Astrophysicists investigating the origins of the Milky Way may have discovered our galaxy’s “old heart” – the original, ancient core around which all of its stars and planets grew.
The collection of 18,000 of the oldest stars in our galaxy are located in the constellation of Sagittarius and come from the protogalaxy of the Milky Way – a primordial mass of gas and dust forming the first stars of a young galaxy – aged from more than 12.5 billion years old. Comprising about 0.2% of the total mass of our galaxy, the cluster is the core around which the entire Milky Way eventually developed, the researchers found. The results were published on September 8 on the preprint server arXivand have not yet been peer reviewed.
To discover the primordial group of stars, astronomers relied on data from the European Space Agency’s (ESA) Gaia Observatory – a 3,594-pound (1,630-kilogram) spacecraft launched in 2013 with the aim of create the most detailed and accurate map of the Milky Way,
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“It has long been believed (based on theories and simulations) that the oldest stars are found at the very center of a galaxy. We have now shown that they are there in large numbers,” said the author. lead author of the study. Hans-Walter Rix, an astronomer from the Max Planck Institute for Astronomy in Heidelberg, Germany, told Live Science. “It’s like doing archeology in an old city. We have shown that the oldest and most primitive ruins are in the ‘modern’ city center.”
To find the ancient core of our galaxy, we started by searching the most populated region, its central bulge, for the tiny proportion of stars about the same age as the Milky Way, about 13 billion years old. ‘years.
To pluck out this small cluster like a needle in a haystack, the researchers gathered data gathered from Gaia on 2 million stars within 30 degrees of the galactic center, looking for lower mass stars and longer life with low metal content. Stars matching this profile originated in a much younger universe that was not yet filled with heavy metals scattered far and wide by supernova explosions.
But that’s only half the story, because metal-poor stars in the Milky Way may also have originated from smaller dwarf galaxies that crashed and merged with our galaxy throughout its lifetime. By examining the trajectories of these stars through space while selecting only those that did not veer into the metal-poor regions of the galaxy, the researchers were able to separate the stars that formed the ancient core from the stars that originated from ‘a dwarf. galaxy.
This left the researchers with some of the original skeleton of the stars around which the Milky Way grew – a population they estimate to be between 50 million and 200 million times more massive than our own sun. Because heavier stars die faster than smaller ones, the remaining stars are on average about 1.5 times lighter than the sun, the researchers said.
“These stars make up about half of the total stellar mass when born,” Rix said. “So about half the stars [from the protogalaxy] survive to this day.”
The researchers’ examination of the now-exposed ancient core of the Milky Way revealed two things. First, since stars in the older protogalaxy orbit much less around the galactic center than younger stars, this confirms past observations that the Milky Way’s core began life stationary, eventually picking up a speed of rotation as the galaxy’s center of mass grew.
And second, despite multiple mergers with smaller galaxies, the tight clustering of stars at the center of the Milky Way indicates that its core has not been invaded by collisions from other galaxies.
“The Milky Way has never been dramatically shaken up,” Rix said. “Our galaxy has lived a sheltered life.”
With further study, researchers hope the ancient core can teach them even more about our galaxy’s early years, such as the types of supernovae that must have exploded at the time of its creation to produce the proportions of primitive chemical elements. that we see today.
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