Astronomers discover oldest stars in Milky Way galaxy

Astronomers have discovered some of the oldest stars in our Milky Way galaxy by determining their locations and velocities.

Just like humans, stars have a life span: birth, youth, adulthood, senior and death.

Scientists at Georgia State University in the US focused on old or "senior citizen" stars, also known as cool subdwarfs, that are much older and cooler in temperature than the Sun.

In a study, published in The Astronomical Journal, astronomers conducted a census of our solar neighbourhood 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 neighbourhood," which is about 80 light years in radius.

"The reason my horizon is more distant is that there are not a lot of senior citizens (old stars) in our solar neighbourhood," said lead author of the study Wei-Chun Jao, from the Georgia State University.

"There are plenty of adult stars in our solar neighbourhood, but there's not a lot of senior citizens, so we have to reach farther away in the galaxy to find them," said Jao.

The astronomers first observed the stars over many years with the 0.9 metre telescope.

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 neighbourhood by 25%. 

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.

Finding old stars could also lead to the discovery of new planets, Jao said.

"Maybe we can find some ancient civilisations around these old stars. Maybe these stars have some planets around them that we don t know about," Jao added.