New discoveries in the early history of the Milky Way

Guo Shoujing telescope under the Milky Way. (Guo Shoujing telescope official website / photo)

On a clear night, the beautiful Milky Way once triggered countless reverie of mankind. For more than a hundred years, with the continuous development of astronomy, we have gained a deeper understanding of the Milky Way. But many of the mysteries surrounding the Milky Way have yet to be solved.

On March 24, 2022, Dr. Maosheng Xiang and Professor Hans-Walter Rix of the Max Planck Institute for Astronomy in Germany published their latest research on the early formation and evolution of the Milky Way in the form of a cover article in the journal Nature. They give a time-series image of the evolution of the Milky Way over 13 billion years, especially revealing significant events that occurred in the early stages, rewriting our understanding of the early history of the Milky Way.

Go back 13 billion years

Their study is based on sky survey observations obtained by the National Astronomical Observatory of the Chinese Academy of Sciences ( LAMOST ) and the European Space Agency 's Gaia Space Telescope . Both telescopes, as representatives of ground-based sky surveys and space surveys, have carried out fruitful sky surveys in recent years: the Guo Shoujing telescope has released tens of millions of stellar spectral data, while the Gaia space telescope has obtained information on the position and motion of 1.4 billion stars. Based on these data, Xiang Maosheng and Ricks obtained the most accurate information on the age of large samples of stars to date.

For the study, they chose subgiant stars, a class of stars that can easily determine their time of formation. Subgiants are stars in the intermediate stages of the evolution of the main sequence of stars into red giants, and their observable information such as temperature and luminosity is extremely sensitive to initial age, so their age can be accurately determined by measuring this information. Xiang Maosheng and Ricks used the sky survey data of the Guo Shoujing telescope to determine the atmospheric parameters of 7 million stars, and obtained high-precision stellar luminosity and orbital kinematic parameters based on the sky survey data of the Gaia Space Telescope. Among them, they selected 250,000 subgiant stars and determined their precise ages with an average age accuracy of 7%, compared with the typical age error of 20% or greater for previous large samples of stars.

The spatial coverage of these 250,000 stars reaches 30,000 light years, and the abundance of metallic elements ranges from 1/300 of the solar metal content to 3 times the solar metal content, which is the first time that the high-precision age of a large sample of stars has been obtained in such a wide space range and stellar metal abundance in the Milky Way, laying a solid foundation for studying the formation and evolution of the Milky Way. It is important to note that, contrary to the usual definition, in astronomical studies, researchers refer to elements other than hydrogen and helium as "metallic elements."

Xiang Maosheng and Ricks also divided the 250,000 stars into two groups based on their position in the Milky Way. If we look down on the Milky Way, the Milky Way looks like a disk with spiral arms; but if we look sideways at the Milky Way, the Milky Way is biconvex, similar to two omelettes placed back to back together. The astronomers synthesized observations to give an image of the structure of the Milky Way: the Milky Way includes the central region of the silver nucleus (the center of the Milky Way – in which the silver center is located), the outer silver disk, and the larger silver halo. The silver disc consists of a relatively thick disc and a relatively thin and more extended disc, and the spherical structure that surrounds the silver disc is called a silver halo.

In their grouping, one group is stars formed in the thin disk of the Milky Way, and the other group is stars formed in the thick disk and halo of the Milky Way. They found that stars at different locations in the Milky Way also differ significantly in age.

In the paper, they describe a timeline of the formation and evolution of the Milky Way: Stars in the Milky Way's thick disk began to form 13 billion years ago, 800 million years after the Big Bang. Over the next 5 billion years, thick disks and silver halos in the Milky Way gradually formed, an early stage in the evolution of the Milky Way. 8 billion years ago, it became a watershed in star formation. After this, the star-forming activity of the thick disk and the silver halo ceased, and the stars in the thin disk of the Milky Way began to form, which was the late stage of the evolution of the Milky Way, which continues to this day.

They noted in the study that throughout the early stages, the content of metallic elements in thick discs increased by a factor of 30. At the beginning of this period, the content of metallic elements in a newborn star is only 1/10 of the content of the Sun, and by the end of the period, the content of metallic elements in the star is 3 times that of the Sun. Therefore, they concluded that the age of the thick disk star is closely related to the content of metallic elements.

Galactic collisions

Xiang And Ricks also found that while the thick disk star formation activity lasted 5 billion years, most of the thick disk stars came from a concentrated eruption 11 billion years ago. They believe that 11 billion years ago, there was another major event in the Milky Way, that is, the collision and merger of the Milky Way and a dwarf planet. In their view, it is not a coincidence that concentrated star formation and galaxies collide almost simultaneously in time, but that the collision of two galaxies stimulates star-forming activity in a thick disk to produce so many new stars. The Milky Way's rate of star formation also reached its highest record at this time.

