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For around 20 years, astronomers have struggled to find an ancient group of stars mixed in with the gas, dust, and newer stars of our galaxy’s bulge. These “fossil” stars preceded the Milky Way and should have been discernible by their distinctive chemistry and orbits. Yet until recently, only a small number of them had ever been found.
Now, a determined effort using data-intensive machine learning has unearthed a trove of them, bringing into focus their features and fates. The methods used in their discovery have enabled scientists to update their understanding of the Milky Way’s formation and of disk galaxies in general.
Competing Theories
Astronomers believe that the Milky Way was preceded by something called a proto-galaxy—a violent, chaotic place containing young stars with wild orbits. Its origin story starts out credibly enough. After the Big Bang, dark matter coalesced in our region of space. The dark matter attracted ordinary matter. The first waves of stars then arose, but how these stars got there was anyone’s guess.
“People didn’t have a really good idea of what the proto-galaxy looked like,” said Vedant Chandra, an astrophysicist at Harvard University and one of the lead authors on a recent paper detailing the ancient star discoveries.
By the 2000s, scientists had settled on two formation theories. Either the proto-galaxy gave birth to the Milky Way’s first stars internally, as gas coalesced into stars, or it cannibalized other galaxies, ripping out stars and siphoning off dark matter. To settle the question, astronomers would need to isolate the Milky Way’s earliest star population. Studies identified candidate stars, but if the internal-nursery theory was correct, a much larger fossil population lay undiscovered.
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There once was a cosmic seed that sprouted the Milky Way galaxy. Astronomers have discovered its last surviving remnants.
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