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Summer: star clusters and nebulae are most abundant at this time of year.

DURING SUMMER the Milky Way arches high across the sky, hosting a plethora of fine star clusters and nebulae. Just north of the Milky Way lie the high-flying constellations Lyra and Hercules, which include two of the sky's best globular star clusters, two of the brightest planetary nebulae, and some truly remarkable double stars.

Lyra is easy to locate because of its dazzling star Vega and its distinctive triangle-and-parallelogram shape. Big Hercules is trickier because its stars are medium-faint. But it's fairly easy to locate to the east of Lyra and above Rasalhague, the brightest star in its sector of the sky.

Rasalgethi, Hercules' brightest star, lies quite close to Rasalhague. It's a magnificent double star at 120x or more: a bright star with a red, orange, or yellow tint accompanied by a fainter companion that appears greenish to many people.

Just 1.7 [degrees] northeast of Vega you'll find Epsilon (e) Lyrae, better known as the Double Double. It's a widely separated pair of tight doubles forming a quadruple star system overall. Binoculars easily separate Epsilon into two stars--in fact a few very sharp-eyed people can separate them without optical aid. I can usually make out all four through a telescope at 100x when the atmosphere is steady.

This area contains many other remarkable doubles, some of which are listed in the table on the facing page.

Messier 57, the Ring Nebula, is small and intense, so it punches easily through heavy light pollution. But for the same reasons, it looks almost exactly like a faint star through binoculars and finderscopes. Look 60% of the way from Sulafat to Sheliak, which form the opposite end of the Lyra pattern from Vega.

Like all planetary nebulae, M57 is an envelope of glowing gas cast off by a star as it nears the end of its active life. It's best observed at 60x or higher. The hollow inside the namesake Ring is wonderful, but it may be tricky to spot in bright skies. Through my 70-mm refractor from my city home, M57 is a solid disk when I look directly at it, but the hollow spot appears when I use averted vision, looking a little off to the side. It's a charming effect, like a cork popping out of a bottle.

NGC 6210 is another bright and even smaller planetary nebula. It's easy to see because its light is highly concentrated, but hard to distinguish from a star at magnifications of 50x or lower. You might recognize it by its distinctive, slightly bluish or greenish color. Magnifications around 200x on nights of steady seeing should reveal its tiny elliptical disk.

The other great deep-sky showpiece in this area is Messier 13, the Great Cluster in Hercules. It's quite easy to spot once you've located the four faintish stars of the Keystone, which marks Hercules' kilt or skirt. (Hercules is upside-down in the sky; Rasalgethi marks his head.)

Using binoculars, your finderscope, or a low-power eyepiece, look for a glow in the Keystone's west side 1/3 of the way from Eta (n) to Zeta (Q. M13 is a globular cluster containing more than a quarter-million stars--a completely different kind of object from the open star clusters described on page 58, which contain only a few dozen to a few thousand stars.

Depending on your telescope's aperture, the magniication, and how dark your skies are, M13 may appear anywhere from an undifferentiated glow to a glorious ball containing hundreds of faint stars, like a sugar pile in moonlight.

Near my city home I need an 8-inch scope to see M13's dozen brightest stars, about the same number I can see through my 70-mm refractor under dark skies. High magniications--as much as 35x per inch of aperture--increase the number of visible stars, and averted vision is a huge help. One of the charms of globular clusters is having faint stars twinkling in and out of visibility as your eye scans over the cluster. They tease your eye, disappearing when you look straight at them and appearing as you look away.

Messier 92 is Hercules' other great globular cluster. It would probably be much more famous if it weren't overshadowed by its larger, brighter neighbor.

M92 is quite similar to M13--equally intense and roughly as easy (or hard) to resolve into individual stars. But M92 is only about 60% as big as M13. Look for it 40% of the way from Iota ([iota]) to Eta, or starhop carefully to it from Pi ([pi]).

Additional Double Stars in Hercules and Lyra

Object         RA                       Dec.

8 Her          [17.sup.h] 15.0.sup.m]   +24[degrees] 50'

p Her          [17.sup.h] 23.7.sup.m]   +37[degrees] 09'

95 Her         [18.sup.h] 01.5.sup.m]   +23[degrees] 36'

100 Her        [18.sup.h] 07.8.sup.m]   +26[degrees] 06'

O[SIGMA] 358   [18.sup.h] 35.9.sup.m]   +16[degrees] 59'

[zeta] Lyr     [18.sup.h] 44.8.sup.m]   +37[degrees] 36'

Object         Description

8 Her          Bright star with wide, faint companion;
               well split at 80x.

p Her          Tight, slightly uneven pair at 80x.

95 Her         Similar bluish stars well split at 60x.

100 Her        Identical stars just split at 16x, wide at 60x.

O[SIGMA] 358   In Hercules. Identical stars; tough split at 180x.

[zeta] Lyr     Gorgeous wide double, can split
               with 10x binoculars.
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Title Annotation:Observing: Deep-Sky Seasons
Author:Flanders, Tony
Geographic Code:1USA
Date:Jan 1, 2013
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