A brand new evaluation of almost 1 / 4 million stars places agency ages on probably the most momentous pages from our galaxy’s life story.
Far grander than most of its neighbors, the Milky Way arose way back, as lesser galaxies smashed collectively. Its thick disk — a pancake-shaped inhabitants of previous stars — originated remarkably quickly after the Big Bang and properly earlier than many of the stellar halo that envelops the galaxy’s disk, astronomers report March 23 in Nature.
“We are now able to provide a very clear timeline of what happened in the earliest time of our Milky Way,” says astronomer Maosheng Xiang.
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He and Hans-Walter Rix, each on the Max Planck Institute for Astronomy in Heidelberg, Germany, studied nearly 250,000 subgiants — stars which are rising bigger and cooler after utilizing up the hydrogen gasoline at their facilities. The temperatures and luminosities of those stars reveal their ages, letting the researchers observe how completely different epochs in galactic historical past spawned stars with completely different chemical compositions and orbits across the Milky Way’s middle.
“There’s just an incredible amount of information here,” says Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not concerned with the research. “We really want to understand how our galaxy came to be the way it is,” she says. “When were the chemical elements of which we are made created?”
Xiang and Rix found that the Milky Way’s thick disk obtained its begin about 13 billion years in the past. That’s simply 800 million years after the universe’s beginning. The thick disk, which measures 6,000 light-years from prime to backside within the solar’s neighborhood, saved forming stars for a very long time, till about 8 billion years in the past.
During this era, the thick disk’s iron content material shot up 30-fold as exploding stars enriched its star-forming gasoline, the group discovered. At the daybreak of the thick disk period, a new child star had solely a tenth as a lot iron, relative to hydrogen, because the solar; by the tip, 5 billion years later, a thick disk star was 3 times richer in iron than the solar.
Xiang and Rix additionally discovered a decent relation between a thick disk star’s age and iron content material. This means gasoline was totally combined all through the thick disk: As time went on, new child stars inherited steadily larger quantities of iron, regardless of whether or not the celebs fashioned near or removed from the galactic middle.
But that’s not all that was taking place. As different researchers reported in 2018, one other galaxy as soon as hit our personal, giving the Milky Way many of the stars in its halo, which engulfs the disk (SN: 11/1/18). Halo stars have little iron.
The new work revises the date of this nice galactic encounter: “We found that the merger happened 11 billion years ago,” Xiang says, a billion years sooner than thought. As the intruder’s gasoline crashed into the Milky Way’s gasoline, it triggered the creation of so many new stars that our galaxy’s star formation fee reached a report excessive 11 billion years in the past.
The merger additionally splashed some thick disk stars up into the halo, which Xiang and Rix recognized from the celebs’ larger iron abundances. These “splash” stars, the researchers discovered, are a minimum of 11 billion years previous, confirming the date of the merger.
The thick disk ran out of gasoline 8 billion years in the past and stopped making stars. Fresh gasoline across the Milky Way then settled right into a thinner disk, which has given beginning to stars ever since — together with the 4.6-billion-year-old solar and most of its stellar neighbors. The skinny disk is about 2,000 light-years thick in our a part of the galaxy.
“The Milky Way has been quite quiet for the last 8 billion years,” Xiang says, experiencing no additional encounters with massive galaxies. That makes it completely different from most of its friends.
If the thick disk actually existed 13 billion years in the past, Xiang says, then the brand new James Webb Space Telescope (SN: 1/24/22) could discern comparable disks in galaxies 13 billion light-years from Earth — portraits of the Milky Way as a younger galaxy.