30% of Milky Way Stars Moved Far from Their Home Orbits

This brief animation shows how stars can change orbits. The animation shows two pairs of stars (marked as red and blue). Within each pair of stars, both start in the same orbit. At random times, one star of the pair changes orbit, moving in our out in our Galaxy. Lines trace the path of each star. Because of the greater density of stars in the inner Milky Way, more stars migrate out than in.

Credit: Sloan Digital Sky Surveys

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 says.

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,” says 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 of “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 in other regions — 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 what part of the Galaxy the star was born in.

The image shows two pairs of stars (marked as red and blue) in which each pair started in the same orbit, and then one star in the pair changed orbits. The star marked as red has completed its move into a new orbit, while the star marked in blue is still moving.

Credit: Dana Berry / SkyWorks Digital, Inc.; SDSS collaboration

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,” says 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.

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,” says Steven 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.”