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Globular cluster NGC 6441, globular cluster NGC 6496, and the Cat's Eye Nebula (NGC 6543)

July 2–9, 2015: The Silver Nuggest Cluster (NGC 6441) and globular cluster NGC 6496 in Scorpius offer small-telescope owners nice views, while large-telescope owners can seek out the Cat's Eye Nebula (NGC 6543) in Draco.
This week's large-telescope target, the Cat's Eye Nebula (NGC 6543), lies 5° east-northeast of magnitude 3.2 Zeta (ζ) Draconis.
Astronomy: Roen Kelly
Each week, Astronomy magazine Senior Editor Michael E. Bakich, a master at explaining how to observe, posts a podcast about three or more objects or events you can see in the sky.

Targets for July 2–9, 2015

Small telescope:
The Silver Nugget Cluster (NGC 6441)
Small telescope:
Globular cluster NGC 6496
Large telescope:
The Cat's Eye Nebula (NGC 6543)
Treasure in the sky
This week’s first small-telescope target is the Silver Nugget Cluster in Scorpius. Also known as NGC 6441, this globular cluster shines at magnitude 7.2 and measures 7.8' across.

You’ll find this object 3.3° due east of magnitude 1.6 Shaula (Lambda [λ] Scorpii), right next to the 3rd-magnitude star G Scorpii. Although this orange star just 4' west of the cluster’s center provides a nice contrast, move it out of the field of view when you’re ready to study the cluster. From a dark site, through a 4-inch telescope at 150x, the core appears bright and concentrated with a thin, slightly irregular, halo surrounding it. Larger scopes make the halo easier to see.

The magnitude 10.0 star GSC 7389:2031 lies a bit more than 1' southwest of the globular’s core. NGC 6441 is one of only four globular clusters known to contain a planetary nebula, but you’ll need a 25-inch or larger scope, a finder chart, ultra-high magnification, and superb seeing to spot it.

Astronomy magazine Contributing Editor Stephen James O’Meara gave this cluster the common name I list here. He thought that, under moderate magnification, the stars of NGC 6441 and the single star G Scorpii looked like silver and gold nuggets lying face up in the sand.

Concentrate hard
This week’s second small-scope object is globular cluster NGC 6496 in Scorpius. It sits 4.1° east-southeast of magnitude 1.9 Theta (θ) Scorpii, right on that constellation’s border with Corona Australis.

When you first train your telescope on this 5.6'-wide object, you’ll spot the orange magnitude 4.8 star SAO 228562, which lies 0.4° to the west-southwest. Admittedly, it makes the scene more attractive overall, but move the star out of the field of view when you observe the globular, which glows at magnitude 8.6. That makes NGC 6496 only 3 percent as bright as the star.

Even a 3-inch telescope will let you see this target. Use a magnification around 100x.

And, although this object is a globular cluster, it’s one of the loosest (that is, least concentrated) in the sky. Even with a good imagination, you won’t see a central condensation, the hallmark of most globulars. Instead, look for the half dozen or so magnitude 11 and 12 stars that mark its location.

This week’s large-telescope target is the magnificent Cat’s Eye Nebula in Draco. This planetary nebula also goes by NGC 6543 and Caldwell 6.

You’ll find it just a bit more than 5° east-northeast of magnitude 3.2 Zeta (ζ) Draconis. Through telescopes as small as 4 inches in aperture, this colorful magnitude 8.1 object looks blue, blue-green, greenish-blue, or green, depending on your eyes’ color sensitivity.

But to see more than just color, use a magnification of at least 200x in an 8-inch or larger telescope. You’ll first notice some hazy spiral structure around the bright central star. A faint outer shell 5' across surrounds NGC 6543. This halo contains more mass than the core, which only spans about 20". Past observers have misidentified a bright part of the halo as a galaxy. That section even carries its own designation — IC 4677.
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Observing Talk
After you listen to the podcast and try to find the objects, be sure to share your observing experience with us by leaving a comment at the blog or in the Reader Forums.


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