Our Part of the Milky Way Is Four Times Bigger Than We Thought

Radio astronomers announce a major new finding that changes how scientists view the structure of our home galaxy.

The section of the Milky Way galaxy that contains our solar system is four times larger than astronomers previously thought.

Published in the journal Science Advances this week, a new study reported that our surrounding area of stars, gas, and dust—called the Local Arm, Orion Spur, or Orion–Cygnus Arm—is actually about 20,000 light-years long.

The immediate implications are that the galaxy is actually a little more symmetrical and regular than scientists previously thought, says one of the study's co-authors, Mark J. Reid of the Harvard-Smithsonian Centre for Astrophysics. Although a better understanding of our galaxy's structure doesn't necessarily mean gravity or other forces acting on us are different from what we expect, it could help us better understand large-scale features in the future, notes Reid.

The Local Arm is still considerably shorter than the Milky Way’s four main arms, which wind around the galaxy's centre for roughly 80,000 light-years. The total diameter of the galaxy is often reported as about 100,000 light-years, although Reid says more recent evidence suggests it is probably more like 70,000 light-years, at least to the edge of where star density starts to really drop off.

"When we actually measured distances in the Local Arm we were surprised," says Reid. "A lot of the material that we thought was in a nearby arm was actually in the Local Arm."

The new paper also found that the Local Arm has a similar pitch angle and rate of star formation as the larger arms (the two biggest arms are known as the Scutum–Centaurus and Carina–Sagittarius Arms). That suggests that the Local Arm is more like a true arm, albeit smaller, and less like a spur, which tends to lie in a cross-cutting direction to the main arms.

The Milky Way contains more than a billion stars (and possibly hundreds of billions), as well as lots of gas and dust. Our galaxy is in the shape of a flattened disk (it’s only about a thousand light-years thick), with an overall spiral shape—a relatively common form in our universe of many galaxies.

Our solar system is located roughly three-quarters of the way to the Milky Way’s edge, says Reid. It takes the sun about 250 million years to make one rotation around the galaxy.

To measure the Local Arm, the team of scientists from five countries looked at radio wave emissions collected by the Very Long Baseline Array, a series of 10 radio telescopes that are spread out over a wide area, from Hawaii to the Virgin Islands. The telescopes are coordinated from a base in New Mexico.

In 2010, the telescopes were put to use in an ambitious project to better map the 3-D structure of our galaxy, with the goal of helping scientists better understand how it works.

"Since we are inside of it we can’t really see what it looks like," says Reid. "It's sort of like 500 years ago, when people wanted to know what the rest of the Earth looked like. So our project hopes to help us better understand the nature of the Milky Way by measuring distances between regions."

That data will also help scientists refine our estimates of how much mass the galaxy contains.
Denilso Camargo of Federal University of Rio Grande do Sul in Porto Alegre, Brazil, told the New York Times that the new paper "provides important contributions to the better understanding of our galaxy." Camargo did not contribute to the paper.

In May, other scientists announced that our galaxy is also slimmer than expected, a mere 700 billion times the mass of our sun.

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