The distant stars analysed, of the RR Lyrae type, provide a very powerful tool to study the galactic halo and corroborate current theses about the size and mass of our galaxy.
Astronomers from the University of California (UC) in Santa Cruz, USA, have discovered in the stellar halo of the Milky Way the farthest star from Earth in the galaxy. It is a RR Lyrae type star, which astronomers study to measure distances. The star is more than a million light-years from Earth, almost half the distance from our neighboring galaxy, Andromeda, which is located about 2.5 million light-years away, they reported Monday.
Estimate stellar distances
The characteristic pulsations and brightness of RR Lyrae stars make them excellent “standard candles” for measuring galactic distances. These new observations allowed the researchers to trace the outer limits of the Milky Way’s halo. “This study is redefining what constitutes the outer limits of our galaxy“, said Raja GuhaThakurta, a professor at UC Santa Cruz. “Our galaxy and Andromeda are so large that there is hardly any space between the two galaxies,” he said.
“The halo is the hardest part to study because the outer limits are so far away,” GuhaThakurta explained. He warned that our galaxy’s stellar halo envelops the disk, which is only about 100,000 light-years across, while the halo extends hundreds of thousands of light-years in all directions and contains the oldest stars in the galaxy. “Stars are very few compared to the high stellar densities of the disk […] but the halo is dominated by dark matter and actually contains most of the galaxy’s mass,” the astronomer revealed.
The doctoral student who led the new research, Yuting Feng, says that other studies had theoretically calculated that the stellar halo should extend about a million light-years from the galactic center. For its part, the new study revealed that the 208 RR Lyrae stars, detected by Feng and his colleagues, They are located between 65,200 and 1,043,200 light years away from the center of our galaxy..
RR Lyrae are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle. “The way their brightness varies resembles an electrocardiogram, they are like the heartbeat of the galaxy“, GuhaThakurta describes. “Also, if you measure its average brightness, it is the same from star to star. This combination is fantastic for studying the structure of the galaxy,” she said.
“We were able to use these variable stars as reliable trackers to pin down distances,” Feng said. “Our observations confirm the theoretical estimates of the size of the halo, making it an important result.“, he stated. Astronomers can identify an RR Lyrae star from its characteristic pulsations and then use its observed brightness to calculate how far away it is.
“This robust sample of distant RR Lyrae stars gives us a very powerful tool to study the halo and test our current models of the size and mass of our galaxy.”