A multinational team of astronomers with the Sloan Digital Sky Survey-III (SDSS) has produced a new map of our home Milky Way Galaxy and determined that nearly 30 % of stars have dramatically changed their orbits.
This image shows how stellar orbits in the Milky Way Galaxy 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. Image credit: Dana Berry / SkyWorks Digital, Inc. / SDSS collaboration.
“In our modern world, many people move far away from their birthplaces. Now we are finding the same is true of stars in our Galaxy – about 30% of the stars have traveled a long way from where they were born,” explained Michael Hayden of New Mexico State University, team member and first author of a paper published in the Astrophysical Journal (arXiv.org preprint).
To build a new map of our Galaxy, the team used the SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe almost 70,000 red giant stars during a four-year period.
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,” Mr 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. 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,” said co-author Prof Jon Holtzman, also from New Mexico State University.
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 Milky Way, 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.
Prof Holtzman, Mr Hayden and co-authors used APOGEE data to map the relative amounts of 15 separate elements, including carbon, silicon, and iron, for their stellar sample.
What they found surprised them – up to 30% of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions.
“While on average the stars in the outer disk of the Milky Way have less heavy element enrichment, there is a fraction of stars in the outer disk that have heavier element abundances that are more typical of stars in the inner disk,” said co-author Dr Jo Bovy from the University of Toronto’s Institute for Advanced Study.
When the astronomers 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 this study is the first clear evidence that migration occurs throughout the Milky Way Galaxy.
“The migration process we describe took place over the life of the disk of the Milky Way, so over the last 10 billion years. We found the evidence for the process within our survey data taken from 2011 to 2014 and continuing,” said Prof Steve Majewski from the University of Virginia.
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Michael R. Hayden et al. 2015. Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk. ApJ 808, 132; doi: 10.1088/0004-637X/808/2/132
