According to astronomers’ findings, the Milky Way has abandoned its “poor old heart” in the region surrounding the constellation Sagittarius. The early star population that the rest of the Milky Way formed around was brought together more than 12.5 billion years ago, and new data from the Gaia spacecraft reveal the complete breadth of what appears to be the galaxy’s initial nucleus. This information was obtained from the Gaia spacecraft.

Hans-Walter Rix, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, said that 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. “Gaia has shown that there they are,” Rix said.

Rix and colleagues report in a study that was published on September 7 at that the ancient core of the Milky Way is a round protogalaxy that spans nearly 18,000 light-years and possesses approximately 100 million times the mass of the sun in stars. This is equivalent to approximately 0.2 percent of the Milky Way’s current stellar mass.

According to Vasily Belokurov, an astronomer from the University of Cambridge who was not involved in the effort, “this investigation really helps to tighten up our understanding of this very, very, very young stage in the life of the Milky Way.” “About this time in the life of the Milky Way, not a whole lot of information is available,” he explains. The new study provides “a bird’s-eye view of the overall structure,” whereas previous research only provided “glimpses” of the population in question.

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Because they originated in a densely populated metropolis that earlier stellar generations had enriched with those metals through supernova explosions, the vast majority of stars in the central region of the Milky Way are rich in metals. This is because earlier stellar generations had enriched the metropolis with those metals. But Rix and his colleagues intended to identify stars that were so devoid of metals that they had to have been created a very long time before the rest of the stellar dwellers of the galaxy. This is what Rix refers to as “a needle-in-a-haystack exercise.”

The information from the Gaia spacecraft, which was sent into space in 2013 on a mission to map the Milky Way, was consulted by his team (SN: 6/13/22). The astronomers looked through over 2 million stars within a large region surrounding the galactic center, which is located in the constellation Sagittarius. They were searching for stars with metal-to-hydrogen ratios that were no greater than 3 percent of those of the sun.

After then, the astronomers looked at how those stars move through space, and they only kept the ones that don’t disappear into the huge ring of metal-poor stars that encircles the disk of the Milky Way galaxy. The researchers explain that the ultimate result is a sample of 18,000 old stars that represents the kernel around which the entire galaxy developed. Rix’s estimation places the protogalaxy’s mass anywhere between 50 million and 200 million times that of the sun, taking into account the fact that dust obscures some of the stars.

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According to Rix, “That’s the original core,” and it is the location that is home to the Milky Way’s oldest stars, which possibly have ages that are greater than 12.5 billion years old. Before the Milky Way’s first disk, also known as the so-called thick disk, came into existence, the protogalaxy was formed when numerous enormous clumps of stars and gas merged together a very long time ago (SN: 3/23/22).

Because of its compact nature, the protogalaxy has been subject to only minimal disruptions since it first formed. There have been collisions with other, smaller galaxies, which have increased the mass of the Milky Way. However, “we didn’t have any later mergers that profoundly delved into the core and rattled it up,” as Rix explains, because that would have caused the core to be larger presently.

The newly discovered information on the protogalaxy even captures the beginning stages of the Milky Way’s spin-up, or the process by which it transformed from an object that did not rotate into one that does. The stars that are the most ancient in the proto–Milky Way scarcely spin around the core of the galaxy and instead dive in and out of it, whereas the stars that are slightly younger show more and more movement around the center of the galaxy as time goes on. According to Belokurov, who noticed the similar spin-up in research that he and a colleague revealed in July, “This is the Milky Way trying to become a disk galaxy.”

The Milky Way is a massive galaxy that rotates quite quickly at this point; during one hour, our solar system travels 900,000 kilometers in space as we circle the galactic core. The latest research, however, demonstrates that the Milky Way was once only a humble protogalaxy whose stars are still shining brightly today. These are the same stars that scientists may now examine for additional information regarding the origin and early development of the galaxy.

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