A new analysis of nearly 250,000 stars puts the determined age on the most important page in our galaxy’s story of life.
The Milky Way is far grander than most of its neighbors, and appeared a long time ago as smaller galaxies collided together. Its thick disc—a pancake-like cluster of old stars— Originated shortly after the Big Bang Long before most of the stellar halo surrounding the galaxy’s disk, astronomers reported on March 23 nature.
“We are now able to provide a very clear timeline of what happened at the earliest in our galaxy,” astronomer Xiang Maosheng said.
Both he and Hans-Walter Rix at the Max Planck Institute for Astronomy in Heidelberg, Germany, study nearly 250,000 subgiant stars—stars that become larger and cooler after they run out of hydrogen fuel at their centers. The temperature and luminosity of these stars reveal their ages, allowing researchers to trace how different periods in galactic history produced stars with different chemical compositions and orbits around the galactic center.
“There’s a lot of information here,” said Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not involved in the study. “We really want to understand how our galaxy got what it is,” she said. “When were the chemical elements that make up us created?”
Xiang and Ricks discovered that the Milky Way’s thick disk began about 13 billion years ago. That’s just 800 million years after the universe was born. The thick disk measures 6,000 light-years from top to bottom near the sun, forming stars over a long period of time until about 8 billion years ago.
During this period, the iron content of the thick disk soared by a factor of 30, as the exploding star enriched its star-forming gas, the team found. At the dawn of the thick disk era, a newborn star had only one-tenth the iron content of the Sun relative to hydrogen. By the end, 5 billion years later, a thick disk star had three times the iron content of the Sun.
Xiang and Ricks also found a strong relationship between the age of thick-disk stars and their iron content. This means that the gas is thoroughly mixed throughout the thick disk: Newborn stars inherit more and more iron over time, regardless of whether the stars formed near or far from the galactic center.
But that’s not all. As other researchers reported in 2018, Another galaxy once hit our ownmost of the Milky Way’s stars are in its halo, which engulfs the disk (Serial Number: 11/1/18). Halo stars have almost no iron.
The new work modifies the date of this great galactic encounter: “We found that the merger happened 11 billion years ago,” Xiang said, a billion years earlier than thought. As the invader’s gas hits the Milky Way’s gas, it triggers the creation of so many new stars that our galaxy’s star formation rate reached a record high 11 billion years ago.
The merger also splashed some thick disk stars into the halo, which Xiang and Rex determined from the star’s higher iron abundance. The researchers found that these “splash” stars were at least 11 billion years old, confirming the date of the merger.
The thick disk ran out of gas and stopped making stars 8 billion years ago. The fresh gas surrounding the Milky Way then sank into a thinner disk, and stars have been born since then—including the Sun and most of its stellar neighbors 4.6 billion years ago. In our own galaxy, this thin disk is about 2,000 light-years thick.
“For the past 8 billion years, the Milky Way has been quiet,” Xiang said, having never encountered a large galaxy again. This sets it apart from most of its peers.
Xiang said that if the thick disk really existed 13 billion years ago, then The new James Webb Space Telescope (Serial Number: 1/24/22) can be discerned in a galaxy 13 billion light-years from Earth—a portrait of the Milky Way as a young galaxy.