Yale Researcher Identifies Stars That Give Clues to the Milky Way

A Yale researcher has found up to 100 new and very distant RR Lyrae stars in the Milky Way that can give valuable clues about the galaxy’s history and composition.

Using a large format camera in a telescope at the Llano del hato Observatory in the Venezuelan Andes, Kathy Vivas, a doctoral candidate in Yale’s Astronomy Department, and Astronomy Professor Robert Zinn were able to monitor hundreds of thousands of stars in the “halo,” or the external part of the galaxy.

“As many as 100 new and very distant RR Lyrae stars were discovered,” she said in a paper presented January 12 at the American Astronomical Society meeting in Atlanta. “This will allow, for the first time, the study of the structure and properties of the whole halo, not only its innermost parts.”

The current theory is that the halo was the first component formed in the Milky Way and that the disk and spiral arms, where the solar system is located, came much later. The Milky Way galaxy is at least 12 billion years old.

The stars in the halo of the galaxy are the oldest. A study of their distribution in the sky, their chemical composition, and how they are moving provides clues as to when and how the halo was formed, Vivas said.

“There has been increasing evidence that the Milky Way could have been literally eating small nearby satellite galaxies and many of the stars that now belong to the galaxy could have originated in a different place,” Vivas said. “What fraction of the whole halo population was due to this galactic cannibalism is still unknown.”

RR Lyrae stars, which are named for their prototype in the constellation Lyra, are known to be tracers of the galactic halo. Although there are many of these stars in the galaxy, very few have been discovered far from the Sun.

“This new survey is finding very distant RR Lyrae stars, between 13,000 and 220,000 light years from the Sun,” Vivas said.

She said the observation was made possible by combining a wide field telescope with a mega-pixel camera, which allows astronomers to cover large regions of the sky in a short time. The camera, one of the largest in the world, was recently built in a collaboration between scientists and engineers from the United States and Venezuela. The project was headed by Charles Baltay, chairman of the Physics Department at Yale.

The brightness of RR Lyrae stars varies greatly over a half day period, which means the same region of the sky needs to be observed several times. Automatic computer programs compare the brightness of individual stars in the different images and selects those which present variations to separate the RR Lyrae stars from other stars.