An astronomer has developed a new technique for calculating the masses… (Getty Images )
White dwarf stars are dying stars — burned-out cinders that have exhausted the hydrogen that sustains them. But scientists may soon count on these stellar flameouts to unravel the history of the Milky Way.
In a study published online Wednesday by the journal Nature, astronomer Jason Kalirai described a new technique for calculating the masses and ages of old stars based on the masses of the white dwarfs they have become.
The new information will help researchers better understand the formation of Earth's galaxy. "If we want to assess when components of the Milky Way formed, we need the ages of the stars," said Kalirai, who is based at the Space Telescope Science Institute in Baltimore.
Kalirai focused on white dwarfs in a part of the Milky Way known as the halo — a vast sphere, about 500,000 light-years in diameter, that envelops the galaxy's more familiar bulge and disk. The halo is composed mostly of very old stars, for which scientists have had a hard time establishing a precise age using traditional techniques that rely on measurements of brightness and temperature.
To get his sharper age estimate, Kalirai first looked at light from newly formed white dwarf stars in Messier 4, a nearby cluster of stars known to be about 12.5 billion years old. Their spectra contained information about hydrogen that he used to determine the stars' masses.
The age of a star is directly related to its mass. The more massive a star, the more pressure at its core and the faster it burns through its hydrogen supply. Less massive stars burn more slowly.
Turning to four newly formed white dwarfs in the Milky Way's halo, Kalirai found that they were more massive than newly formed white dwarfs in Messier 4. That allowed him to calculate that their progenitor stars were also heavier, which means the halo stars were younger — about 11.4 billion years old, give or take 700 million years.
Timothy Beers, director of the Kitt Peak National Observatory in Tucson, said that Kalirai's new age estimate for the inner halo stars supported the findings of his own research, which suggests that the halo contains a population of more massive stars in the interior and an older, smaller cohort farther out. That would mean that it formed through two steps involving an assemblage of massive, metal-rich stars in the inner halo and a separate, continual accretion of smaller objects that created the outer halo.
"Increasing the precision in the age estimate helps us resolve the entire sequence," said Beers, who wrote an editorial in Nature that accompanied the study. "This tool provides the most precise way we know to measure the ages of these populations."
Rosemary Wyse, an astronomer at Johns Hopkins University in Baltimore who also was not involved in the study, said that using white dwarfs to measure stellar ages was a decades-old idea, but that it's only become possible recently, as data from more powerful telescopes have begun pouring in.
Kalirai used telescopes at the Keck Observatory in Hawaii and the European Southern Observatory in Chile.
Wyse said she was looking forward to seeing more age estimates of stars calculated with the new technique, including measurements of white dwarfs in the outer halo.
Kalirai said that was exactly what he planned to tackle next.
"If the outer halo is older than the inner halo, we should be able to measure that directly," he said.