Milky Way stars found nearly halfway to Andromeda Galaxy

A SLEW OF NEWFOUND STARS LINGERING in the fringes of our galaxy are so far away, they could redefine the outer limits of the Milky Way. The most distant of these stars is more than 1 million light-years from the galactic center. That’s almost halfway to our largest neighbor, the Andromeda Galaxy, which is some 2.5 million light-years away.

The stars identified in this study — a class of variables known as RR Lyrae stars — were plucked from a survey by the Canada-France-Hawaii Telescope on Mauna Kea. The dataset focused on the Virgo Cluster of galaxies, which lies outside our own Local Group. But the Milky Way stars were waiting in the foreground of the data, the researchers say. The results were presented in January at the 241st meeting of the American Astronomical Society in Seattle, Washington.

RR Lyrae stars are valuable to astronomers because they all have roughly the same mass and luminosity — and also exhibit characteristic pulsations, resulting in predictable variations in brightness that make them easy to identify. This makes RR Lyrae stars an excellent type of distance indicator that astronomers call a standard candle. Because their intrinsic brightness is known, it is easy to determine their distance based on how faint they appear to be.

The 208 RR Lyrae stars detected by the team are located in the Milky Way’s outer halo and lie between about 65,000 to 1.05 million light-years from the galactic center. This is consistent with models that predict the outer halo should extend that far, Yuting Feng, a doctoral student at the University of California, Santa Cruz, who led the study, tells Astronomy. “So that’s an important result.”

But those same models predict that beyond 1 million lightyears, stars should get much sparser. The fact that Feng and his colleagues have been able to see so many stars near this theoretical border suggests that this dip in stellar density might occur farther out. If that’s the case, there’s a chance that the halos of the Milky Way and Andromeda already overlap. “It challenges the notion that galaxies are islands with big, empty space between them,” Raja GuhaThakurta, Feng’s advisor, says.

“I won’t be surprised if there are a few dozens of stars which are being contested by the two galaxies,” Feng adds.

NASA, ESA, AND A. FEILD (STSCI) URBAN SPRAWL. The Milky Way’s outer halo extends some 1 million lightyears from the core of our galaxy — and a few newfound stars are living right at the theoretical outskirts.