NGC 6633: This Captain Hook Cluster Pulls You In

The constellation Ophiuchus hosts a wide variety of astronomical objects, including a nice collection of attractive star clusters. Not far from the beautiful IC 4665 and Graff’s Cluster nearby, lies NGC 6633 ready for summertime observers to enjoy it.

NGC 6633’s diameter is nearly the size of the full moon in the sky, and a rich collection of 38 known stars makes the cluster shine at a magnitude of +4.6. Additionally, according to Michael Bakich of Astronomy magazine, a group of 50 dimmer stars resides behind the cluster. In fact, Bakich warns observers to keep magnification somewhat low (around 100x) so one doesn’t “disperse the stars so much that you look ‘through’ the cluster.” ¹ On the other hand, if you have a pair of binoculars with an aperture of 50mm or larger, give it a try at finding NGC 6633. As you look for the cluster, keep in mind it lies in a busy field of stars, so it can be a little tricky to find.

To me, younger star clusters like NGC 6633 and nearby IC 4665’s are brilliant objects not to be missed. These two, for instance, come alive because they are large and spread out across the night sky, at least the size of the full moon. And, they are home to many vibrant blue-white younger stars that shine with great intensity.

Historically speaking, the Swiss astronomer Philippe Loys de Chéseaux discovered NGC 6633 in 1745. It was recorded again on the night of July 31, 1783, by Caroline Herschel. ²

My Observations

This is another brilliant star cluster in this region of the sky. While my sketch is done with a 20mm eyepiece, I’m enjoying going wider up to my 32mm eyepiece. I’m slowly moving the scope back and forth, scrolling through the stars. For some reason, this cluster reminds me of looking at an eddy in a small stream of water. A great summer target for sure!

Key Stats

Constellation	Ophiuchus
Best Viewing	Summer
Visual Magnitude	+4.6
Distance from Earth	1,040 ly
Milky Way Location	Orion Spur
My Viewing Grade	A

Sources and Notes

• The banner photo of NGC 6633 is from a photo by Kurt Zeppetello. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
  
• ¹ Bakich, Michael E. “Open Cluster NGC 6633, a Complex of Dark Nebulae, and Globular Cluster.” Astronomy.com, 13 Aug. 2015, https://astronomy.com/observing/observing-podcasts/2015/08/ngc-6633-a-complex-of-dark-nebulae-and-m54.
  
• ² Hartmut Frommert, Christine Kronberg. NGC 6633, http://www.messier.seds.org/xtra/ngc/n6633.html.
  
• Sketch by Wayne McGraw
  
• AKA: Tweedledum Cluster, Collinder 380 or Melotte 201, HD 169959, Bee