You can take the star out of the galaxy's heart, but not the heart of the galaxy out of the star. Stars carry with them a chemical trace of the region in which they were born, but almost a third of Milky Way giant stars end up a long way from home.
The Sun circles the galactic center at a speed of 230 kilometers per second (143 miles/second), completing an orbit four times each billion years. Other orbital periods vary with distance from the core. However, it used to be thought that most maintained regular paths, with nearby stars such as Sirius and Vega keeping us company at a fairly constant distance.
Some stars buck this trend, most famously Barnard’s Star: Currently almost six light-years away, it will be 3.75 light-years from the Sun in 11,800 years, reflecting the difference in the two stars' paths. Several other nearby red dwarfs show similar behavior, but now astronomers from 26 institutions have measured the frequency of such movement galaxy-wide, using almost 70,000 red giants.
The sample size is a huge increase from previous studies, which have usually involved just a few hundred nearby objects, the authors note in a paper published in The Astrophysical Journal.
Lead author Michael Hayden, a Ph.D. student at New Mexico State University, said, "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 where they were born."
The giant (sorry) sample was observed using the Sloan Digital Sky Survey APOGEE spectrograph as part of a quest to understand how the galaxy evolved. The team observed the spectral lines in the sample stars to find their chemical composition.
"From the chemical composition of a star, we can learn its ancestry and life history," said Hayden. Co-author and professor Jon Holtzman explained, "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." With each generation of stars, more heavy elements are created, so the proportion of these elements in a star provides an indication of how many rounds of star formation have occurred in the region around the star’s birthplace.
Stellar generations run faster closer to the center of the galaxy, thanks to the greater concentration of raw materials, so the chemical composition of a star can give us a good indication of how far out it formed.
When Hayden and Holtzman looked at the concentrations of 15 elements in their huge stellar sample, they found 30% had compositions very different from what would be expected if they were born close to their current location.
On the other hand, the compositions could be explained if a large number of stars undergo radial mixing towards and away from the galactic center, a theory previously proposed but not universally accepted.
The researchers regard this as the first of many revelations that will come from the survey of so many stars.
Animation showing how stars that are born close together can separate with sudden shifts towards and away from the center of the galaxy. Credit: Dana Berry/SkyWorks Digital, Inc.; SDSS collaboration