In the quest to find the outer limits of our galaxy, astronomers have discovered over 200 stars that form the Milky Way’s edge, the most distant of which is over one million light-years away—nearly halfway to the Andromeda galaxy.
The 208 stars the researchers identified are known as RR Lyrae stars, which are stars with a brightness that can change as viewed from Earth. These stars are typically old and brighten and dim at regular intervals, which is a mechanism that allows scientists to calculate how far away they are. By calculating the distance to these RR Lyrae stars, the team found that the farthest of the bunch was located about halfway between the Milky Way and the Andromeda galaxy, one of our cosmic next-door neighbors.
“This study is redefining what constitutes the outer limits of our galaxy,” said Raja GuhaThakurta in a press release. GuhaThakurta is professor and chair of astronomy and astrophysics at the University of California Santa Cruz. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”
The Milky Way galaxy consists of a few different parts, the primary of which is a thin, spiral disk about 100,000 light-years across. Our home solar system sits on one of the arms of this disk. An inner and outer halo surround the disk, and these halos contain some of the oldest stars in our galaxy.
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Previous studies have placed the edge of the outer halo at 1 million light-years from the Milky Way’s center, but based on the new work, the edge of this halo should be about 1.04 million light-years from the galactic center. Yuting Feng, a doctoral student at the university working with GuhaThakurta, led the study and is presenting the findings this week at the American Astronomical Society meeting in Seattle.
“We were able to use these variable stars as reliable tracers to pin down the distances,” said Yuting Feng, a doctoral student at the university working with GuhaThakurta. “Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.”
Space is vast and lonely—but we can feel a bit cozier knowing that our galactic neighbor is closer than we thought.