It's easy to think of stars as being fixed in place, because that's how we see them in the sky. But like Earth and the other planets, they have orbits. And it turns out those orbits can change dramatically. In creating a new map of the Milky Way as part of the Sloan Digital Sky Survey (SDSS), scientists recently discovered that around 30 percent of the stars in our galaxy have done exactly that – they've moved into a totally new orbit.

The scientists came upon this revelation by studying the chemical composition of each star, which is evident in the spectra – or the range and intensity of light wavelengths coming from the star – with different lines in a spectrograph corresponding to elements and compounds.

"Stellar spectra show us that the chemical makeup of our galaxy is constantly changing," explains New Mexico State University professor Jon Holtzman, who was involved in the study. "Stars create heavier elements in their core, and when the stars die, those heavier elements go back into the gas from which the next stars form."

The amount of heavy elements in each star tells astronomers which part of the galaxy it was born in, like a stellar fossil record. But data from the SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph, which studied 100,000 stars during a four-year period, suggests that as many as 30 percent of stars have moved far from their birthplace.

The scientists believe that the pattern can be explained by a model in which stars gradually move closer or farther from the center of the galaxy as they get older. This radial migration is thought to be caused by irregularities in the Milky Way's galactic disk – the spiral arms of gas, dust, and young stars.

But this discovery is just the tip of the iceberg for APOGEE data.

"Once we unlock the full information content of APOGEE, we will understand the chemistry and shape of our galaxy much more clearly," said APOGEE's principal investigator Steven Majewski.

A paper describing the study was published in The Astrophysical Journal.