We inhabit a giant spiral-shaped galaxy that glows with hundreds of billions of stars, a colossus so massive that at least two dozen lesser galaxies revolve around it. But how did this enormous entity arise? Clues come from the Milky Way's oldest and wisest stars—those in the stellar halo, the galactic component that envelops the bright disk housing the sun.
Halo stars stand out because they formed before supernova explosions had scattered a large amount of heavy elements into the galaxy, so halo stars possess little iron. The brightest halo members are iron-poor globular star clusters, spectacular objects that can pack hundreds of thousands of old stars into a sphere just a few dozen light-years across.
Now the Hubble Space Telescope has found that an individual halo star is even older than these ancient star cities and is thus an ideal time capsule passed down from the Milky Way's birth. "This is a really elegant piece of work," says Gerry Gilmore, an astronomer at the University of Cambridge in England who was not affiliated with the researchers. "They've done as good a job as it is possible to do."
Despite its importance, the stellar halo constitutes just a thousandth of the Milky Way's total mass. Although the halo extends far beyond the disk, most of its stars lie closer to the galaxy's center than we do, so globular clusters abound in constellations toward the galactic center, such as Scorpius and Sagittarius.
Don VandenBerg, an astronomer at the University of Victoria in British Columbia, and his colleagues measured the ages of two halo stars in the constellation Libra. Neither star belongs to a globular cluster, and both are subgiants—stars that are transitioning from the main-sequence stage in which our sun shines to the red-giant stage, when a star is much larger. The astronomers chose these stars because at a given temperature subgiants of different ages have different luminosities, so measuring the latter reveals the former.
VandenBerg's team used Hubble to determine that one of the halo stars, named HD 140283, is about 190 light-years from Earth. The distance revealed how much light the star emits. Models of stellar evolution indicate the star should attain this luminosity at an age of 14.3 billion years. That's slightly older than the 13.8-billion-year-old universe but the stellar age is uncertain by 0.8 billion years, so there's no conflict.
The star is much older than a globular cluster with the same chemical composition, however. "I think there's a real age difference," VandenBerg says. The cluster, named M92, lies in the constellation Hercules and is only about 12.5 billion years old—some 1.5 billion years younger than the star. Both have the same low iron abundance, about one two-hundred-fiftieth that of the sun.
It's the same story for the other star, named HD 132475, which is younger and richer in iron. It is some 320 light-years away and around 12.6 billion years old—about a billion years older than globular cluster M5, whose iron abundance of about one thirtieth of our sun’s matches that of the lone halo star. Thus, as the astronomers report in the September 10 issue of The Astrophysical Journal, both stars apparently formed well before the clusters they resemble. "It makes sense astrophysically," Gilmore says. Early on, he explains, the galaxy probably couldn't make big clusters but instead only individual stars and small stellar groups. Stars form when clouds of gas collapse. But to collapse a cloud must cool; in the modern Milky Way carbon and oxygen atoms radiate heat, cooling clouds to frigid temperatures. But the early galaxy had little carbon or oxygen. As a result, Gilmore says, something as grand as a globular cluster could emerge only after supernovae had cast these two crucial elements into space. So the first objects the Milky Way formed were instead individual stars.
Still, the new finding rests on just two stars. "One has to be cautious," VandenBerg warns. Fortunately, the situation will soon improve, because the European Space Agency’s Gaia spacecraft is measuring distances of countless stars, including subgiants in the halo. "We'll go from having two stars to having 20 million stars with precision numbers," Gilmore says. "And that's going to rewrite the book."
By the decade's end, the Gaia data should either confirm or refute the new work. Globular clusters are stunning sights through a backyard telescope but the most telling clues to the galaxy's earliest epoch may come from the lone stars scattered throughout its halo.