Volume 603 Issue 7902, 24 March 2022

Star dater’s guide to the Galaxy

The cover image shows a view of the Milky Way captured at Nambung National Park in Western Australia. To understand how the Galaxy formed requires precision age dating of the stars that it contains. In this week’s issue, Maosheng Xiang and Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany, present an analysis of the birth dates for nearly 250,000 stars in their subgiant evolutionary phase, when they can serve as precise stellar clocks. The researchers found that the individual ages of the stars ranged from about 1.5 billion to more than 13 billion years old. Tripling the age-dating precision for such a large stellar sample allowed the researchers to infer the sequence of events that initiated our Galaxy’s formation. Using this information, Xiang and Rix were able to determine that the oldest part of our Galaxy’s disk had already begun to form about 13 billion years ago, just 800 million years after the Big Bang, and that the formation of the inner Galactic halo was completed some 2 billion years later.