Study of the Milky Way offers new insights into the make-up of our galactic home

The analysis suggests that the thickest component of the star disk in our galaxy’s core — an example of what astronomers call a spiral galaxy — began forming 13 billion years ago, about 800 million years after the Big Bang. The spherical halo of stars surrounding the disk was not formed until about two billion years later.

“We didn’t know that the disc preceded the corona,” said Amina Helmy, an astronomer at the University of Groningen in the Netherlands, who was not involved in the new research. “If you asked anyone before 2018, they would say corona is older.”

Research in previous decades indicated that corona stars were older than their disk counterparts. But the 2018 release of data collected by the European Space Agency’s Gaia telescope casts doubt on that understanding.

Dr. Helmy said she was shocked by new discoveries about the Milky Way, which spans 100,000 light years and contains vast clouds of gas and dust along with an estimated 100 billion to 400 billion stars and at least many planets. .

Determining the ages of those stars, some of which are more than 13 billion years old, is very difficult, she added. But without this information, astronomers cannot determine when certain events occurred in the early history of our galaxy.

Said Timothy Beers, an astrophysicist at the University of Notre Dame who wrote the Nature article accompanying the new study but was not involved in the research.

If astronomers can learn the intrinsic brightness or luminosity of a star, as well as its temperature and chemical composition, they can estimate its age based on theoretical models that show how stars evolve over time as they consume hydrogen fuel.

For the new study, Maoxing Chiang and Hans-Walter Rex, two astronomers at the Max Planck Institute for Astronomy in Germany, used that methodology to determine the exact ages of 247,104 stars in the inner corona and disk of our galaxy that were at a certain evolutionary stage. .

While this is a small part of the estimated number of stars in the Milky Way, it is 100 times more than the stars previously used to date stars of this type.

they used Gaia telescope orbiting the sunwhich charts the motion, position and distance of stars, to determine their luminosity, and a ground-based spectrometer telescope in China to measure the temperature of stars and the composition of the elements.

Once the ages of the stars had been determined, the study authors looked at the orbits of the stars to determine if they were in the disk or the halo — disk stars orbit the galactic center in semicircular orbits, while halo stars do not.

The first stars to form in the halo coincided with the initial formation of the thickened disk, but the new study shows that most of the Milky Way’s halo stars joined the galaxy about 11.2 billion years ago — when a smaller galaxy called Gaia-Enceladus was known to have merged with our country. After this merger, the aura assembly ended.

Dr. Helmy said that the results “show beyond a reasonable doubt that there is a disk present in our galaxy at the time of the merger,” adding that the existence of a disk at that time has been the subject of debate for a long time.

Dr. Chiang said he hopes to examine more stars from a wider swath of our galaxy. The new study only examined stars from a small region surrounding the Sun, which lies about 25,000 light-years away Milky Way Center.

He likened his work to archaeology: “We are lucky, because stars are like fossils that help us study the galaxy’s past.”

write to Aylin Woodward at [email protected]