Sloan Digital Sky Survey (SDSS) scientists who created a new map of the Milky Way found that nearly one-third of the stars in the solar system have radically changed their orbits, the researchers reported in the latest edition of The Astrophysical Journal.
In the study, lead author Michael Hayden, a graduate student in astronomy at New Mexico State University, and an international team of experts spent four years using the SDSS’s Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe more than 100,000 stars and found that about 30 percent of them were no longer in their original locations.
The construction of this new stellar map required the researchers to measure the elements in each star’s atmosphere, Hayden explained. By studying their chemical composition, they can learn the ancestry and life history of these stars, and this chemical data is obtained from the measurements of the light each one gives off at different wavelengths (also known as spectra).
Chemical composition of stars evidence of radial migration
Spectra show lines which correspond to different elements and compounds, and by reading these lines, astronomers can determine the chemical composition of each star. These stellar spectra revealed that the chemical makeup of the galaxy is in a constant state of flux, the study authors said, as heavier elements are formed in stellar cores, then released once older stars star to die off.
Those elements then enter the gas clouds that are used to form the next generation of stars, causing those stars to have a higher percentage of heavier elements than their predecessors. Some regions of the galaxy form stars more quickly than others, causing new star generations to form and increasing the average heavy element content of those stars.
Using this knowledge, astronomers can determine what part of the galaxy a star was born in by tracing the amount of heavy elements in that star, the researchers said. Hayden and his fellow researchers used APOGEE data to map the amounts of 15 separate elements inside stars from all over the galaxy, and found that up to 30 percent had compositions indicating that they had been formed in parts of the galaxy that were far away from their present locations.
Further analysis of these elemental abundance patterns revealed that most of the observations could be explained by a model in which stars migrate radially, or move closer to or farther away from the galactic center with time. These motions are believed to be caused by irregularities in the galactic disc, and while evidence of stellar migration had previously been found in stars near the sun, this is the first evidence that this phenomenon occurs all over the galaxy.
Discussing the research with Vanderbilt’s Dr. Jonathan Bird
Dr. Jonathan Bird, a postdoctoral fellow in astronomy at Vanderbilt University who specializes in galaxy formation and one of the researchers behind the new study, was kind enough to discuss the work with redOrbit via email while on a work trip in Germany. He played an essential role in determining the signature of the “fossil records” of the stars collected by the SDSS.
The study, Dr. Bird explained, “describes an observational result – how the chemistry of stars changes as we look in different regions of the galaxy.” He “provided theoretical context for the observational result” in two major ways. First, he checked simulations to ensure that the general observational results could be recreated by these so-called “wandering stars.”
Once the simulations showed that this was indeed possible, the team developed what Dr. Bird called a “toy model” (a simplified model which used only the relevant physical phenomena and which ignored everything else) in order to see how the stars would behave. This toy model indicated that wandering stars were indeed the solution to the chemical composition problem.
Dr. Bird also said that he “provided a lot of theoretical context to the findings. There are still many different theories of how galaxies come together and this paper provides an important observational constraint on certain classes of these galaxy formation theories. In the manuscript, I discussed this and how our results also have something to say about the galaxy’s past.”
Dr. Bird on the significance of the study and the APOGEE technology
He called the results “quite significant, because we really didn’t know much about the chemical composition of stars outside of our ‘solar neighborhood.’ There were hints that ‘wandering stars’ were needed to explain some results earlier to this, but we now know that wandering stars must happen, and because we now have information from other parts of the galaxy, we can begin to constrain how common wandering stars are.”
This is essential, the Vanderbilt researcher told redOrbit, because wandering stars made it more difficult to distinguish between the galaxy’s past and present. If stars remained where they were born, scientists would have an easier time piecing together the galaxy’s history. This new study, he said, can be combined with future findings to allow astronomers to describe the galaxy’s wandering stars in more detail, and to “rewind the clock” to locate their original birthplaces.
Dr. Bird also touted the APOGEE instrument, which can observe light for 300 stars at a time and which “breaks ground” as a spectrograph capable of operating in the infrared spectrum as well as the visible spectrum. Infrared wavelengths allow scientists to see through that dust prevents them from seeing stars at visual wavelengths, which he said is “a VERY big deal” and the main reason why APOGEE “is discovering brand new things about the galaxy.”
He noted that APOGEE is currently in its second phase, APOGEE-2, and that the survey was given funding to continue its work for another six years to continue studying stars from Apache Point Observatory in New Mexico, as well as funding to create a copy of the spectrograph to be used at the Las Campanas Observatory in Chile. In March, after completing this paper, Dr. Bird was elected APOGEE-2 Science Working Group Chair, he added.
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Image credit: Dr. Jonathan Bird
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