In their latest study, they calculated that the collision time between the Milky Way and the dwarf planets was 1 billion years earlier than previously estimated. On October 31, 2018, astronomers Amina Helmi and others at the University of Groningen in the Netherlands published their findings on the early merger events of the Milky Way, which they believed occurred 10 billion years ago, published in the journal Nature.

For the study, Hemi and colleagues analyzed the velocities and positions of tens of thousands of stars in the Milky Way within 33,000 light-years of the Sun, using the same data from the Gaia Space Telescope. They found that, unlike the Sun and other stars in the Milky Way, which rotate around the galactic center, there are about 30,000 stars moving in the opposite direction, that is, appearing to move backwards. Hermi points out that when these stars move in opposite directions, this tells us that they can hardly form in the same place as other stars in our galaxy.

At the same time, they also found a second piece of evidence. They used data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to analyze the stars. The experiment, part of the Sloan Digital Sky Survey (SDSS), used the spectra of stars to determine their chemical composition and age. Hemi et al. found that these backward-moving stars have lower levels of heavy elements than stars like the Sun, suggesting that they formed early in the universe, before massive stars and supernovae spread heavy elements in galaxies. As a result, they are convinced that these stars formed elsewhere outside the Milky Way.

By comparing the observations to computer simulations, Hemi et al. calculated that about 10 billion years ago, the Milky Way collided with a smaller galaxy. The galaxy is about 20 to 25 percent the size of the Milky Way and has a mass of about 600 million times the mass of the Sun. They also believe that the collision could also explain why the Milky Way contains different components: the thick disk contains older stars, the young stars in the thin disk make up the spiral arm and the halo, and many of the stars in the current halo come from engulfed galaxies.

The team named the engulfed galaxy Gaia-Enceladus. In Greek mythology, the giant Enceladus was the son of gaia, the god of the earth, and Uranus. Legend has it that he was buried at the foot of Mount Etna in Italy, so the earthquake that occurred in this area was attributed to him. Hemi et al. named it because the galaxy Enceladus is buried in the Milky Way, also in the gaia space telescope data, and it is one of the reasons for the galaxy's tens of billions of years of turmoil.

Restore big history

Although humans have always been curious about the Milky Way overhead and have made many speculations about the origin of the Milky Way, for a long time, humans thought that the Milky Way was the entire universe. It was not until the 20th century that astronomers, with the help of more powerful observation tools, gradually gained a more scientific understanding of the Milky Way. On April 26, 1920, astronomers Harlow Shapley and Heber Curtis held a "Great Astronomical Debate of the Century" at the Smithsonian Museum of Natural History in Washington, D.C. Ultimately, the two men won and lost several debates in this debate: Curtis's idea that the Andromeda Nebula is an extra-Milky Way independent galaxy was later confirmed by Edwin Hubble's observations that the Milky Way is indeed not the whole universe; and Shapley correctly proposed that the Sun is far from the center of the Milky Way.

Now we realize that the Milky Way is not only not the whole of the universe, but just an ordinary galaxy in the universe. The Milky Way contains hundreds of billions of stars, and the Sun is just one of them. However, as the galaxy in which we live, studying the origin and evolution of the Milky Way is of great significance for us to understand the origin and evolution of other galaxies in the universe and the origin of life, so it has always been the focus of astronomers' research, and astronomers have been trying to restore the history of the Milky Way for tens of billions of years.

At present, astronomers believe that the Milky Way as a whole is in a relatively calm state during the late stages of evolution, and there have been no more violent events that collide with other galaxies. However, this state does not last forever. Astronomers predict that in 500 million years, the Milky Way will swallow up the Sagittarius dwarf galaxy, which is currently 50,000 light-years away from the center of Silver. Meanwhile, the Andromeda Galaxy, which is currently about 2.5 million light-years from Earth, is moving toward the Milky Way at a speed of 300 kilometers per second relative to the Sun, and the two will collide in about 4 billion years, eventually merging into one elliptical galaxy.

Astronomy studies are like time travel, both looking to the endless future and looking back at the distant past. Xiang and Ricks' research points to the Milky Way's earliest stars formed 13 billion years ago, while the James Webb Space Telescope, launched in December 2021, was able to look into the depths of the universe more than 13.5 billion light-years from Earth. Therefore, Xiang Maosheng believes that the telescope may find the same structure as the thick disk of the Milky Way that has just formed, thus verifying their conclusions.

In reconstructing the history of the Milky Way, Xiang And Rick's research has helped us sketch a clearer picture of the formation and evolution of the early Milky Way. This is a new beginning, and in this history book of the Galaxy, more wonderful stories will be presented before our eyes.

Southern Weekend contributed to Ju Qiang