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The Sciences

The Archer

Gaze toward the Milky Way's core, around which we imperceptibly revolve


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August means grand, and this month the sky features something truly august: the center of the galaxy. All naked-eye objects visible from the United States - absolutely everything on Earth and in the starry firmament (except for the Andromeda galaxy, now rising in the northeast) - revolve around this single spot in the summer sky. Yet a glance upward into the south reveals . . . absolutely nothing. You'll simply have to take it on faith that it's there.

You'd expect such a busy region to be bright and easy to spot. But the center of the galaxy is impossible to see, buried behind layers of obscuring dust. We don't even come close. The farthest objects discernible in visible wavelengths lie a few thousand light-years in the direction of the galactic center, which, according to astrometric data from the Hipparcos satellite, supposedly lay at a distance of 26,000 to 28,000 light-years away. Newer estimates put the true figure at around 23,000 light-yearsowhich means the entire galaxy, and maybe the whole universe, is about 10 percent smaller than we thoughtobut the core remains just as invisible.

Our track to the galactic nucleus is in Sagittarius, which reaches its highest position this month. Due south at nightfall, just a third of the way up the sky from most of the United States, Sagittarius resembles the classical "archer" only to highly suggestible minds. The constellation really looks like a teapot. In fact, mapmakers have given up trying to construct anything else out of this grouping. Extending that image, "steam" othe Milky Wayoappears to rise from the "spout" and continues upward across the entire sky.

The spot where the steam leaves the spout marks the center of our galaxy. Our solar system revolves around this point somewhere between 115 and 150 miles per second. One revolution takes 220 million years to complete, an interval sometimes called a galactic year (the longest cyclic period in all timekeeping). Tracking the orbit requires patience: we've rotated less than a degree since human brains first arrived on the scene to be tormented by such immense notions.

To the teapot's right floats the striking tail of Scorpius; together they form a vivid grouping. Here binoculars demonstrate their celestial value. Ordinary 7x35 binoculars, and especially 7x50s, will reveal star clusters and intriguing nebulas crowded into the region between the tail and the galactic core.

Telescope owners can aim bigger guns at these strange blurry blobs. One metamorphoses into a globular cluster of several hundred thousand suns, the awesome M22. Another three remain nebulous but change into streaky designs well known to observersothe Swan, the Trifid, and the nearby Lagoon nebula, which also encloses a loose, lovely star cluster.

The region surrounding our galaxy's core lies, by chance, directly in line with the plane of our solar system. No planets currently sit there, but every December 21, on the winter solstice, the sun marks the spot, telling us that Earth's southern axis angles in just that direction. Our world would have to tilt an extra 62 degrees, however, for the galactic nucleus to sit straight above the South Pole.

This month the moon floats just above the galaxy's core on August 4 and acts as a helpful marker; it illuminates the spot even more closely on August 31. On those nights, one could contemplate the scene dimensionally: there's the moon, less than two seconds distant at light speed, and behind it the locus of all galactic motion beckons mysteriously, more than a trillion times farther away.

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