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UVA astronomers track stars in our galaxy as they change orbit

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uva-logoRight now, in a galaxy right here, stars are changing their orbits.

Scientists from the University of Virginia and other schools, working with the Sloan Digital Sky Survey, have created a new map of the Milky Way and determined that 30 percent of stars have dramatically changed their orbits. This discovery of significant stellar migration brings a new understanding of how stars are formed and travel throughout the galaxy.

To build a map of the Milky Way, the scientists used the SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe 140,000 stars during a three-year campaign. This project was led by astronomers from the University of Virginia, New Mexico State and the Institute for Advanced Study in Princeton, New Jersey, and included a collaboration of dozens of astronomers from around the world.

“The migration process we describe took place over the life of the disk of the Milky Way, so over the last 10 billion years,” said Steve Majewski, a professor of astronomy at U.Va. “We found the evidence for the process within our survey data taken from 2011 to 2014 and continuing.”

Future studies by astronomers using data from SDSS promise even more new discoveries.

“These latest results take advantage of only a small fraction of the available APOGEE data,” said Majewski, the principal investigator of APOGEE. “Once we unlock the full information content of APOGEE, we will understand the chemistry and shape of our galaxy much more clearly.”

The APOGEE survey is made possible by a unique, state-of-the-art, multi-object infrared spectrograph designed and assembled in the labs of the U.Va. Department of Astronomy, Majewski said. This continues to pay dividends in helping astronomers understand the nature of the stellar populations in the galaxy.

“The U.Va. Department of Astronomy is currently in the process of building a second APOGEE spectrograph to be used on a telescope in Chile to explore the parts of the Galaxy only accessible from the Southern Hemisphere,” Majewski said.  “These new results from the first instrument operating from New Mexico only whet our appetites for the new discoveries that await APOGEE’s probe of the Southern Milky Way.”

Majewski said the second instrument will be commissioned in Chile next year.

“The instrument team from the Department of Astronomy, led by faculty members John Wilson and Michael Skrutskie, should be proud of the numerous discoveries about the Milky Way and its constituent stars and planets being made in such a short time since we commissioned the APOGEE instrument on the Sloan Telescope in New Mexico,” Mejewski said.

“In our modern world, many people move far away from their birthplaces, sometimes halfway around the world,” said Michael Hayden of New Mexico State University, the lead author of the new study. “Now we’re finding the same is true of stars in our galaxy — about 30 percent of the stars in our galaxy have traveled a long way from the orbits in which they were born.”

The key to creating and interpreting this map is measuring the elements in the atmosphere of each star. “From the chemical composition of a star, we can learn its ancestry and life history,” Hayden said.

The chemical information comes from spectra, which are detailed measurements of how much light the star gives off at different wavelengths. Spectra show prominent lines that correspond to elements and molecules present. Reading the spectral lines of a star can tell astronomers what the star is made of.

“Stellar spectra show us that the chemical makeup of our galaxy is constantly changing,” said Jon Holtzman, an astronomer at NMSU who was involved in the study. “Stars create heavier elements in their cores, and when the stars die, those heavier elements go back into the gas from which the next stars form.”

As a result of this process, called chemical enrichment, each generation of stars has a higher percentage of heavier elements than the previous generation did. In some regions of the galaxy, star formation has proceeded more vigorously than elsewhere — and in these more vigorous regions, more generations of stars have formed. Thus, the average amount of heavier elements in stars varies across different parts of the galaxy.  Astronomers can use the amount of heavy elements in a star to determine in which part of the galaxy the star was born.

Hayden and colleagues used APOGEE data to map the relative amounts of 15 separate elements, including carbon, silicon and iron, for stars all over the galaxy. What they found surprised them — up to 30 percent of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions.

“While on average the stars in the outer disk of the Milky Way have less heavy element enrichment, there is a fraction of stars in the outer disk that have heavier element abundances that are more typical of stars in the inner disk,” said Jo Bovy of the Institute for Advanced Study and the University of Toronto, another key member of the research team.

When the team looked at the pattern of element abundances in detail, they found that much of the data could be explained by a model in which stars migrate into new orbits around the galactic center, moving nearer or farther with time. These random in-and-out motions are referred to as “migration,” and are likely caused by irregularities in the galactic disk, such as the Milky Way’s famous spiral arms. Evidence of stellar migration had previously been seen in stars near the sun, but the new study is the first clear evidence that migration occurs for stars throughout the galaxy.

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