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star Watch

Milky Way’s hub looms in the south after dark

We’re not quite at the midpoint of summer yet. We’ll cross that mark on Tuesday at 7:54 p.m., the halfway point between the June solstice and the September equinox.

But we’re close enough to the middle of summer for all practical purposes. One sign of this is that the constellation Sagittarius is now glimmering at its highest due south right after dark.

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Sagittarius, in classical sky lore, was supposed to be a centaur shooting an arrow, and in a really dark sky with enough faint stars you can connect the dots to make a distorted, semi-abstract stick figure of a man-horse with a bow and arrow. It even matches up with the ancient Greeks’ descriptions of where his body parts and bow were supposed to be.

But don’t try to piece out such a thing in the skies under which most of us live. The light pollution over Greater Boston hides all but the stars plotted here. Even for those, you may need sharp eyes, a shadowy viewing spot with no glare from lights, and a few minutes for your eyes to adapt to the dark.

What you’ll see is not a distorted centaur, but a perfect teapot, with a triangular spout on the right that’s tilting and beginning to pour. Catch it while you can. Being so far south, the Sagittarius Teapot cruises into good evening view for only about a month each year.

If you have trouble finding it, start from a brighter landmark: the star Altair more than halfway up the sky in the southeast. No other star there is so bright. Look for the Teapot at the lower right from Altair by about four fist-widths at arm’s length.

Hidden riches

As a constellation of the zodiac, Sagittarius often contains a visiting planet or, for a couple days every month, the moon. But not now.

In a dark enough sky, you can see the Milky Way arching overhead and down through Sagittarius. That is where it becomes the richest in faint stars.

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It does, however, contain a rich abundance of fainter, farther things that make it an astronomer’s playground, for everyone from naked-eye skygazers to researchers working at the cutting edges of star formation, galaxy structure, and black holes.

Start with the Lagoon Nebula. It’s a vast, swirly pool of glowing mist and dark clouds speckled with newborn stars condensing from its thickest clumps. On a moonless night far from city skyglow, you can see the Lagoon with the unaided eye as an enhanced spot of the Milky Way. Even near the city, you can detect it with binoculars. Use the star patterns in the Teapot to guide your way, as shown here. To keep from getting lost, keep in mind that a binocular’s field of view is about half as wide as the Teapot.

Swarms of fainter deep-sky wonders fill Sagittarius when using a telescope. But the most important things here are out of sight.

In a dark enough sky, you can see the Milky Way arching high overhead and down through Sagittarius. And that is where the Milky Way becomes widest, brightest, and richest in faint stars. In 1785, when the Milky Way was all the universe that astronomers knew, William Herschel made telescopic star counts in its different parts. He concluded that Sagittarius was where its riches extended farthest from us.

In 1920 Harlow Shapley at Harvard realized that the distant globular star clusters scattered across the sky are arrayed around a center that seemed to be in Sagittarius. He concluded that the constellation contains the center of our Milky Way galaxy.

He was right. But the signs were subtle. The centers of other galaxies glow brightly. Why doesn’t ours do the same, since it’s hundreds of times closer?

The answer is that if you’re inside a galaxy like ours, you’ve got the worst possible vantage point. The Milky Way’s spiral arms are riddled with dark dust clouds, and these block our view of the good stuff. You can see some of these dark clouds silhouetted along the Milky Way’s midline in a good night sky. These, as well as dust spread more thinly, block our view of the Milky Way’s rich center just off the Teapot’s spout.

Infrared and radio telescopes can now peer through the obscuring dust to the frenetic activity at the galactic center. At the very core, astronomers found quite a trophy: a supermassive black hole containing 4 million times the mass of the Sun. That’s 1.3 trillion Earth masses. Its presence is revealed by effects of its gravity, including nearby stars whipping in fast orbits around what seems to be a dark point of nothingness.

Infrared and radio are also revealing the hidden far side of the Milky Way and the complete layout of our galaxy’s spiral arms.

It’s too bad that so much is obscured from eyeball view. If it weren’t, the Milky Way would be so bright that it would shine right through the glow of Boston. And the ancients might have viewed Sagittarius not as a centaur but as, obviously, the Center of All.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.
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