Milky Way Intimacy - Part 2

In the last Sky Watch article we were trying to wrap our minds around what we see when we look toward the center of our Milky Way Galaxy. As I explained, we live amongst a swirling disc of stars with a gigantic bulge in the middle, at the center of which is a supermassive-black hole, which has been devouring all that ventures near it for billions of years. Most of our view in that direction is blocked by the clouds of dust and gas that will one day be the stuff of new stars and planets. As I said, our perspective is like sitting on a raisin inside a giant cinnamon roll. Sorting out the layers of this pastry from the inside is indeed one of humanity’s greatest achievements.

These two articles build on a series of enlightening articles published by Craig Crossen in Sky & Telescope magazine entitled “Observing the Milky Way.” Over the course of the past year, he took us on a step-by-step tour of the entire Milky Way, providing depth and insight I’ve never had before. I want to share with you how to trace out small portions of two of the arms of our galaxy.

Between the galactic center and us lies several sweeping arms that contain millions of stars. The nearest one to us is called the Sagittarius-Carina Arm. (Note that these names have not yet been standardized; I’m using the ones that Mr. Crossen used in his article.) In the direction of the galactic center, the Sagittarius-Carina Arm lies about four thousand light-years away from us. We can identify it by “connecting the dots” with several of its most beautiful features.

(Visualizing the following discussion may be difficult without a star chart. If you don’t have one handy, you can easily find one online.)

Low in the south, at the point where Scorpius’ tail begins to curve to the left, is a knot of faint stars known as NGC 6231, or the Northern Jewel Box. This star cluster is the centerpiece of a larger group known as the Scorpius OB1 Association, “one of the richest concentrations of extremely hot and luminous O-type giants and supergiants known in our galaxy.” It’s one of the most stunning binocular sights. Indeed, all of the features mentioned in this article can be seen with a decent pair of binoculars. Of course, a telescope will bring out more striking features.

NGC 6231 lies to the right of the galactic center. That part of the Sagittarius-Carina Arm arches out away from the center of the galaxy and is about six thousand light-years from us. The further we look to the right, the further away the arm recedes from us.

Looking up and to the left of the Jewel Box (in the evening summer sky) is a large, dark cloud of dust and gas. To its left is the famous “steam” rising out of the “teapot” of Sagittarius. It is natural for our eyes to gravitate toward the streaks of bright stardust on either side of these avenues of darkness. It’s important to realize that we are indeed looking at something, not nothing. These dust lanes mark the true centerline of the galactic plane. Moreover, these dust clouds are darker than the rest of the night sky, demonstrating that outer space isn’t as pitch black as we think it is!

While the centerline of the galaxy is visually obscured, we can still trace the galactic arms via the lighted portions above and below. To the left of the Great Rift in Sagittarius is a string of nebulae (gas and dust clouds) that connect the dots that outline the Sagittarius-Carina Arm.

Our next waypoint is M8, the Lagoon Nebula. One of the brightest emission nebulae, it lies just below the galactic plane. M8 is to the left of the galactic center. Here the Sagittarius-Carina Arm begins to bend away from us and in toward the central bulge.

Further up and to the left is M17 (the Swan Nebula) and M16 (the Eagle Nebula). Here the Sagittarius-Carina Arm is really curving back around the galactic nucleus. M17 is about five thousand light-years distant, but nearby M16 is seven thousand light-years away. As we look at these two in the same binocular field, we’re looking tangentially along an inner edge of a galactic spiral arm, with M16 two thousand light-years behind M17.

The next arm in toward the center, the next layer in the cinnamon roll, if you will, is the Norma Arm. It is largely obscured by the Sagittarius-Carina Arm; yet there are enough holes through which we can make out a great deal.

M24, the Small Sagittarius Star Cloud, is a bright patch just to the right (on the galactic plane) of M17. Yet, it is two to three times further away (10,000-16,000 light-years).

If that isn’t deep enough, we can actually see our galaxy’s central bulge. The Great Sagittarius Star Cloud--the steam rising out of the Teapot--is a clear shot of at least 20,000 light years toward the center of our galaxy. We’re lucky to have such a window on the inner grandeur of our galaxy.

I’ve only touched upon a few highlights of the splendor of our galaxy. Craig Crossen’s Sky & Telescope articles take us clear around the Milky Way, even those parts in the Southern Hemisphere that we cannot see. Sky & Telescope is considering releasing his articles as a monograph. If they do, I’ll certainly let you know.

I’m planning to present a lecture on this topic at the Brite Lake Star Party on August 23rd. Bring your telescope, binoculars, star charts, friends, and family for a night of wonder! The gates open at 7:30 p.m. The event is sponsored by the Antelope Valley Astronomy Club, the Greater Tehachapi Astronomy Club, the Tehachapi Valley Recreation and Parks District, and your local NASA-JPL Solar System Ambassadors. See you there! Until then, keep an eye above the horizon!