Astronomers have gained a fresh understanding of the Milky Way’s formation following an analysis of nearly a quarter of a million stars among the many billions that make up our home galaxy.
The research, published Wednesday in the journal Nature, addresses the order in which the Milky Way’s major constituent parts came together to form the colossal spiral-shaped collection of stars that contains our own solar system.
The analysis suggests that the thicker component of the disk of stars at the core of our galaxy—an example of what astronomers call a barred spiral galaxy—began forming 13 billion years ago, about 800 million years after the Big Bang. It wasn’t until about 2 billion years later that the spherical halo of stars surrounding the disk finished forming.
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“We didn’t know that the disk preceded the halo,” said Amina Helmi, an astronomer at the University of Groningen in the Netherlands who wasn’t involved in the new research. “If you had asked anybody before 2018, they would say the halo is older.”
Research conducted in previous decades suggested that halo stars were older than their disk counterparts. But the 2018 release of data collected by the European Space Agency’s Gaia Telescope cast doubt on that understanding.
Dr. Helmi said she was stunned by the new findings about the Milky Way, which spans a distance of 100,000 light-years and which contains vast clouds of gas and dust along with an estimated 100 billion to 400 billion stars and at least as many planets.
Pinpointing the ages of those stars, some of which are more than 13 billion years old, is notoriously difficult, she added. But without that information, astronomers can’t determine when certain events happened in our galaxy’s early history.
“If you’re going to say, tell me a story of how the galaxy came to be, we really couldn’t do that without a precise clock for individual stars,” said Timothy Beers, a University of Notre Dame astrophysicist who wrote a Nature article accompanying the new study but wasn’t involved in the research.
If astronomers can figure out 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 their hydrogen fuel.
For the new study, Maosheng Xiang and Hans-Walter Rix, two astronomers at the Max Planck Institute for Astronomy in Germany, used that methodology to determine the precise ages of 247,104 stars in our galaxy’s inner halo and disk that were in a particular evolutionary phase.
While that is a tiny fraction of the estimated number of stars in the Milky Way, it is 100 times more stars than have been used previously for star-dating of this type.
They used the sun-orbiting Gaia Telescope, which maps the movement, position and distance of stars, to determine luminosity, and a ground-based spectroscopy telescope in China to measure the stars’ temperature and elemental makeup.
Once the stars’ ages had been determined, the study authors looked at the stars’ orbits to determine whether they were located in the disk or halo—disk stars rotate around the galaxy’s center in nearly circular orbits, while halo stars do not.
The earliest stars that made up the halo coincided with the initial formation of the thick disk, but the new study showed most of the Milky Way’s halo stars joined the galaxy around 11.2 billion years ago—when a smaller galaxy called Gaia-Enceladus is known to have merged with ours. After that merger, halo assembly ended.
The findings “show beyond doubt that there was a disk present in our galaxy at the time of the merger,” Dr. Helmi said, adding that the presence of a disk at that time has long been debated.
Dr. Xiang said he hopes to examine more stars from a broader swath of our galaxy. The new study only examined stars from a small area surrounding the sun, which sits about 25,000 light-years from the Milky Way’s center.
He likened his work to archaeology: “We’re lucky, because the stars are like fossils that help us study the galaxy’s past.”
Write to Aylin Woodward at Aylin.Woodward@wsj.com
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Milky Way Study Yields Fresh Insights Into Formation of Our Galactic Home
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