The Most Distant Stars In Our Galaxy Are Halfway To Andromeda, Say Scientists


Have you ever seen the Andromeda galaxy? Go outside after dark this month and look high up in the northeast sky and you’ll find the W-shaped constellation of Cassiopeia. Think of its second V-shape as an arrow. It points straight at a fuzzy patch that, under the darkest skies though more likely through binoculars, looks like what it is—a very large and very close galaxy.

Also called M31—and our closest giant neighboring galaxy at just 2.5 million light years away—Andromeda is home to at least a trillion stars. Like the Milky Way, it’s a spiral galaxy, but the distance between the two local giants may not be as great as previously thought.

Presented at the American Astronomical Society meeting in Seattle last week, a new study claims to have discovered about 200 ancient stars in the Milky Way’s halo, the most distant of which is more than a million light-years from Earth. That’s almost half the distance to Andromeda.

So where do the two galaxies end—and perhaps meet? “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, who was part of the research team. “Our galaxy and Andromeda are both so big, there’s hardly any space between the two galaxies.”

What is a “RR Lyrae” star? Consider this: not all stars shine as bright as each other. In short, a star may look bright because it is very luminous or because it is very close. That’s a conundrum for astronomers trying to figure out distances in the cosmos. However, an “RR Lyrae” star pulsates in a certain way, so astronomers can identify them and calculate how far away they are.

The stars in question are known as “RR Lyrae” stars, a type of variable star that blinks so predictably that they’re known as “standard candles.” They allow astronomers to accurately measure galactic distances solely by their brightness.

RR Lyrae are old stars with very specific physical properties that cause them to expand and contract in a regularly repeating cycle. They typically reside in ancient stellar populations over 10 billion years old.

“The way their brightness varies looks like … 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 addition, if you measure their average brightness, it is the same from star to star. This combination is fantastic for studying the structure of the galaxy.”

The discovery confirms our Milky Way galaxy to be a lot larger than it appears. The galaxy’s disk is about 100,000 light years across, but the halo—which contains the oldest stars in the galaxy—extends for hundreds of thousands of light-years in every direction. Maybe as much as a million light-years.

“The halo is the hardest part to study because the outer limits are so far away,” said GuhaThakurta. “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 research is based on data from the Next Generation Virgo Cluster Survey (NGVS), a program using the Canada-France-Hawaii Telescope (CFHT) to study a cluster of galaxies well beyond the Milky Way.

xWishing you clear skies and wide eyes.



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