One-third of Milky Way Stars have Changed their Orbits.

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Nearly one-third of the stars in our own galaxy have dramatically changed their orbits, say a team of researchers from the New Mexico State University with the Sloan Digital Sky Survey [SDSS] who have created a new map of the Milky Way.

“In our modern world, many people move far away from their birthplaces, sometimes halfway around the world, 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,” said Michael Hayden, astronomy graduate student and lead author of the new study.

To build the new map of the Milky Way, the scientists used the Sloan Digital Sky Survey [SDSS] telescope's Apache Point Observatory Galactic Evolution Explorer [APOGEE] spectrograph to observe 100,000 stars during a 4 year period.

The key to creating and interpreting this map of the galaxy 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," Hayden said.

The chemical information comes from spectra, which are detailed measurements of how much light the star gives off at different wavelengths. Spectra show prominent lines that correspond to elements and compounds. Astronomers can tell what a star is made of by reading these spectral lines.

“Stellar spectra show us that the chemical makeup of our galaxy is constantly changing,” said Jon Holtzman, NMSU astronomy professor who was involved in the study. “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.”


A single frame from an image shows how stellar orbits in the Milky Way can change. It shows two pairs of stars (marked as red and blue) in which each pair started in the same orbit, and then one star in the pair changed orbits. The star marked as red has completed its move into a new orbit, while the star marked in blue is still moving.

As a result of this process of “chemical enrichment,” each generation of stars has a higher percentage of heavier elements than the previous generation did. 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 new stars have formed. 

This means the average amount of heavier elements in stars varies among different parts of the galaxy. Astronomers then can determine what part of the galaxy a star was born in by tracing the amount of heavy elements in that star.

Hayden and his colleagues used APOGEE data to map the relative amounts of 15 separate elements, including carbon, silicon, and iron for stars all over the galaxy. 

What they found surprised them, up to 30 percent of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions. 

When the team looked at the pattern of element abundances in detail, they found that much of the data could be explained by a model in which stars migrate radially, moving closer or farther from the galactic center with time. These random in-and-out motions are referred to as “migration,” and 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 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,” said 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.”


article reference:- newscenter.nmsu.edu
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