The Milky Way left its “poor old heart” in and across the constellation Sagittarius, astronomers report. New information from the Gaia spacecraft reveal the complete extent of what appears to be the galaxy’s unique nucleus — the traditional stellar inhabitants that the remainder of the Milky Way grew round — which got here collectively greater than 12.5 billion years in the past.
“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 there they are,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.
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The Milky Way’s historical coronary heart is a spherical protogalaxy that spans practically 18,000 light-years and possesses roughly 100 million instances the mass of the solar in stars, or about 0.2 % of the Milky Way’s present stellar mass, Rix and colleagues report in a examine posted September 7 at arXiv.org.
“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 on the University of Cambridge who was not concerned within the work. “Not much is really known about this period of the Milky Way’s life,” he says. “We’ve seen glimpses of this population before,” however the brand new examine provides “a bird’s-eye view of the whole structure.”
Most stars within the Milky Way’s central area abound with metals, as a result of the celebs originated in a crowded metropolis that earlier stellar generations had enriched with these metals by supernova explosions. But Rix and his colleagues needed to search out the exceptions to the rule, stars so metal-poor they will need to have been born nicely earlier than the remainder of the galaxy’s stellar denizens got here alongside — what Rix calls “a needle-in-a-haystack exercise.”
His workforce turned to information from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way (SN: 6/13/22). The astronomers searched about 2 million stars inside a broad area across the galaxy’s heart, which lies within the constellation Sagittarius, searching for stars with metal-to-hydrogen ratios not more than 3 % of the solar’s.
The astronomers then examined how these stars transfer by area, retaining solely those that don’t dart off into the huge halo of metal-poor stars engulfing the Milky Way’s disk. The finish outcome: a pattern of 18,000 historical stars that represents the kernel round which your entire galaxy blossomed, the researchers say. By accounting for stars obscured by mud, Rix estimates that the protogalaxy is between 50 million and 200 million instances as large because the solar.
“That’s the original core,” Rix says, and it harbors the Milky Way’s oldest stars, which he says in all probability have ages exceeding 12.5 billion years. The protogalaxy shaped when a number of massive clumps of stars and gasoline conglomerated way back, earlier than the Milky Way’s first disk — the so-called thick disk — arose (SN: 3/23/22).
The protogalaxy is compact, which suggests little has disturbed it since its formation. Smaller galaxies have crashed into the Milky Way, augmenting its mass, however “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 information on the protogalaxy even seize the Milky Way’s preliminary spin-up — its transition from an object that didn’t rotate into one which now does. The oldest stars within the proto–Milky Way barely revolve across the galaxy’s heart however dive out and in of it as a substitute, whereas barely youthful stars present an increasing number of motion across the galactic heart. “This is the Milky Way trying to become a disk galaxy,” says Belokurov, who noticed the identical spin-up in analysis that he and a colleague reported in July.
Today, the Milky Way is a huge galaxy that spins quickly — every hour our photo voltaic system speeds by 900,000 kilometers of area as we race across the galaxy’s heart. But the brand new examine exhibits that the Milky Way bought its begin as a modest protogalaxy whose stars nonetheless shine right now, stars that astronomers can now scrutinize for additional clues to the galaxy’s delivery and early evolution.