Our Galactic Neighborhood Is Bigger Than Scientists Once Thought

Goodbye spur, hello spiral arm

Milky Way
Our part of the Milky Way is more impressive than anyone thought. kobaken++ (Flickr/Creative Commons)

What shape is the Milky Way? For years, scientists have thought it was a spiral, like most other known galaxies. They also thought that our neighborhood, an area also known as the Local Arm, is just a little spur that circles around the center of the galaxy in perfect form. But new observations suggest that’s not true, reports Eva Botkin-Kowacki for The Christian Science Monitor. New analysis suggests that the Local Arm is much bigger than anyone thought, and the Milky Way looks pretty different than suspected.

In the study, recently published in the journal Science Advances, researchers confirm that the Local Arm is a major spiral structure instead of the smaller, secondary spiral spur it was once believed to be. Though it still seems to be shorter than the main arms of the galaxy, new measurements suggest that it’s more than 20,000 light years long—four times the previous measurement.

This discovery was made possible by the National Radio Astronomy Observatory’s Very Long Baseline Array, which consists of ten telescopes that are scattered around the world. It’s been fully online since 1993, when all ten began simultaneously taking measurements. The telescopes observe radio waves, recording all kinds of astronomical phenomena that could never have been spotted by the human eye. In this case, the telescopes were trained on the Orion Arm, another name for the Local Arm.

They were in search of star-forming regions, a classic way to trace the spiral arms that are packed with gas and dust—the makings of new stars. But the researchers found something unexpected: When they measured star-forming regions they once thought were in the Perseus Arm, a more distant, major spiral arm of the galaxy, they discovered that they were all part of the Milky Way instead. 

The results suggest that the Local Arm is comparable in size and star formation rates to major spiral arms nearby. And researchers now think that our galactic neighborhood seems to be a massive patchwork of features. 

"The new measurements suggest the Milky Way is not a grand design spiral with well-definied arms, but a spiral with many branches and subtle spurs," writes Rebecca Boyle for New Scientist

Mark Reid, an astronomer at the Harvard-Smithsonian Center for Astrophysics, tells Botkin-Kowacki that the reason they never observed the structure before was because observations are taken from inside the galactic disk. When viewed from within, it’s hard to see the spiral pattern, and dust makes it even harder to tell what’s happening. Since we’re on the inside, it’s impossible to see more than a certain angle of a certain part of the galaxy—a quandary that has led to ongoing controversy about what the galaxy looks like in the first place.

Thanks to the new, more precise observations and calculations, researchers were able to get a more complex picture of the galaxy—one that seems more like a patchwork quilt than a perfect spiral. Until they’re able to get observations from the outside, scientists will have to content themselves with measurements taken from within. But don’t worry; those measurements are becoming more and more precise. And with the help of spacecraft like ESA’s Gaia satellite, which recently provided observations for the most complete map of the galaxy to date, the picture becoming clearer every day.

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