Have you ever wondered how big our galaxy really is? Well, you might be surprised to learn that some of the stars in the Milky Way are so far away that they are almost halfway to our neighboring galaxy, Andromeda. That’s right, the outermost fringes of our galaxy extend for more than a million light-years in every direction, reaching nearly half the distance to Andromeda, which is about 2.5 million light-years away.
This astonishing discovery was made by astronomers who found more than 200 variable stars called RR Lyrae stars in the Milky Way’s halo, which is a spherical shell of old stars and dark matter surrounding the galaxy’s disk. RR Lyrae stars are useful for measuring galactic distances because of their predictable pulsations and brightness. They are also very old, dating back to the early stages of the galaxy’s formation.
“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. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”
The researchers used data from the Next Generation Virgo Cluster Survey (NGVS), a project meant to study a group of galaxies far beyond the Milky Way. They were able to identify 208 RR Lyrae stars in the Milky Way’s halo, ranging in distance from about 20 to 320 kiloparsecs (one kiloparsec equals 3,260 light-years). The most distant ones are over a million light-years away.
“We were able to use these variable stars as reliable tracers to pin down the distances,” said Yuting Feng, a doctoral student at UC Santa Cruz and the lead author of the study. “Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.”
The halo is not only huge, but also mysterious. It contains most of the mass of the galaxy, but most of it is invisible dark matter. The stars are very sparse compared to the high stellar densities of the disk and the bulge. The halo also contains clues about the history and evolution of the galaxy.
“The halo is the hardest part to study because the outer limits are so far away,” GuhaThakurta said. “The stars are very sparse compared to the high stellar densities of the disk and the bulge, but the halo is dominated by dark matter and actually contains most of the mass of the galaxy.”
The new findings were presented at the 241st meeting of the American Astronomical Society in Seattle on January 9 and 11. They shed new light on the structure and extent of our galaxy, as well as its relationship with Andromeda. The two galaxies are expected to collide in about 4 billion years, creating a new giant galaxy.
So next time you look up at the night sky and marvel at the beauty of our galaxy, remember that some of its stars are closer to another galaxy than they are to us. And that’s just one of the many wonders of our cosmic neighborhood.
– Astronomers find the most distant stars in our galaxy halfway to Andromeda, ScienceDaily and University of California – Santa Cruz, January 9, 2023
– Milky Way’s farthest stars reach halfway to Andromeda, EarthSky, January 17, 2023
– Newly Found Stars are Technically in the Milky Way, but They’re Halfway to Andromeda, Universe Today, January 10, 2023
– Astronomers discovered the most distant stars in our galaxy, halfway to Andromeda, Tech Explorist, January 10, 2023