Our intergalactic neighbourhood is more prominent than we thought

Good news, everyone! We’re not intergalactic hicks after all! Turns out that our solar system is located in a large spiral branch called the Local Arm, a prominent feature of the Milky Way galaxy.

Top image: The left side shows what we used to think the Milky Way looked like, the right side shows the recently updated version. Image credit: Robert Hurt, IPAC; Bill Saxton, NRAO/AUI/NSF.

Astronomers have speculated that our solar system is situated in a kind of short “spur” — a minor offshoot located between the galaxy’s two long spiral arms, the Perseus Arm and the Sagittarius Arm. And indeed, given that we’re located within the Milky Way itself, astronomers have had a hard time painting an accurate picture of what our galaxy actually looks like. It's been 150 years since Stephen Alexander first suggested that the Milky Way is a spiral — but no consensus exists on the exact nature of its spiral arms.

Our intergalactic neighbourhood is more prominent than we thought

But now, by using a technique called the trigonometric parallax method, a team of astronomers led by Xu Ye have been able to get a better sense of our location relative to other stars. The technique works by using the National Science Foundation's Very Long Baseline Array (VLBA) to measure changes in our position as seen from opposite ends of Earth’s orbit (i.e. measurements made in June and December from 2008 to 2012). The array consists of 10 radio telescopes spread throughout the United States, Hawaii, and the Virgin Islands.

When combined with other historical data, these parallax measurements allowed the astronomers to study the distribution and 3-dimensional motions of star forming regions in the spiral arm over the entire northern sky.

“Our results suggest that the Local arm does not have the large pitch angle characteristic of a short spur,” they write in the study. “Instead its active star formation...suggest that it is more like the adjacent Perseus and Sagittarius arms; perhaps it is a branch of one of these arms.”

They note that it could also be a bifurcation of the Carina-Sagittarius arm, or an independent arm segment altogether.

Contrary to previous results, the astronomers now believe that the Local arm is closer to the Perseus than to the Sagittarius arm, suggesting that a branching from the former may be more likely.

As for the Local arm itself, our neighbourhood is about 5 kpc in length (16,308 light-years) and about 1 kpc in width (3,261 light-years).

Movin' on up!

You can read a preprint of the article: “On the Nature of the Local Spiral Arm of the Milky Way.” The paper is scheduled to appear in the upcoming edition of the Astrophysical Journal.