NGC 6645–the cosmic maw

August 19, 2012

I was out hunting Herchels last night and observed the open cluster NGC 6645 for the first time. From my notebook:

Fairly large open cluster. Not sparse, loads of stars, but all are about equally dim–none really jump out. Has an unusual dark spot in the middle like a mouth or the entrance to a cave. That plus five radiating chains of stars make it look like a howling monster.

This didn’t occur to me until today, but I finally realized which monster it put me in mind of–the Beholder from D&D. This sketch gives a sense of its appearance, although it does vary a bit with aperture–that artist saw “a pretty faint unresolved gray haze with about 30 dim stars visible” whereas the 10-inch resolved it completely, with mini-clusters around the central hole and long chains of stars radiating away like nerve endings from a cartoon neuron.

Anyway, this open cluster is well-placed in the early evening, it’s not hard to find, and it has a lot of character. Well worth tracking down. You can even generate a custom finder chart for it using the interactive star chart at NGC891.com, which I just learned about.

What does it look like to you?


  1. I’ll try to run this one down tonight as it is high enough above my treeline horizon to be in semi-dark skies. How about posting your sketch of NGC 6645? And maybe observing (and sketching) it in your TS 70 for comparison?


  2. Looks very interesting (circle of stars with empty center), I’ll have to check it out.

    Been observing mainly open clusters, globular clusters and planetary nebulae lately, giving the more difficult galaxies a rest.

    That’s because I’ve been observing in more light-polluted conditions (red and red/orange zone) up in Simi-Valley. The desert area around Palm Springs/Joshua Tree has been affected by clouds and thunderstorms lately.

  3. Hi Matt.

    Clear skies last night, so I managed to run down NGC 6645. As you said, not hard to find (that I found is is proof). Your metaphor is apt; it does look like a wide open mouth, defined by what I found to be a half dozen relatively bright stars. “Cosmic Maw” fits it well or, if we want to hit the turbo button in our imagination, a celestial version of that German expressionist classic “The Scream” by Edvard Munch as the “mouth” shapes are similar.

    I didn’t find 6645 to be a particularly stunning cluster, more an interesting one given its black, empty center made more dramatic by your label than it otherwise might have been, and one I likely would have missed had it not been for your post. So I get to check off one more.

    A bonus was that the “Maw” set me up for M18 which is just a few degrees to the west. Overall, a much richer cluster than 6645 that took me a lot longer to sketch.

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