Astronomers have discovered a slew of stars lingering in the far fringes of our galaxy. And when we say far, we mean far.
The most distant of these stars is located more than 1 million light-years away. That’s almost halfway to our largest galactic neighbor, the Andromeda Galaxy, which is located some 2.5 million light-years away.
“This study is redefining what constitutes the outer limits of our galaxy,” said Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz, in a news release. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”
The results were presented on January 9 and 11 at the 241st meeting of the American Astronomical Society (AAS) in Seattle, Washington.
Living on the edge
The stars identified in this study are a special class of star known as RR Lyrae stars. These stars are important to astronomers because they exhibit characteristic pulsations and variations in brightness.
“The way their brightness varies looks like an EKG — they’re like the heartbeats of the galaxy — so the brightness goes up quickly and comes down slowly, and the cycle repeats perfectly with this very characteristic shape,” GuhaThakurta said in the release. “In addition, if you measure their average brightness, it is the same from star to star.”
This predictable behavior makes RR Lyrae stars an excellent type of distance indicator that astronomers call a “standard candle.” Because their intrinsic brightness is known, it is relatively easy to determine their distance, with errors ranging from just 3 to 5 percent, GuhaThakurta told Astronomy at the meeting.
Unlike the Sun, these newfound stars do not reside in the Milky Way’s familiar thin disk, which is about 100,000 light-years across and home to most of our galaxy’s young stars. Instead, these RR Lyrae stars live in the Milky Way’s expansive halo, a spherical shell of mostly ancient stars that encapsulates our galaxy’s spiral arms and central bulge.
