If we could view our home galaxy, the Milky Way, from afar, this Hubble Space Telescope portrait of UGC 12158 is probably very close to how it would look.

Refining Our Place in the Milky Way

Has our solar system’s address just changed?

Using somewhat unscientific language in a news release in June, the National Radio Astronomy Observatory, in Charlottesville, Virginia, announced that ‘our solar system’s Milky Way neighbourhood just went upscale.’

Today’s astronomy textbooks all show our Sun and solar system residing in a ho-hum distant suburb of the Milky Way Galaxy, far from the glowing downtown hub. We live more than halfway out from the galactic core in a region once known as the Orion Spur, nestled between two major spiral arms.

If we could view our home galaxy, the Milky Way, from afar, this Hubble Space Telescope portrait of UGC 12158 is probably very close to how it would look. Although UGC 12158 is 50 percent bigger, its shape and spiral structure bear a striking resemblance to the Milky Way Galaxy.

A new high-resolution radio telescope study, which shows that the region where the Sun lives is actually part of a larger, previously poorly surveyed structure radio astronomers have dubbed the Local Arm, has led to a remapping of our sector of the galaxy. We’re still tucked between the inner Sagittarius Arm and the beefy outer Perseus Arm, but our stellar byway has more muscle than we thought.

This has been a tricky bit of interstellar cartography because we reside inside the pancake-shaped Milky Way Galaxy, which is laden with dust clouds that block the view of much of the galaxy in visible light. But radio and infrared light can penetrate the dusty smog and let us see clear across the galaxy.

In 2008, infrared observations from NASA’s Spitzer Space Telescope resulted in a revised map of the Milky Way that showed two major arms — Centaurus and Perseus — attached to the ends of a thick central bar. (The Perseus Arm is labelled in the illustrations below; the Centaurus Arm is not but is easily identified emerging from the upper end of the galaxy’s central bar.)

Milky Way schematics
Because Earth is located inside our home galaxy, the Milky Way, the task of mapping the galaxy’s structural details has occupied astronomers for well over a century. Today, the major spiral arrangement with its central bar is understood, but some details remain fuzzy. The latest revision in the ongoing mapping process is shown below at right, where the extent and position of our Local Arm has been added. Compare with the previous version at left. Illustrations courtesy NRAO

The major arms consist of the highest densities of both young and old stars. The minor arms are primarily filled with gas and pockets of star-forming activity.

From 2008 to 2012, radio astronomers mapped our galactic neighbourhood using the Very Long Baseline Array, 10 radio tele­scopes spanning more than 8,000 kilometres. The array yields images as sharp as what would be provided by a single continent-sized dish antenna. With this capability, astronomers were able to use simple trig­onometric parallax (using the Earth’s orbit as the baseline) to measure distances to nearby star-forming regions. The radio telescopes didn’t look directly at stars but picked up emissions of water and methanol molecules in precursor star-birth nebulas that boost microwave frequencies.

As it plies its 250-million-year orbit about the galactic centre, the Sun is just passing through the Local Arm. The name “Local Arm” bothers me. It’s as mundane as “Local Group”—the name given to the small neighbourhood of galaxies that we inhabit.

As the mapping of the galaxy’s spiral arms has developed over the past 70 years or so, the structure where our Sun resides has often been called the Orion Arm, or the Orion Spur, when it was thought to be a stump rather than a full arm. Now, because it appears to have more heft, Orion Arm seems more appropriate than Local Arm. But, for the present, we appear to be stuck with Local Arm.

Perusing the Hubble Space Telescope archive of galaxies, I found a look-alike cousin to our new picture of the Milky Way—a spiral in the constellation Pegasus catalogued as UGC 12158, above. Like the Milky Way, it has a small bar structure in the centre and an arrangement of several spiral arms. UGC 12158 is 140,000 light-years across, compared with the Milky Way Galaxy’s 100,000 or so, and 400 million light-years distant. The light we are now receiving left UGC 12158 just as life was first crawling out of our oceans.

Ray Villard is news director for the Space Telescope Science Institute at Johns Hopkins University, in Baltimore, Maryland.