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Open cluster NGC 6025, globular cluster NGC 6139, and planetary nebula NGC 6153

August 11–18, 2016: Open cluster NGC 6025 in Triangulum Australe offers small-telescope owners nice views, while large-telescope owners can seek out globular cluster NGC 6139 and planetary nebula NGC 6153 in Scorpius.

Triangulum-Australe
Open cluster NGC 6025 lies at the northern edge of Triangulum Australe, right at that constellation’s border with Norma.
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 August 11–18, 2016

Small telescope: Open cluster NGC 6025
Large telescope: Globular cluster NGC 6139
Large telescope: Planetary nebula NGC 6153
Downloadable File(s)

Southern gems
This week’s small-telescope target is open cluster NGC 6025 in the far-southern constellation Triangulum Australe. Observers also know it as Caldwell 95.

It lies at the northern edge of Triangulum Australe, right at that constellation’s border with Norma. You can find it 3.1° north-northeast of magnitude 2.8 Beta (β) Trianguli Australis.

Under a dark sky, most observers can spot magnitude 5.1 NGC 6025 without optical aid. Through a 6-inch telescope, you’ll count roughly 40 stars between magnitudes 7 and 11 spanning a region 12' across.

Through a 14-inch or larger scope, crank up the magnification, and look 20' south-southeast of the cluster’s center for the magnitude 14.6 spiral galaxy PGC 56940.

See this globular
This week’s first large-scope object is globular cluster NGC 6139 in Scorpius the Scorpion. To locate it, point your telescope 4.8° west-southwest of magnitude 3.0 Mu (μ) Scorpii.

A 4-inch telescope easily reveals magnitude 9.1 NGC 6139 against a reasonably rich background. But simply seeing this cluster and seeing detail in it are two different things.

You’ll need at least a 14-inch telescope and a magnification above 300x to resolve even a few stars in this compact object, which measures 8.2' across. What you will see is a concentrated core surrounded by an unevenly lit halo.

Extrasolar planetary
This week’s second large-telescope target is planetary nebula NGC 6153, which also lies in Scorpius. It glows at magnitude 10.9 and measures 25" across.

You’ll find it 4.5° west-southwest of magnitude 3.0 Mu (μ) Scorpii. Alternatively, point your telescope 1.6° south-southeast of our previous object, NGC 6139, and you’ll land on this planetary nebula.

Through an 8-inch or larger telescope, you’ll need magnifications above 200x to reveal this object as anything other than a bloated “star.” An Oxygen-III filter helps. Look for unevenness in the edge and an ever-so-slightly darker central region.

Expand your observing at Astronomy.com

The Sky this Week
Get a daily digest of celestial events coming soon to a sky near you.

Observing Basics
Find more guidance from Senior Editor Michael E. Bakich with his Observing Basics video series.
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