Many scattered stars in the center of the Milky Way represent the remnants of the old galactic core, from when our galaxy was just born. A team of researchers has identified 18,000 stars from the galaxy’s early days, when it was just a compact collection of proto-galaxies.
To identify all 18,000 ancient stars of the Milky Way, researchers used measurements from the most accurate three-dimensional map of the galaxy, as well as a neural network to probe the chemical composition of more than two million of these stars, according to Science Alert.
The clues that led to the detection of these stars were identified in previous studies.
“But our results add significantly to the existing picture, showing that there is indeed one iceberg closely related in situ (ed. in the natural environment), whose peaks were previously recognized”, writes a team coordinated by astronomer Hans-Walter Rix from the Max Planck Institute for Astronomy in Germany.
The research was published in The Astrophysical Journal.
What does ESA’s space observatory reveal about the origin of stars in the Milky Way?
The 13-billion-year history of the Milky Way is a huge and “savory” puzzle that needs to be reconstructed from the current state of the galaxy.
Populations of stars can be related to each other based on common features such as motions and chemical compositions, a property known as metallicity. This is where the European Space Agency’s (ESA) Gaia Space Observatory comes in.
The satellite has been in Earth’s orbit around the Sun for years, keeping a close eye on the stars and making measurements of their three-dimensional positions and motions in the galaxy. In addition, Gaia has made measurements that allow the metallicity of stars to be estimated.
Metallicity can bind stars together, as those with a similar composition may have been born at the same time and place. But it can also tell us roughly how old a star is, because certain elements simply weren’t present in the Universe until there were stars to create them.
The beginnings of the Universe, a very small diversity of matter
Shortly after the initial Big Bang explosion, which occurred 13.8 billion years ago, there was not much elemental diversity in the Universe.
The early universe contained mostly hydrogen and very little helium. When the first stars formed from clusters in this environment, their hot, dense cores began to break apart atoms to form heavier elements: hydrogen into helium, helium into carbon, and even 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 scatters their fusion products into space.
Energetic supernova explosions also produce heavier metals such as gold, silver and uranium. Young stars absorb these elements as they form.
The age of a star, determined by the level of metal it contains
The later a star forms in the Universe, the more metals it is likely to contain. Thus, a higher metallicity means a younger star, and “metal-poor” stars are considered older. But not all stellar orbits are the same as they travel around the galactic center.
Rix and his colleagues used the Gaia data to examine red giant stars within a few thousand light-years of the Milky Way. They identified two million stars, whose light was analyzed by a neural network that was able to identify metallicities.
Thus, they found a population of stars with similar ages, abundances and orbits, suggesting that they were present before our galaxy filled with stars and swelled from collisions with other galaxies, starting about 11 billions of years.
Stars formed in proto-galaxies, remnants of 12.5 billion years ago
We know that the oldest stars in the Milky Way predate the first major collision with a galaxy called Gaia-Enceladus, but this population in the galactic center appears to be a coherent population of them.
Rix called these ancestral stars the “poor old heart” of the Milky Way, because the stars in question are metal-poor, very old, and can be found in the heart of the galaxy. The population represents, the researchers say, the remnants of proto-galaxies.
The oldest stars were not born in these proto-galaxies, but represent the generation of stars that formed when the proto-galaxy’s stars died. The researchers found that they are more than 12.5 billion years old.
The fascinating discovery raises a lot of questions, which researchers hope to investigate.