These stars also appear to have a completely different origin than their neighbors.
An international team of astronomers has discovered that near the center of our galaxy — about 2,000 light-years within the Milky Way — there is a population of ancient stars. In fact, these stars are more than 10 billion years old, and their orbits in space hint that they have preserved the early history of the formation of our galaxy.
In a first, the team, led by Dr. Andrea Kunder of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany, has kinematically separated this ancient population from the rest of the stars that currently dominate the mass of the central galaxy by using the AAOmega spectrograph on the Anglo Australian Telescope located near Siding Spring, Australia, focusing on a well-known class of stars called RR Lyrae variables.
These stars pulsate in brightness roughly once a day, which makes them a bit more difficult to study than their unchanging counterparts. However, they have the advantage of being “standard candles.” Standard candles are believed to emit the exact same level of brightness at all times, meaning physicists can use them as markers to measure other cosmic phenomenon against.
RR Lyrae stars are only found in stellar populations more than 10 billion years old, and since their level of brightness remains relatively constant, they can be used to determine near exact distance estimations. This means that astronomers can use the age of these stars as a means to explore the conditions in the central part of our Milky Way when it was first formed.
Our Milky Way consists of multiple generations of stars that span from the time of its formation to the present. Since heavy elements, referred to by astronomers as “metals,” are brewed in stars, subsequent star generations become more and more metal-rich. Therefore it is expected that older components of our Milky Way should be metal-poor stars.
However, it turns out most of our galaxy’s central region is dominated by metal-rich stars — meaning they have approximately the same metal content as our sun — and fall within a American football-shaped orbit known as the “bar.” Stars in the bar were found to orbit in roughly the same direction around the galactic center, so it was believed that all stars in the center would rotate in this way.
But to the astronomers surprise, RR Lyrae stars do not follow the football-shaped orbits, but rather have large random motions more consistent with have being formed at a far distance from the center of the Milky Way. "We expected to find that these stars rotate just like the rest of the bar" said lead investigator Kunder in a press release.
Since RR Lyrae stars are moving, their pulsations result in changes in their apparent velocity over the course of the day. Accounting for this, the team was able to show that the velocity dispersion, or random motion, of the RR Lyrae star population was very high compared to the other stars in the Milky Way’s center.
Coauthor Juntai Shen of the Shanghai Astronomical Observatory explained, "They account for only one percent of the total mass of the bar, but this even more ancient population of stars appears to have a completely different origin than other stars there, consistent with having been one of the first parts of the Milky Way to form."
The team’s next steps will be to measure to exact metal content of the RR Lyrae population in hopes of finding additional clues to discover the mysterious history of these stars.
The study was published in The Astrophysical Journal Letters.
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