The term “Galaxy” is derived from the Greek word gala, which means “milk.” Studying the Milky Way provides your science students with a new way to decipher the breathtaking views they see in the night sky, allowing them to better understand how the Universe formed and continues to evolve. With their billions of stars, gas, and dust, galaxies like the Milky Way trace the structure of the visible Universe.
According to astronomers, the Milky Way left its “poor old heart” in and around the constellation Sagittarius. New Gaia data reveal the full extent of what appears to be the galaxy’s original nucleus — the ancient stellar population around which the rest of the Milky Way grew — which came together more than 12.5 billion years ago.
“People have long speculated that such a vast population [of old stars] should exist in the center of our Milky Way, and Gaia now shows that they do,” says astronomer Hans-Walter Rix of Germany’s Max Planck Institute for Astronomy.
The Milky Way’s ancient heart is a round protogalaxy that spans nearly 18,000 light-years and contains roughly 100 million times the mass of the sun in stars, or about 0.2 percent of the Milky Way’s current stellar mass, Rix and colleagues report in a study published on arXiv.org.
This study really helps to firm up our understanding of this very, very, very young stage in the Milky Way’s life. We’ve seen glimpses of this population before, but the new study provides a bird’s-eye view of the entire structure.Vasily Belokurov
“This study really helps to firm up our understanding of this very, very, very young stage in the Milky Way’s life,” says Vasily Belokurov, an astronomer at the University of Cambridge who was not involved in the research. “Not much is known about this period of the Milky Way’s life,” he says. “We’ve seen glimpses of this population before,” but the new study provides “a bird’s-eye view of the entire structure.”
Most stars in the Milky Way’s central region abound with metals, because the stars originated in a crowded metropolis that earlier stellar generations had enriched with those metals through supernova explosions. But Rix and his colleagues wanted to find the exceptions to the rule, stars so metal-poor they must have been born well before the rest of the galaxy’s stellar denizens came along — what Rix calls “a needle-in-a-haystack exercise.”
His team turned to data from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way. The astronomers searched about 2 million stars within a broad region around the galaxy’s center, which lies in the constellation Sagittarius, looking for stars with metal-to-hydrogen ratios no more than 3 percent of the sun’s.
The astronomers then examined how those stars move through space, retaining only the ones that don’t dart off into the vast halo of metal-poor stars engulfing the Milky Way’s disk. The end result: a sample of 18,000 ancient stars that represents the kernel around which the entire galaxy blossomed, the researchers say. By accounting for stars obscured by dust, Rix estimates that the protogalaxy is between 50 million and 200 million times as massive as the sun.
“That’s the original core,” Rix says, and it harbors the Milky Way’s oldest stars, which he says probably have ages exceeding 12.5 billion years. The protogalaxy formed when several large clumps of stars and gas conglomerated long ago, before the Milky Way’s first disk — the so-called thick disk — arose.
Because the protogalaxy is compact, little has disturbed it since its formation. Smaller galaxies have collided with the Milky Way, increasing its mass, but “we didn’t have any later mergers that deeply penetrated into the core and shook it up, because then the core would be larger now,” Rix says.
The new data on the protogalaxy even capture the Milky Way’s initial spin-up — its transformation from an object that didn’t rotate to one that now does. The oldest stars in the proto-Milky Way barely revolve around the galaxy’s center, instead diving in and out of it, whereas slightly younger stars show increasing movement around the galactic center. “This is the Milky Way trying to become a disk galaxy,” says Belokurov, who saw the same spin-up in research that he and a colleague.
Today, the Milky Way is a massive galaxy that spins rapidly — our solar system travels 900,000 kilometers per hour as we race around the galaxy’s core. The new study, however, shows that the Milky Way began as a small protogalaxy whose stars can still be seen today, stars that astronomers can now examine for more information about the galaxy’s birth and early evolution.