Throughout human history philosophers, theologians, and scientists assumed Earth was the center of the universe. Ptolemy’s geocentric model of the universe was used to prepare astrological charts for over 1,500 years. (No wonder the astrologers couldn’t make any successful predictions!)
The Copernican model of a sun-centered universe took hold only 400 years ago. And, less than 100 years ago, many astronomers thought that the sun was in the center of our Milky Way galaxy.
Fast-forward to today and astronomy textbooks all show our sun and solar system residing in a ho-hum back-ally neighborhood of the galaxy. We live halfway between the galactic core and the galactic center, on the edge in a region called the Orion Spur that is nestled between two major spiral arms.
But this week a cutesy press release from the National Radio Astronomy Observatory in Charlottesville, Va., announced that “our Solar System’s Milky Way neighborhood just went upscale.”
A high-resolution radio probe of our galaxy shows that the sun presently lives in the middle of a much larger structure the radio astronomers have dubbed the Local Arm. We’re still nestled between the inner Sagittarius Arm and beefy outer Perseus Arm, but our stellar byway has more muscle now.
This has been a tricky bit of interstellar cartography because we live inside the pancake-shaped galaxy. Dust clouds and star clouds block the view of most of the galaxy in visible light. But radio and infrared light can penetrate the dusty smog and see clear across the galaxy.
In 2008, NASA’s Spitzer Space Telescope observations led to a remapping of the Milky Way that showed two major arms, Scutum-Centaurus and Perseus, attached to the ends of a thick central bar. Two other spiral arms, Norma and Sagittarius, were demoted to minor arms because they are less distinct. 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 neighborhood using the precision Very-Long Baseline Array, ten radio telescopes spanning over 5,000 miles. 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 straightforward trigonometric parallax (using 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 that boost microwave frequencies.
The sun is just passing through the Local Arm along a 250 million year orbit about the galactic center. Still, we need a better name than Local Arm. That’s as mundane as the Local Group — the name for the backwater neighborhood of galaxies that we inhabit.
It’s a little too early and presumptive to call it the Federation Arm, as Trekkies might like. The easiest name with some gravitas would be the Orion Arm, which is already being used. Because it looks like our stellar arm does a dog-leg off of the Perseus Arm, how about the name Canis-Cruris Arm? (Latin for dog-leg.) The constellation Canis Major, the Great Dog, is alongside Orion in the winter sky.
Perusing the Hubble Space Telescope archive of galaxies I have found a look-alike cousin to our new view of the Milky Way. It is a modest-looking spiral in the autumn constellation Pegasus, cataloged as UGC 12158 (above). Like the Milky Way it has a small bar structure in the center and arrangement of several spiral arms like ours.
Too bad it’s 400 million light-years away. We couldn’t have any conversation with a pen-pal civilization located in the similar neighborhood in that galaxy. The light any extraterrestrials are receiving now left our galaxy just as life was first crawling out of our oceans.
Image credit: NASA, ESA, NRAO