Astronomers discover the "poor ancient heart" of the Milky Way Galaxy
There are a few stars scattered around the center of the Milky Way, which are the remains of an ancient galactic core, when our galaxy was still newborn.
Using measurements from the most accurate 3D map of the galaxy ever compiled, as well as a neural network to probe the chemical compositions of more than two million stars, a team of astronomers has identified 18,000 stars from the beginning of our galaxy, when it was just a compact collection of primitive galaxies that came together.
Our galaxy, the Milky Way, formed gradually over the course of almost the entire history of the universe, which extends over a period of 13 billion years.
Over the past decades, astronomers have been able to reconstruct different eras of galactic history in the same way that archaeologists reconstruct the history of ancient cities, as some buildings come with specific construction dates.
Star groups can be linked based on common features such as their motions and chemical compositions, a property known as metallicity. Here comes the role of the European Space Agency's Gaia Space Observatory.
The satellite has been in Earth's orbit around the sun for years, carefully tracking the stars and taking measurements of their three-dimensional positions and movements within the galaxy. In addition, the Gaia observatory takes measurements that allow estimates of the stars' metallicity.
Metals can bind stars together. Because stars with a similar composition could have been born in the same place at the same time. But it can also tell us approximately how old a star is, because some elements did not exist in the universe until stars appeared to form them.
After the Big Bang 13.8 billion years ago, there wasn't much in the way of racial diversity.
The primordial universe consisted mainly of hydrogen, with a little helium, but not much else. When the first stars formed from clumps in this medium, their hot, dense cores began smashing atoms together to form heavier elements: hydrogen into helium, helium into carbon, and so on, all the way up to iron for the most massive stars.
Once stars reach the limit of their ability to fuse atomic nuclei, they die, often in a process such as a supernova. Active supernova explosions produce heavier metals, such as gold, silver, and uranium. Young stars then take up these elements as they form.
The later a star formed in the universe, the more likely it was to contain metals. Thus, high metallicity means a younger star, and “metallicity-poor” stars are thought to be older. But not all stellar orbits are the same as they make their way around the galactic center.
When you find a group of stars with similar metal content, on a similar orbital path, it is likely that this group of stars is a population that has been together for a very long time, perhaps since the beginning of formation.
Astronomer Hans-Walter Rex of the Max Planck Institute for Astronomy and his colleagues used Gaia data to look at red giant stars within a few thousand light-years of the Milky Way. They identified two million stars, the light from which was analyzed by a network that can identify metals.
They found a group of stars with similar ages, abundances and orbits, indicating that they existed before the Milky Way Galaxy became filled with stars and bulged due to collisions with other galaxies, starting about 11 billion years ago.
We know that the oldest stars in the Milky Way preceded the first major collision, with a galaxy called the “Sausage Galaxy” or “Gaia-Enceladus,” but this population at the center of the galaxy appears to be a cohesive group of them.
Rex called it “poor old heart.” For the Milky Way Galaxy, because it is poor in metals, very old, and can be found in the heart of the galaxy. Scientists say the inhabitants are the remnants of primordial galaxies.
These clusters of stars that formed in the early universe were not fully developed galaxies, but rather their “seeds.”
Early in the Milky Way's life, three or four of these seeds came together to form the core of what would become our home galaxy.
The “poor old heart” stars were not born in these primitive galaxies, but rather they are the generation of stars that formed when the stars of the primitive galaxy died. Scientists found that it is more than 12.5 billion years old.
The results of the study were published in The Astrophysical Journal.
Source: ScienceAlert