed," said Turner. "What this illustrates is what an explosive period of discovery we're in astronomy because of these new instruments."

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