Stars, stars, stars: which season offers more naked-eye stars than any other?
WHAT MORE could a lover of the night sky ask for than lots of stars? We enjoy planets, of course, along with meteors, comets, deep-sky objects, and the Moon. But stars are astronomy's centerpiece. The very word astronomy means "star ordering."
October evenings offer several ways to see an abundance, even a multitude, of stars.
Richest sky. It's sometimes said that you can behold 2,000 stars on a fine, dark night. That figure takes into account two important facts: Only half of the celestial sphere is above the horizon at a given time, and atmospheric extinction dims stars low in the sky. But what is the total number of stars visible to the naked eye during all seasons and from all latitudes on Earth?
The Millennium Star Atlas (available from Sky Publishing) plots 8,768 stars brighter than magnitude 6.5, the traditional naked-eye limit in a clear sky completely free of light pollution or moonlight. But some people can detect stars as dim as 7.5. There are 26,533 stars brighter than that!
Even accepting the eye's limited field of view, October evenings are not a bad time to see the most naked-eye stars; spring evenings have far fewer. Our monthly star chart (facing page) shows that the lower part of the sky from south to east is occupied by big, sparsely starred constellations, but this is offset by the profusion of naked-eye stars running high overhead along the Milky Way.
Richest constellations. Looking straight up on an October evening, you can behold Cygnus, the northern constellation with the most stars brighter than 6.5 (262 of them). Cygnus is second only to far-southern Centaurus, which has 281. But that makes sense because Centaurus is larger. If you extend the magnitude limit down to 10.0--what you might be able to see in large binoculars or a small telescope--Cygnus wins out. According to the Millennium Star Atlas database, Cygnus contains 14,376 stars brighter than 10.0, compared to 13,779 for Centaurus. Only four other constellations, led by Puppis, have more than 10,000 stars brighter than 10.0.
On the very clearest, darkest nights, I've seen parts of Cygnus pebbled, beaded, and curdled with naked-eye stars. But certain other, smaller parts of the sky have an even higher star density than Cygnus. The best example is tiny Crux, the Southern Cross. As the atlas's introduction points out, by sweeping in Crux with 7 x 50 binoculars you might see 3,000 stars in a single view!
Rich telescopic fields. With a small telescope you can enjoy hundreds of stars in and around the Double Cluster in Perseus, now partway up the northeastern sky. A telescope of 10-inch aperture will show a similar number in a smaller area, like that of the ultrarich open cluster Messier 11 in Scutum, now in the southwest.
If you don't insist on seeing many individual stars and will settle for the glow produced by a hundred thousand or so, check out a globular cluster such as M13, M22, M2, or M15 in the October sky (see page 59). For a glow containing hundreds of billions of stars, the Andromeda Galaxy (M31) is now climbing high in the northeast.
Fred Schaaf welcomes your mail at email@example.com.
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|Title Annotation:||Northern Hemisphere's Sky|
|Publication:||Sky & Telescope|
|Date:||Oct 1, 2007|
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