Team discovers a second halo orbiting Milky Way (12/16/2007)

Artist's rendition of the Milky Way galaxy and the two halos that orbit it. A team of scientists, including some from MSU, recently discovered that two halos – not one, as originally thought – orbit our galaxy. Image courtesy of the Sloan Digital Sky Survey

Research by a team of astronomers, including a Michigan State University professor and his students, has found that two halos of stars - not just one - rotate around our Milky Way galaxy.

The research, published in the Dec. 13 edition of the journal Nature, also detailed the motions of the two halos, which were found to be moving in opposite directions.

"By examining the motions and chemical makeup of the stars, we can see that the inner and outer halos are quite different beasts, and they probably formed in different ways at different times," said Daniela Carollo, who was a visiting scientist at MSU last year, when this research took place. Carollo, a researcher at Italy's Torino Observatory, is completing a doctoral program at the Australian National University.

"Although it was once considered a single structure, an analysis of nearly 20,000 stars shows that the halo is clearly divisible into two, broadly overlapping components," said Timothy Beers, MSU Distinguished Professor of physics and astronomy. "This discovery gives us a much clearer picture of the formation of the first objects in our galaxy and in the entire universe."

The halos are composed of stars and are generally moving in opposite directions, Beers said.

The 20,000 stars that were part of this research were observed by the Sloan Digital Sky Survey, an ambitious project that involves more than 300 astronomers from 25 institutions around the world, including MSU.

Beers and Carollo had originally planned to do a traditional analysis of a section of the night sky.

"But," said Carollo, "as we looked at the velocity and chemical composition of the halo data more closely, the evidence for distinct inner and outer halos became unmistakable."

According to the researchers, the inner-halo stars are "chemically younger" than the stars in the outer halo, containing three times more heavy atoms - such as iron and calcium - than the "chemically older" stars in the outer halo.

The first stars that formed after the Big Bang were composed mainly of helium and hydrogen. After extremely short lives these stars exploded as supernovae and began to pollute the universe with heavier metals such as iron, carbon, nitrogen and oxygen. Consequently, older stars have very low levels of these metals.

"This research opens a potential window for exploring the very early universe," Beers said. "Not only can we chemically identify these very old stars, but we can use this knowledge to identify from the motion of the stars which ones are more likely to be members of the outer halo.

"This also provides additional evidence on how our galaxies formed and evolved," he said.

Our sun orbits around the center of the galaxy at an amazing 500,000 miles per hour. The inner halo rotates in the same direction but much slower - at around 50,000 miles per hour. The outer halo spins in the opposite direction at approximately 100,000 miles per hour.

Other members of the research team included MSU postdoctoral research fellow Sivarani Thirupathi, current MSU graduate student Young Sun Lee, and former MSU graduate student Brian Marsteller, as well as John Norris of the Australian National University, and Masashi Chiba at Tohoku University in Japan.

Note: This story has been adapted from a news release issued by Michigan State University

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