New Milky Way map shows 30% of stars have changed orbits

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Relocating to a new place doesn’t seem to be a terrestrial phenomenon only as a new map of the Milky Way reveals that in the cosmic world, stars too relocate by changing their orbits.

According to a recently prepared map of the Milky Way by scientists with the Sloan Digital Sky Survey (SDSS), 30 per cent of stars in our galaxy have traveled a long way from the orbits they were born and have dramatically changed their orbits. Researchers say that their discovery brings to the forefront a new understanding of how stars are formed, and how they travel throughout our galaxy.

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 key to creating and interpreting this map is measuring the elements in the atmosphere of each star.

“From the chemical composition of a star, we can learn its ancestry and life history,” says Michael Hayden of New Mexico State University (NMSU), the lead author of the new study.

To collect chemical information about the star, researchers use spectra – detailed measurements of how much light the star gives off at different wavelengths. Spectra show prominent lines that correspond to elements and molecules present. Reading the spectral lines of a star can tell astronomers what the star is made of.

Jon Holtzman, an astronomer at NMSU says that the chemical makeup of our galaxy is changing constantly with stars create heavier elements in their cores and when they die those heavier elements go back into the gas from which the next stars form.

This process, which astronomers call chemical enrichment, makes each new generation of stars have heavier elements than the previous generation. In some regions of the galaxy, star formation has proceeded more vigorously than in other regions — and in these more vigorous regions, more generations of stars have formed. Thus, the average amount of heavier elements in stars varies across different parts of the galaxy. Astronomers can use the amount of heavy elements in a star to determine what part of the galaxy the star was born in.

Researchers, for the study, mapped 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.

Jo Bovy of the Institute for Advanced Study and the University of Toronto, another key member of the research team explained that the outer disk of the Milky Way has less heavy element enrichment; however, they found a fraction of stars in that area that were abundant in heavier elements – something that is typical of stars in the inner disk.

Researchers note that this particular ‘migration’ of stars into new orbits around the galactic center, moving nearer or farther with time are likely caused by irregularities in the galactic disk, such as the Milky Way’s famous spiral arms.

Evidence of stellar migration had previously been seen in stars near the Sun, but the new study is the first clear evidence that migration occurs for stars throughout the galaxy.

Future studies by astronomers using data from SDSS promise even more new discoveries. “These latest results take advantage of only a small fraction of the available APOGEE data,” says Steven Majewski, the Principal Investigator of APOGEE. “Once we unlock the full information content of APOGEE, we will understand the chemistry and shape of our galaxy much more clearly.”

The findings have been published in The Astrophysical Journal.