Astronomers have discovered more than 200 distant variable stars known as RR Lyrae stars in the stellar halo of the Milky Way.

The most distant of these stars lies more than a million light-years from Earth, almost half the distance to our neighboring galaxy, Andromeda, which is about 2.5 million light-years away. 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 map out the outer limits of the Milky Way’s halo. “This study is redefining what constitutes the outer limits of our galaxy,” said Raja Guha-Thakurta, professor and chair of astronomy and astrophysics at the University of California Santa Cruz (United States) in a statement. “Our galaxy and Andromeda They are both so large that there is hardly any space between the two galaxies.”

“This study is redefining what constitutes the outer limits of our galaxy,” said Raja Guha-Thakurta, professor and chair of astronomy and astrophysics at the University of California Santa Cruz (United States) in a statement. “Our galaxy and Andromeda They are both so large that there is hardly any space between the two galaxies.” GuhaThakurta explains that the stellar halo component of our galaxy is much larger than the disk, which is about 100,000 light-years across. Our solar system resides in one of the spiral arms of the disk. In the middle of the disk is a central bulge, and around it is the halo, which contains the oldest stars in the galaxy and stretches hundreds of thousands of light-years in all directions.

“The halo is the most difficult part to study because the outer limits are so far away,” GuhaThakurta said. “Stars are very few compared to the high stellar densities of the disk and bulge, but the halo is dominated by matter. dark and actually contains most of the galaxy’s mass.” Yuting Feng, a doctoral student working with GuhaThakurta at UCSC, led the new study and will present their findings at the American Astronomical Society meeting. According to Feng, previous studies had calculated that the stellar halo should extend to about 300 kiloparsecs or 1 million light-years from the galactic center. The 208 RR Lyrae stars detected by Feng and his colleagues were at distances between 20 and 320 kiloparsecs.

“We were able to use these variable stars as reliable tracers to pinpoint distances,” says Feng. “Our observations confirm the theoretical estimates of the size of the halo, so it is an important result.” The findings are based on data from the Next Generation Virgo Cluster Survey (NGVS), a program that uses the Canada-France-Hawaii Telescope (CFHT) to study a galaxy cluster far beyond the Milky Way. The survey was not designed to detect RR Lyrae stars, so the researchers had to dig them out of the data set. The Virgo Cluster is a large galaxy cluster that includes the giant elliptical galaxy M87.

“To get a deep exposure of M87 and its surrounding galaxies, the telescope also captured foreground stars in the same field, so the data we used is sort of a byproduct of that study,” Feng explains. According to GuhaThakurta, the excellent quality of the NGVS data allowed the team to obtain the most reliable and accurate characterization of the RR Lyrae at these distances. RR Lyrae are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle.