The Milky Way's 'thick disk' is 2 billion years older than scientists thought

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The thick disk began forming stars only 0.8 billion years after the Big Bang. Miscalculating someone's age can be embarrassing...especially when you've been out of business for a few billion years. That may be the case with our own Milky Way, suggests research published March 23 in the journal Nature. In the new study, scientists extrapolated the ages of nearly 250,000 stars in the Milky Way using brightness, location, and chemical composition data collected by two powerful telescopes: the European Space Agency's (ESA) Gaia Orbital Observatory, and the Gaia Large Sky Observatory. The Fiber Spectroscopy Telescope (LAMOST) in China. The team discovered that thousands of stars in a part of the Milky Way known as the "thick disk" began forming about 13 billion years ago — 2 billion years earlier than expected, and just 0.8 billion years after the Big Bang. "Our results provide fascinating details about this part of the Milky Way, such as its birthday, star formation rate, and mineral enrichment history," said lead study author Maosheng Xiang, an astrophysicist at the Max-Planck Institute for Astronomy in Heidelberg, Germany. He said in a statement. "Putting these discoveries together using Gaia data is revolutionizing our picture of when and how our galaxy formed." Illustration showing the anatomy of the Milky Way, with a large bulge in the center and two disks of stars (the thick disk and the thin disk) on either side. (Photo credit: Stefan Payne-Wardenaar/MPIA) in focus The Milky Way is a spiral galaxy about 105,000 light-years in diameter, but not all parts of this spiral are uniform in thickness, composition, or stellar density. Near the center of our galaxy is a massive bulge of stars (and possibly a supermassive black hole whose gravity holds the galaxy together). The galactic disk runs on either side of this bulge and is made up of two main sections. One side of the disk - the "thin disk" - contains most of the stars we can see from Earth, mixed with clouds of star-forming gas. Meanwhile, the "thick disk" is twice as high as the thin disk, but its radius is much smaller and contains only a small portion of the stars we can see in the sky, according to the European Space Agency. It is also believed that this part of the Milky Way is much older - devoid of gas and ended in the days of star formation. In their new study, the researchers looked at stars throughout the Milky Way, focusing on a specific type of star called a sub-giant. These are stars that have stopped generating energy in their cores and are slowly turning into red giants (massive stars on the way to collapsing into white dwarfs). The giant phase is a relatively short period of stellar evolution, which means astronomers can estimate the ages of these stars more accurately, according to the researchers. Because older stars tend to glow in a certain range of brightness and have lower metal content (i.e. elements heavier than hydrogen and helium) than younger stars, the team was able to date their star sample by running data from both telescopes through a computer simulation. The researchers found that the stars in the galaxy's thick disk were actually much older than the stars seen elsewhere—surprisingly, those stars were billions of years older than previous studies suggested. According to researchers, this discovery could rewrite the history of our galaxy. The age differences between the stars in the thin and thick disks suggest that our galaxy formed in two separate phases. First, 0.8 billion years after the Big Bang, stars began forming in the thick disk. This star's formation accelerated dramatically about two billion years later when a dwarf galaxy called Gaia Sausage collided with our young galaxy, ushering in the second phase of galaxy evolution. During this second phase, the thick disk rapidly filled with stars, while the first wave of star formation began in the thin disk. The study authors hope to fill in the details of this story further, after the third data set of Gaia's moon was released in June. "With each new analysis and data release, Gaia allows us to piece together the history of our galaxy in greater detail," Timo Prosti, ESA's Gaia Project scientist who was not involved in this study, said in the statement.

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