Last week, you began a tour of some of the finer sights in and around the Milky Way in the constellation Sagittarius. This week, let’s look slightly westward to see another handful of splendid sights along the Sagittarius Arm of the Milky Way. The tour follows the objects in the white font in the above image. Those in blue font were covered in last week’s tour.
The base of operations for this tour is the grand constellation Scorpius, the Scorpion. The long, winding constellation is one of the few that obviously resembles its namesake. The claws of the fearsome celestial arachnid face westward towards the relatively sparse star fields of the constellation Libra. At the heart of the scorpion lies the bright red-orange supergiant star Antares. And to the east lies the winding tail that passes through increasingly rich star fields towards the heart of the Milky Way Galaxy.
Antares is the brightest star in the constellation Scorpius and one of the brightest stars embedded in the sweeping arc of the Milky Way. It’s beautiful from the northern hemisphere, and like much of Scorpius, even more dazzling from the south where it lies almost directly overhead in June and July.
Many new stargazers mistake Antares for the planet Mars. So did the ancient Greeks who named it ant-Ares, which means “compared to Ares” (Ares was the Greek name for Mars, the god of war).
Like all red supergiants, Antares will soon run out of fuel. It has burned hydrogen, then helium, then carbon in its core. Then it will burn oxygen and neon and magnesium into heavier elements like iron. But it will go no further: fusing iron into heavier elements creates no energy. So when the fuel finally runs out, Antares will collapse and detonate as a supernova, the biggest explosion known in the universe. From our sky, it will shine bright enough to cast shadows in the dead of night for many weeks.
The star has a companion, a blue-giant star called Antares B, that’s nearly 3 arc-seconds away from the reddish Antares A, but some 370x fainter. Seeing the fainter star is not easy… it’s like trying to look for a firefly in the glow of a bright streetlight. Still, it’s worth a try. You’ll need a good 8-10″ scope to separate the two stars, and a magnification of perhaps 200x. It’s not a sight for beginners!
As you look at Antares in a pair of binoculars or in a telescope, you will notice a ball of stellar fuzz just 1.5º to the west. This is the lovely globular cluster Messier 4. A small telescope, one with 4″ or more aperture, will resolve some stars, at least in the halo. And an 8” scope will resolve the cluster to its core and reveal hundreds of yellow-white stars. Most observers notice a loosely defined “bar” of stars in M4 that runs north-south. To some, the cluster looks a little like a cat’s eye, with the bar taking the form of the eye’s pupil. The cluster is somewhat reminiscent, though a little smaller, than the cluster Messier 22 covered in last week’s tour.
Now to the splendid complex of bright stars and star clusters known as the False Comet. This sparkling group of associated stars in a rich section of the Milky Way presents the uncanny appearance to the naked eye of a small comet. It’s a beautiful sight for observers far enough south to see it. It lies too low to the horizon (or below the horizon) for observers north of about 40ºN latitude.
The False Comet is a group of two star clusters next to a string of brighter stars in the tail of the constellation Scorpius. The collection starts at zeta (ζ) Scorpii and spans some 2° of sky. The aggregation appears like a small comet with a curved tail pointing northward into the Milky Way.
Easily visible with the naked eye, the “False Comet” has been known since antiquity, although comet hunter Charles Messier was too far north to include it in his famous catalog. The star cluster NGC 6231 forms the “head” of the comet; the large open cluster Trumpler 24 forms the tail. While cataloged as separate clusters, these stars are physically associated and formed out of the same massive nebula only 6-8 million years ago. The collection is roughly 6,000 light years from Earth.
Now to a pair of the prettiest and brightest open clusters in the sky, clusters tangled in the Milky Way east of the tail of Scorpius. Messier 6 is the smaller of the two clusters. Look for it about 4o north of the bright star Shaula (λ Scorpii) in the Scorpion’s tail. M6 is a respectable 4th magnitude, though its light spreads over an area as large as the full Moon. You can see the cluster without optics in dark sky; a full Moon or city lights make it harder to see, especially if the cluster is low on the horizon. M6 is often called the Butterfly Cluster, and a glance through a small telescope reveals why. At 40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with two irregular loops of stars on either side (the wings). A little imagination reveals the butterfly’s “antennae” to the northeast.
Look 3.5º southeast of Messier 6 to find the cluster Messier 7 set in one of the richest sections of the Milky Way. Though they’re close in the sky, the two clusters are not physically associated. M7 is closer, just 800 light years away. To the unaided eye, in the words of Stephen J. O’Meara, the spray of light from M7 looks “like the eruption of distant fireworks.”
M7 appears larger and brighter than M6, though both were known in antiquity. M7 is often called Ptolemy’s cluster after the famous 1st-century astronomer Claudius Ptolemy. M7 is the southernmost Messier object, so it’s rarely seen well at northern latitudes, and it presents a real challenge for observers in northern Europe. The view of this cluster from the southern hemisphere, however, where it’s high overhead, is jaw-dropping.
Finally turn your gaze to the dark lanes and shapes that permeate the Milky Way. Well northeast of the spout of the Sagittarius “Teapot” lies a particularly stark collection of dark, dusty interstellar clouds that are collectively called the Pipe Nebula because of its resemblance to a smoking pipe. The tube of the pipe runs east-west, and the cup lies at the east end of the pipe pointing north. The nebula is a snap to see in images like the one at the top of the page, but it takes some patience to spot it visually with your unaided eye. Dark sky is essential.
And if you have a telescope, search off the western end of the Pipe Nebula for the modest globular cluster Messier 19. The cluster isn’t much to look at compared to the other sights on this tour, especially compared to the globulars M22 and M4, but it does have one remarkable feature: it is remarkably elliptical or oval-shaped compared to other such objects.
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