Milky Way Galaxy's Local Arm Far Larger Than Previously Thought

Astronomers once thought that our Solar System's Milky Way galaxy resided in a small structure called the Local Arm, a branch located between the two major spiral arms of our home galaxy. Now, though, they've discovered that the Local Arm is far larger than previously thought and is more like the adjacent major arms rather than a small spur.

It's hard to know the shape of our own galaxy. After all, it's difficult to view something that you're actually inside. Yet researchers have estimated the shape in the past by measuring cosmic distances. Despite this, though, astronomers have not been able to be absolutely certain about our galaxy's shape.

In order to actually find out what shape our galaxy takes, researchers had to turn to the National Science Foundation's Very Long Baseline Array (VLBA). This instrument possesses the ability to make the most accurate measurements of positions in the sky available to astronomers. More specifically, the VLBA can allow astronomers to use a technique that yields accurate distance measurements unambiguously through simple trigonometry.

The VLBA can precisely measure tiny shifts in apparent position of an object in the sky. Astronomers can then measure this shift and compare it to the background of a more-distant object. Called parallax, this effect can be demonstrated by holding your finger close to your nose and alternately closing each eye.

So what did the researchers find? They discovered that the Local Arm is actually far larger than previously thought. We reside between two major spiral arms of the galaxy, called the Sagittarius Arm and the Perseus arm. While the Sagittarius Arm is closer to the galactic center, the Perseus Arm is further out in the galaxy. The Local Arm is now thought to be a major structure between the two, and may even be a branch of the Perseus Arm.

"Our new evidence suggests that the Local Arm should appear as a prominent feature of the Milky Way," said Alberto Sanna of the Max-Planck Institute for Radio Astronomy in a news release.

The findings have told us a little bit more about the structure of our own galaxy, and may affect future modeling of the system. The new observations are actually part of an ongoing project called the Bar and Spiral Structure Legacy survey, which seeks to map the Milky Way. This survey could reveal new insights into our galaxy and beyond.