Our home galaxy appears relatively stable and calm today. But it has an eventful history behind it – and astronomers have now shed more light on part of it for the first time. Their spectral analyzes of around 250,000 stars allowed the events to be dated, revealing that the oldest, thickest part of the Galactic star disk formed as early as 13 billion years ago – just 800 million years after the Big Bang. However, the greatest boost in star formation came about two billion years later, when our galaxy collided with a smaller neighboring galaxy and merged. The galactic halo also formed around this time.
Our Milky Way consists of billions of stars of different ages, interstellar gases and plenty of dark matter. She has accumulated all these components in the course of an eventful development. In the beginning there was the merging of gas-rich progenitor galaxies into an initially even smaller and less strongly structured galaxy. During this time, the so-called thick disc of our Milky Way formed, the main part of the star disc, which is around 100,000 light-years wide and 6,000 light-years thick. Later, through merging with other, smaller neighboring galaxies, the halo, which lies like a shell around the star disk, and the so-called thin disk, which is only around 2000 light-years thick, were added. However, when these individual phases took place and what caused them has only been clarified to a limited extent.
Stellar subgiants as dating helpers
Maosheng Xiang and Hans-Walter Rix from the Max Planck Institute for Astronomy now provide a more detailed insight into the early years of our Milky Way, 13 to 8 billion years ago. They have succeeded in dating important phases in the history of galaxies more precisely for the first time. This was made possible by evaluating data from two large sky surveys, the European Gaia mission and the LAMOST survey, a spectral analysis of around nine million stars with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope in China. The combination of both sets of data provided astronomers with information about the position, motion, temperature and chemical composition of stars from different regions of the Milky Way.
Using this data, the researchers were able to identify a type of star that is crucial for dating, the so-called subgiants. These stars have already used up most of the hydrogen in their cores, so nuclear fusion there slows down and the core shrinks. At the same time, nuclear fusion begins in the shell surrounding the star’s core – and with it the transition to the giant star. During this transition phase, which lasts only a few million years, the age of these subgiants can be inferred directly from their surface temperature and brightness. “This makes subgiants valuable dating tools for galactic archeology,” Xiang and Rix explain. The downside, however, is that subgiants are very rare. Therefore, the two astronomers had to analyze the data from millions of stars in order to track down around 250,000 of these subgiants in our galaxy and use them to reconstruct the teenage years of the Milky Way.
Structure Formation and Star Formation Boost
The evaluations showed that the oldest representatives of the subgiants lie in the thick disc of the Milky Way and are up to 13 billion years old. According to this, this part of our galaxy was formed only around 800 million years after the Big Bang. From the high total number of stars formed, the astronomers conclude that the thick disc contained large amounts of gas and thus raw material for new stars from the very beginning. That would also explain their comparatively large thickness. A little later the galactic halo began to form. “The oldest stars in the thick disc are on average one to two billion years older than the main population of halo stars,” the researchers report. The formation of this outer shell of gas and stars was then completed around eleven billion years ago.
Around this time, the rest of the Milky Way also underwent a dramatic change. Because around eleven billion years ago, the data show a conspicuous “production maximum” in star formation, at the same time the orbits of numerous stars suddenly changed. Xiang and Rix attribute this to the fact that the Milky Way collided with the slightly smaller neighboring galaxy Gaia-Enceladus/Sausage around this time. “The obvious interpretation of this temporal coincidence is that disturbance from the Gaia-Enceladus/Sausage galaxy strongly stimulated star formation in the thick disc,” the astronomers write. The shock waves from the collision caused more gas clouds to collapse and new stars to form. This spurt in star formation lasted around five to six billion years, but gradually weakened during that time.
Quiet late phase
Then, eight billion years ago, the turbulent and productive “teenage years” of the Milky Way came to an end. Most of the interstellar hydrogen gas in the thick disk was consumed, leaving few new stars to form there. However, because some fresh gas was still flowing in from intergalactic space, star formation in part of the galactic disk remained active longer – the thin disk formed. With the development of this structure, the long, quiet adult phase of our home galaxy began. Because the Milky Way has not undergone any major collisions since then, its structure has remained largely unchanged to this day. The development of the Milky Way thus corresponds to what models also predict for galaxies: a productive early phase, followed by a quiet, less disturbed late period.
Source: Maosheng Xiang and Hans-Walter Rix (Max Plank Institute for Astronomy, Heidelberg), Nature, doi: 10.1038/s41586-022-04496-5