A team of astronomers has discovered some of the oldest stars in our Milky Way. Just like humans, stars have a life span: birth, youth, adulthood, senior and death. The study led by scientists at Georgia State University focused on old or "senior citizen" stars, also known as cool subdwarfs, which are much older and cooler in temperature than the sun.
In this study, astronomers conducted a census of our solar neighborhood to identify how many young, adult and old stars are present. They targeted stars out to a distance of 200 light years, which is relatively nearby considering the galaxy is more than 100,000 light years across. A light year is how far light can travel in one year. This is farther than the traditional horizon for the region of space that is referred to as "the solar neighborhood," which is about 80 light years in radius.
The astronomers first observed the stars over many years with the 0.9 meter telescope at the United State's Cerro Tololo Inter-American Observatory in the foothills of the Chilean Andes. They used a technique called astrometry to measure the stars' positions and were able to determine the stars' motions across the sky, their distances and whether or not each star had a hidden companion orbiting it.
The team's work increased the known population of old stars in our solar neighborhood by 25 percent. Among the new subdwarfs, the researchers discovered two old binary stars, even though older stars are typically found to be alone, rather than in pairs. The authors then took a careful look at one particular characteristic of known subdwarf stars - how fast they move across the sky.
"Every star moves across the sky," lead author Wei-Chun Jao. "They don't stay still. They move in three dimensions, with a few stars moving directly toward or away from us, but most moving tangentially across the sky. In my research, I've found that if a star has a tangential velocity faster than 200 kilometers per second, it has to be old. So, based on their movements in our galaxy, I can evaluate whether a star is an old subdwarf or not. In general, the older a star is, the faster it moves."
Finding old stars could also lead to the discovery of new planets, Jao noted. "Maybe we can find some ancient civilizations around these old stars," Jao said. "Maybe these stars have some planets around them that we don't know about."
The research appears in The Astronomical Journal.