A handful of stars scattered throughout the center of the Milky Way are the remnants of the old galactic nucleus, when our galaxy was still new.
Using measurements from the most precise three-dimensional map of the galaxy ever compiled, as well as a neural network to probe the chemical compositions of more than 2 million stars, a team of astronomers has identified 18,000 stars from our galaxy’s infancy, when it was just a collection. compact of proto-galaxies coming together to dream of bigger things.
Indications of this stellar population have been identified in previous studies.
“But our results,” writes a team led by astronomer Hans-Walter Rix from the Max Planck Institute for Astronomy, “significantly enhances the existing picture by showing that there is, in fact, a tightly packed ‘iceberg’ in situ, the tips of which have been recognized before.”
The 13-billion-year history of the Milky Way is a giant, delicious puzzle that needs to be pieced together from the current state of the galaxy.
The satellite has been sharing Earth’s orbit around the Sun for years, carefully tracking the stars and taking measurements of their three-dimensional positions and motions within the galaxy.
In addition, Gaia takes measurements that make it possible to estimate the metallicity of stars.
Metallicity can bind stars together, because stars that have a similar composition could have been born in the same place at the same time. But it can also tell us roughly the age of a star, because certain elements simply weren’t present in the Universe until there were stars around to form them.
right after the big Bang 13.8 billion years ago, there wasn’t much elemental diversity.
The primordial Universe consisted mostly of hydrogen, with a little helium, and not much else. When the first stars formed from clusters in this medium, their hot, dense cores began to squash atoms into heavier elements: hydrogen into helium, helium into carbon, etc., all the way to iron for the first time. most massive stars.
Once stars reach the limit of their ability to fuse atomic nuclei, they die, often in a process like a supernova which sprays the products of its fusion into space.
Energetic supernova explosions too produce heavier metalssuch as gold, silver and uranium. Baby stars take in these elements as they form.
The later in the Universe a star forms, the more metals it is likely to contain. Therefore, a higher metallicity means a younger star; and “metal-poor” stars are thought to be older. But not all stellar orbits are the same when they wind their way around the galactic center.
When you find a group of stars with a similar metal content, in a similar orbital path, it is reasonable to conclude that this group of stars is a population that has been together for a long time, perhaps since formation.
Rix and his colleagues used data from Gaia to observe red giant stars within a few thousand light-years of the Milky Way. They identified 2 million stars, the light of which was analyzed by a neural network that could identify metallicities.
And they found a population of stars with similar ages, abundances, and orbits, suggesting they were present before the Milky Way became star-filled and bulged. due to collisions with other galaxiesbeginning about 11 billion years ago.
We know that the oldest stars in the Milky Way before the first big collisionwith a galaxy called Gaia-Enceladus, but this population at the galactic center appears to be a coherent population of them.
Rix has called them the “poor old heart” of the Milky Way, because they are metal-poor, very old, and can be found at the heart of the galaxy. The population is, the researchers say, the remnants of protogalaxies.
These packets of stars that formed in the early Universe were not complete galaxies, but their seeds. In the infancy of the Milky Way, three or four of these seeds clumped together to form the nucleus of what would become our home galaxy.
The poor old heart stars were not born in these protogalaxies, but are the generation of stars that formed when the stars in the protogalaxies died. They are, the researchers found, more than 12.5 billion years old.
The fascinating discovery raises many questionsthat the researchers hope to investigate.
What is the spatial distribution of these stars? Do you have any special abundance indices that can give us more information about the initial conditions of the Milky Way? What can its distribution tell us about the History of the collision of the Milky Way?
And, perhaps most pressingly, can they lead us to those stars (smaller, dimmer, and harder to find) that may have been in the earliest protogalaxies when they coalesced in the early stages of the Milky Way’s formation?
It may be old and poor in metals, but the ancient heart of the Milky Way could end up extremely rich in answers about our galactic history.
The research has been published in The Astrophysical Journal.