A group of Max Planck Institute for Astronomy (MPIA) astronomers has discovered the "poor old heart of the Milky Way" after finding a cluster of 18,000 of the galaxy's oldest stars in the constellation Sagittarius that is more than 12.5 billion years old.
The stars account for an estimated 0.2% of the total mass of the galaxy and were discovered to be the original, ancient nucleus from which all of the stars and planets of the Milky Way grew. The findings were released and published in The Astrophysical Journal.
Estimating the Age of Stars Based on Their Metallicity
The Milky Way's 13-billion-year history is a massive, wonderful puzzle that must be reconstructed from the state of the galaxy now. Science Alert reported that star populations may be connected based on shared characteristics such as their movements and chemical compositions, a feature called metallicity. This is where the European Satellite Agency's (ESA) Gaia space observatory comes in.
For years, the satellite has remained in Earth's orbit around the Sun, meticulously watching stars and measuring their three-dimensional locations and movements within the galaxy. It takes measurements to help scientists estimate the metallicity of stars to link them based on their similarity in composition to argue whether they have been born in the same place at the same time.
More so, a star's metallicity tells how old it is because certain elements cannot be found in the present universe until there had been stars around to make them. There was not much elemental variety just after the Big Bang 13.8 billion years ago. The universe in the past was mostly hydrogen, with a trace of helium and not much else.
But when the first stars formed from clusters in this medium, their hot dense cores began to smash atoms together to generate heavier elements, such as hydrogen into helium, helium into carbon, and so on, all the way up to iron for the most massive stars.
When stars approach their limit and die in a supernova-like explosion, they spray the products of nuclear fusion into space, including gold, silver, uranium, and other heavy metals. The later the star originates in the universe, the more metals it has, whereas metal-poor stars are assumed to be older.
They discovered the cluster of stars with comparable ages, abundance, and orbits, and suggested that these stars could have formed before the galaxy was filled by other stars. The team refers to them as "poor old heart" given their low metallicity, old age, and the location at the center of the Milky Way.
Mapping the Poor Old Heart of Milky Way
When looking at the metallicity of a star, proves how easy it is to identify the ancient core of the Milky Way galaxy. The team mapped these stars, which appear to be concentrated around the center of the galaxy.
According to Phys.org, the distances conveniently supplied by observations from Gaia aided in making a 3D reconstruction that shows those stars confined in a small region at the galactic center about 30,000 light-years across.
The cluster of stars compliments a previous study by researchers about the teenage years of the Milky Way. The star cluster just has enough metallicity to help scientists identify the metal-poorest of those stars that later on formed the thick disk in the galaxy.
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