The Milky Way, our galaxy: The bulge is the thick central region. © ESA, Gaia, DPAC, CC BY-SA 3.0 IGO
Our galaxy, aptly named the Milky Way, is generally estimated to be 13.2 billion years old, with the universe estimated to be 13.77 billion years old. So it is not the oldest galaxy but it is part of the first generations.
One of the oldest stars in the Milky Way is logically called the star Methuselah or HD140283. It is a subgiant, so at the beginning of the end of its life, so to speak, relatively close to us, only 202 light years in the direction of the constellation.
This star in the constellation is Methuselah. © ESA / Hubble
It is believed to be a Population II star, that is, a generation of stars that lit up in the Universe about 13.6 billion years ago. Its distinguishing feature is that it is extremely poorly deficient in heavy chemical elements, known in astrophysics as “metals”.
PanGu consists mainly of very old stars, more than 12.5 billion years old!
So the “metallic” content is one of the main markers of the generation to which the stars belong. And indeed, a team of researchers has just shown that a disk-shaped geometric structure is formed by stars that are extremely metal-poor. The authors called this structure PanGu. It has 2 billion solar masses and stars estimated to be 13.5 billion years old!
A graph showing the distribution of stars in this famous disk according to their age and their Fe/H ratio (% of metals, approx.). We see that the majority of them are very old, with birth certificates dating back more than 12.5 billion years and up to 13.5 billion. © Maosheng Xiang et al (Nature Astronomy, 2024)
Of course, we’re still younger than the age of the universe, but that changes a lot. First, the existence of this structure would postpone the birth of our Milky Way, because 2 billion solar masses (currently, what’s more) means that it is not a large globular cluster, but one of the galaxy’s stellar roots. There is one. Furthermore, it will probably force a review of the history of the universe. Indeed, such a large number of such old stars, even if they are “only” population II and not population III (earlier), raises questions about when the first Sun formed.
The merger of our galaxy with a dwarf galaxy Gaia-Enceladus about 10 billion years ago. Therefore, this new structure would be older than this major event. © Gabriel Perez Diaz, SMM, Institute of Astrophysics of the Canary Islands
This disk, which can be described as “primary”, would therefore have existed well before our galaxy’s very ancient merger with Gaia-Enceladus, a dwarf galaxy that we absorbed 10 billion years ago. . Generally, two major stages of evolution are attributed to the Milky Way, the first occurring over 5 billion years. But if this structure is confirmed, then an even older phase will follow and our history will be reversed.
