Astronomy: Understanding the evolution of the Milky Way

The Milky Way may have evolved in different phases, beginning with the formation of disk stellar populations about thirteen billion years ago, suggests a Nature paper. The findings, made through the precise ageing of nearly a quarter of a million stars, improve our understanding of the Milky Way’s assembly history, especially during the early phases of formation.

To understand how our Galaxy was formed, the precise ages of large samples of stars are required, the oldest of which can extend to about fourteen billion years old. Stars in their subgiant evolutionary phases offer a precise stellar clock, as astronomers can directly and precisely measure their age during their brief stellar phase, based on their brightness. However, due to the short lifetime of this evolutionary phase, observations of stars in the subgiant phase are relatively rare, and large surveys have not been available in the past.

To investigate the assembly history of our Galaxy, Maosheng Xiang and Hans-Walter Rix used data from the ESA Gaia space observatory and Large Sky Area Multi-Object Fibre Spectroscopic Telescope in China to identify around 250,000 stars in the subgiant phase, covering a large spatial volume of the Milky Way. The authors estimated the individual ages of these stars and found that they ranged from about 1.5 billion to 13.8 billion years old. They then identified and characterized the origins of the different structural elements in the disk and halo stellar populations. The findings suggest that the formation of the thick disk had already begun around 13 billion years ago, only 800 million years after the Big Bang. The authors calculate that the inner Galactic halo finished assembling — through the merger of the Gaia-Enceladus galaxy with the ancient Milky Way — two billion years later, thought to coincide with the time when most of the stars in the thick disk formed via a burst.

The age, composition and motion of these stars highlight the dynamic processes involved in the formation of our Galaxy, the authors argue. In an associated News & Views, Timothy Beers suggests that as more data for larger samples of stars in the Milky Way become available, our understanding of how our Galaxy formed will only improve.