A cluster of stars scattered throughout the center of our Galaxy are the remnants of the ancient galactic nucleus, when our galaxy was still young.
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 2 million stars, a team of astronomers has identified 18,000 stars from our galaxy’s infancy, when it was just a compact collection of protogalaxies coming together to dream bigger things.
Hints of this stellar population have been identified in previous studies.
“But our results,” writes a team led by astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy, “significantly strengthen the existing picture by showing that there is indeed a tightly bound in situ ‘iceberg’ whose tips were recognized before ».
The 13 billion year history of the Milky Way is a giant, delicious puzzle that must be reconstructed from the state of the galaxy today.
Populations of stars can be linked based on common features such as their motions and chemical compositions, a property known as metallicity. This is where ESA’s Gaia space observatory comes in.
The satellite has shared Earth’s orbit around the Sun for years, keeping a close eye on stars and taking measurements of their three-dimensional positions and movements within the galaxy.
In addition, Gaia takes measurements that allow estimates of the metallicity of stars.
Metallicity can link 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 how old a star is, because some elements simply didn’t exist in the Universe until there were stars around to make them.
Right after the Big Bang 13.8 billion years ago, there weren’t many barriers to elemental diversity.
The early Universe consisted mostly of hydrogen, with some helium, and not much else. When the first stars formed from clusters in this medium, their hot, dense cores began to crush atoms together to form heavier elements: hydrogen into helium, helium into carbon, and so on, 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 supernova-like process that sprays their fusion products into space.
Energetic supernova explosions also produce heavier metals, such as gold, silver and uranium. Baby stars then pick up these elements as they form.
The later in the Universe a star forms, the more metals it is likely to have. 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 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 reasonable to conclude that this group of stars is a population that has been together for a very long time, perhaps since formation.
Rix and his colleagues used Gaia data to examine red giant stars within a few thousand light-years of our Milky Way. They identified 2 million stars, the light from which was analyzed by a neural network that could pinpoint metallicities.
And they found a population of stars with similar ages, abundances and orbits that suggest they were present before the Milky Way was filled with stars and bloated by collisions with other galaxies, beginning about 11 billion years ago.
We know that the oldest stars in our Galaxy predate the first major collision, with 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 Galaxy because they are metal-poor, very old, and can be found in the heart of the galaxy. The population is, the researchers say, the remnants of protogalaxies.
These clusters of stars that formed in the early Universe were not full galaxies, but their seeds. In the Milky Way’s infancy, three or four of these seeds came together to form the core of what would become our galaxy.
The poor old heart stars were not born in these protogalaxies, but are the generation of stars that formed when the protogalaxy stars died. It is, according to the researchers, more than 12.5 billion years old.
The exciting discovery raises many questions, which the researchers hope to investigate.
What is the spatial distribution of these stars? Do they have special abundance ratios that could tell us more about the conditions of the Earl of the Galaxy? What can their distribution tell us about the conflict history of the Galaxy?
And, perhaps most pressingly, can they lead us to those stars – smaller, fainter and harder to find – that might have been in the first protogalaxies when they came together in the early stages of the Milky Way’s formation?
It may be metal-poor and old, but the ancient heart of the Galaxy could turn out to be extremely rich with answers about our galactic history.
The research has been published in The Astrophysical Journal.