Astronomers have discovered more than 200 distant variable stars known as RR Lyra stars in the stellar halo of the Milky Way. The farthest of these stars is one million light-years from Earth, nearly half the size of our neighboring galaxy, Andromeda, which is about 2.5 million light-years away. The characteristic vibrations and brightness of RR Lyra 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 halo.

“This work redefines the outer limits of our galaxy,” said Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz. “Our galaxy and Andromeda are so large that there is almost no space between them.”

GuhaThakurta explained that the stellar halo of our galaxy is much larger than the disk, about 100,000 light-years in diameter. Our solar system is located in one of the spiral arms of the disk. At the center of the disk is a central protuberance surrounded by a halo that contains the oldest stars in the galaxy and stretches hundreds of thousands of light-years in any direction.

“Hale is the hardest part to study as the outer borders are so far away,” GuhaThakurta said. “Stars are very rare compared to the high stellar density of the disk and bulge, but the halo is governed by dark matter and actually contains most of the galaxy’s mass.”

Yuting Feng, a postdoctoral researcher working with GuhaThakurta at UCSC, led the new study and presented his findings in two papers at the American Astronomical Society meeting in Seattle on January 9 and 11. According to Feng, previous modeling studies estimated that the stellar halo should be about 300 kiloparsecs, or 1 million light-years from the center of the galaxy. (Astronomers measure galactic distances in kiloparsecs; one kiloparsec equals 3,260 light-years.) The star 208 RR Lyra, discovered by Feng and colleagues, was between 20 and 320 kiloparsecs.

“We were able to use these variable stars as reliable trackers to determine distances,” said Feng. “Our observations confirm the theoretical estimates of halo size, so this is an important result.”

The findings are based on data from the Virgo Next Generation Cluster Survey (NGVS), a program that uses the Canada-France-Hawaii Telescope (CFHT) to study clusters of galaxies far beyond the Milky Way. The survey was not designed to detect Lyra’s RR stars, so the researchers had to remove them from the dataset. The Virgo Cluster is a large galaxy cluster that includes the giant elliptical galaxy M87.

“To get a deep exposure of M87 and the surrounding galaxies, the telescope also captured foreground stars from the same area, so the data we used is a kind of byproduct of this research,” Feng explained.

The excellent quality of the NGVS data allowed the team to obtain the most reliable and accurate characterization of RR Lyrae at these distances, according to GuhaTakurta. RR Lyres are ancient stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle.

“The way their brightness changes is like an EKG – they are like the heartbeat of a galaxy – so the brightness increases rapidly and gradually decreases and the cycle repeats itself perfectly with this very characteristic shape,” said GuhaThakurta. “Also, if you measure their average luminosity, it’s the same from star to star. This combination is great for studying galaxy structure.”

The sky is full of stars, some brighter than others, but because a star is too bright or too close, it can appear bright and be difficult to distinguish. Astronomers can identify an RR Lyra star from its characteristic vibrations and then use its observed brightness to calculate how far away it is. However, the process is not easy. More distant objects, such as quasars, may appear like RR Lyra stars.

“Only astronomers know how painful it is to get reliable indications of these distances,” Feng said. “This powerful sample of distant RR Lyra stars gives us a very powerful tool to study halos and test our current models of the size and mass of our galaxy.”

This study is based on observations obtained on the Canada-France-Hawaii Telescope (CFHT) operated by the CFHT and CEA/IRFU joint project MegaPrime/MegaCam and the National Research Council of Canada (NRC), Institut National des Sciences. l’Univers of the Center National de la Recherche Scientifique (CNRS) France and University of Hawaii.