This animation shows how stars can change orbits. It 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 or out. Lines trace the path of each star. (Dana Berry / SkyWorks Digital, Inc.; SDSS collaboration)

In the Milky Way galaxy, you have two kinds of stars. You have your stay-at-home stars and you have your wandering stars.

A new map of the Milky Way, created by scientists with the Sloan Digital Sky Survey (SDSS), shows that a surprisingly large proportion, 30 percent, of its stars are wanderers that have dramatically changed orbits during their lifetimes.

“This discovery, published in [the July 29] issue of The Astrophysical Journal brings a new understanding of how stars are formed, and how they travel throughout our galaxy,” an SDSS news release stated.

To build a map of the Milky Way, the scientists used the SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe 100,000 stars during a 4-year campaign.

“The chemical composition of a star is really a “fossil record” of the chemical makeup of the gas cloud in which the star formed. APOGEE’s main mission is to collect these fossils from all over the galaxy,” said Jonathan Bird, a post-doctoral fellow in the Vanderbilt Initiative in Data-intensive Astrophysics, who was heavily involved in the study.

One of the uses of this astro-fossil record is to determine where stars were born. As they burn, stars create heavy elements. When they die, these elements are dispersed into the surrounding area. As a result, new stars that form in the vicinity are enriched in heavy elements. The process of star formation has proceeded at different rates in different parts of the galaxy, so the level of enrichment varies from place to place. This allows astronomers to narrow in on a star’s birthplace and determine whether it has traveled significant distance, or not.

According to Bird, the APOGEE project is transforming the relatively new field of “galactic archeology” that uses fossil records of stars throughout the galaxy to reconstruct the history of the Milky Way. Previously, interstellar dust has blurred the light coming from stars that lie beyond our local, galactic neighborhood to such as degree that astronomers could not get the high-resolution spectroscopic data they needed to determine the provenance of distant stars. However, APOGEE has solved this problem by using infrared light that penetrates the dust and so can provide the precise information about the velocities and chemical composition of stars that astronomers need throughout the galaxy.

As a result, “the APOGEE survey gives us the opportunity to piece together the formation history of the entire galaxy,” Bird said.