It’s hard to think about space on a truly galactic scale. Heck, simple stargazing is difficult enough, so comprehending the motions of enormous celestial bodies across truly incomprehensible distances is nigh impossible. Thankfully, you don’t have to: Scientists at Penn State and the Sloan Digital Sky Survey-III (SDSS) have created the first-ever map of the Milky Way Galaxy that provides evidence that stars migrate throughout it.
“We were able to measure the properties of nearly 70,000 stars in our Galaxy for this particular study using the innovative SDSS infrared spectrograph,” said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State and a coauthor of the study. “This exercise can be described as Galactic archeology. These data reveal the locations, motions, and compositions of the stars, which provide insights into their formation and their history.”
In order to build the map (and the ensuing animation), researchers used something called the Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph. With it, they were able to observe some 10,000 stars over a period of four years.
All told, they estimate that 30% of the stars in the Milky Way Galaxy are somewhere other than where they were originally born. The researchers are able to determine this by measuring the chemical compositions of the stars and the galaxy. Newer generations of stars are richer in heavier chemical compounds than their predecessor, because when old stars die those heavier elements are left in the gas that forms the newer stars. By keeping track of elemental levels in different parts of the galaxy, astronomers can tell where a star originated.
“While on average the stars in the outer disk of the Milky Way have less heavy-element enrichment, there is a small 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.
Most of the migrations can be explained by irregularities in the Milky Way Galaxy, like the spiral arms. The stars appear to move randomly toward and away from the Galactic center, radically in some cases. Though this behavior has been observed in stars nearer our Sun, this is the first evidence that it happens throughout the Galaxy.