Third of stars in Milky Way have dramatically changed orbit

Researchers have created a new map of the Milky Way which shows that nearly a third of the stars have dramatically changed their orbits.
The discovery, by a team of scientists with the Sloan Digital Sky Survey (SDSS), brings a new understanding of how stars are formed, and how they travel throughout our galaxy.
"In our modern world, many people move far away from their birthplaces, sometimes halfway around the world," said Michael Hayden, New Mexico State University (NMSU) astronomy graduate student and lead author of the new study.
"Now we're finding the same is true of stars in our galaxy - about 30 per cent of the stars in our galaxy have travelled a long way from where they were born," Hayden said.

To build a new map of the Milky Way, scientists used SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe 100,000 stars during a 4-year period.
For the last six years, NMSU astronomers in the College of Arts and Sciences, and collaborators, have been using the 2.5-metre SDSS telescope at the Apache Point, located in the Sacramento Mountains to complete experiments that includes studies of Milky Way stars.

The key to creating and interpreting map of the galaxy is measuring the elements in the atmosphere of each star. 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 compounds. Astronomers can tell what a star is made of by reading these spectral lines.
"Stellar spectra show us that the chemical makeup of our galaxy is constantly changing," said Jon Holtzman, NMSU astronomy professor 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," Holtzman said.
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 - and in these more vigorous regions, more generations of new stars have formed.
Astronomers can determine what part of the galaxy a star was born in by tracing amount of heavy elements in that star.
Hayden and his colleagues used APOGEE data to map the relative amounts of 15 separate elements, including carbon, silicon, and iron for stars all over the galaxy.
They found that up to 30 per cent of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions.

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 radially, moving closer or farther from the galactic centre 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, researchers said.