Caldwell 30 (NGC7331)

In early September, I set up my LX200 for CCD imaging. I didn’t plan to image any particular astronomical object. I usually thumb through Stephan James O’Meara’s The Messier Objects and The Caldwell Objects for potential candidates. The books include a wealth of Messier and Caldwell object celestial locations, magnitudes, photographs, histories and observing tips. This night, I fortuitously opened the Caldwell book to Caldwell 30. My eyes were immediately drawn to a label Stephan’s Quintet in the lower right of the included photograph. Stephan’s Quintet is a stunning example of gravity at work on a gigantic scale. Three of the galaxies show evidence of interaction (let’s just say they are mixing it up). Without further delay (or reading of the text) I entered one of the galaxy’s New General Catalog (NGC) number, 7319 into the LX200 hand controller’s GOTO command. My telescope’s computerized targeting slewed the LX200 to a location in northwest Pegasus. When CCD imaging of new targets, I lead off with short exposures of a few seconds to verify the position of the object in the field of view. I may nudge the pointing north-south/east-west to center the object before proceeding with longer exposures. After centering the faint “fuzzies” of the Quintet, I activated the Software Bisque’s CCDSoft program’s auto-guiding mode and took two 60 second images. Satisfied with the auto-guiding accuracy I followed with a 300 second guided image. To assess the resolution of this collection of galaxies, I combined the 3 images to produce a 420 second exposure. See the results in Figure 1. More magnification (in this case removing the focal reducer) would improve the image. Changing the magnification is disruptive time-wise so, with a sigh, I decided to table Stephan’s Quintet for another night.

Looking back at O’Meara’s C30 photograph I realized C30 wasn’t Stephan’s Quintet after all. C30 referred to NGC 7331 just northeast of the Quintet. I entered GOTO NGC 7331 on the LX200 hand controller and listened for the telescope’s drive motor to activate. The scope’s movement was slight. I took a 10 second image to inspect the results of the GOTO pointing. A ghostly elliptical patch of white stretched a full 2/3’s of the north-south field of view. I knew right away that I’d found my target for the night and as my CCD image efforts can go, several more through early November.

I refer the reader to O’Meara’s book for the detailed explanation of this stunning spiral galaxy in Pegasus. Here I’ll relay some interesting information from O’Meara’s lengthy C30 description. C30 or NGC 7331 is located some 4.5 degrees north-northwest of Eta Pegasi (see Matar in Figure 2). Under dark skies this 9.5 magnitude object is a challenge with 7x35 binoculars. NGC 7331 is 47 million light years distant and has a diameter of 130,000 light years. It is equal in size and mass to the Andromeda Galaxy (2.3 million light years). Satellite X-ray observations indicate a possible massive black hole in 7331’s central bulge. There are radio astronomy data indicating a possible connection between 7331 and NGC 7320 in the Quintet (lower left object in Figure 1). I encourage interested readers to review O’Meara’s full text on C30 for themselves.

I took several NGC 7331 image sets between September and November. Early attempts were foiled by a polar alignment problem that became apparent in the long exposures required. After a tweak or two to the telescope’s alignment it all came together on November 6 right in time for election night. For those interested CCD imagers out there I’ll note the imaging details for reference (and hopefully encourage feedback):

All images were aligned, medium and color combined using Software Bisque’s CCDSoft imaging program. I did not apply flats or bias corrections. Subsequent processing using Maxim DL’s Digital Data Processing and AIPWIN Deconvolution algorithms. A final touch up was accomplished in Photoshop Elements (phew!). The optical setup included a 10” LX200 f/6.3 with a 0.63 focal reducer. The CCD camera was an SBIG ST-7XE with CFW8 color filter wheel. Using TheSky Version 6’s Image Link option I determined the image scale is approximately 1.56 arcseconds per pixel. The camera position angle was rotated about 27 degrees west.

The results: If you read my previous article titled “The Pluto Challenge” I again used the plate-solving technique of TheSky program to identify the additional NGC objects. Evident in the resulting image in Figure 2 are background galaxies NGC 7336, 7335, 7340 and 7337 east of 7331 (left to right, top to bottom). 7337 shows a hint of spiral structure in this image. To right (or west) of 7337 there is a faint wisp of an object. Interestingly enough the same object appears in TheSky field but it is unidentified. I cross- referenced the field to the Uranometria 2000.0 Deep Sky Atlas and there was no object listed there either. In comparison to “face-on” spirals, NGC 7331 is tilted to our line of sight. The east side (left) is rotated away and the west towards us. At this angle 7331 shows a nice combination of spiral structure and dust lanes along the outer edges of the spiral arms. The spiral arms are rotating (clockwise) about the central bulge. Compare this image to the “edge-on” spiral NGC 891 (C23) the “Outer Limits Galaxy” in Andromeda and the “face-on” spiral Messier 74 “The Phantom” in Pisces (also known as NGC 628).

I may spend another night with NGC 7331 this year. I’m very pleased with the image here but it would benefit from longer exposures to bring out more detail. It may have to wait until next year as it’s getting harder and harder to ignore Orion’s march to the zenith!

Star-hopping to deep, deep space…

For those who appreciate “star-hopping” their way to NGC 7331 I’ve included a star field map from Software Bisque’s TheSky Version 6 showing the approximate location in Pegasus. If you aren’t familiar with the constellations Pegasus is an excellent one to start your learning. Locating the Great Square of Pegasus can help you adjust the scale of constellations in star maps to the actual sky. Once you’ve located the Great Square of Pegasus you can walk the constellations to the east-northeast into Andromeda, Cassiopeia and Perseus. As the winter months arrive keep watching the east and learn new star fields such as the cluster of the Pleiades (M45) and the constellations Taurus, and mighty Orion.

In November-December between 6-9 pm CST, Pegasus is arriving at and crossing directly overhead and a bit south. Stand outside and face south (or lie down with your feet towards the south). Stretch both arms straight up pointing your index fingers. You are pointing to a location referred to as your zenith. From central Iowa your zenith should is close to declination angle 41-42 degrees on a star map. Lower your arms (southern direction) about 10-15 degrees while watching for two bright stars near each figure tip. Hopefully you’ve spotted them. Spread your arms a bit and look a bit further south. You should see the Great Square of Pegasus take shape. Once you’ve located the square we want to work from the upper right corner star Scheat or Beta Pegasi. In Figure 2, I labeled Scheat as (1). Look for another somewhat dimmer star slightly north and west of Scheat. This is Matar or Eta Pegasi labeled (2). Binoculars are required for the next hop as the stars are all magnitude 6 and higher. Imagine a line drawn from Scheat through Matar. Continue on this line west a short distance past Matar searching for two stars aligned almost perpendicular to the Scheat-Matar line (3). Now for the tough hop. North along this line you will spot a close pair of stars (4). I positioned Matar (2) in my 7x35 binocular’s lower left field of view and this doublet (4) in the upper right. With this arrangement you have encircled the field containing NGC 7331 and Stephan’s Quintet. I added a dashed circle to the star map to show the exact location of NGC 7331.

O’Meara laid down the challenge to observe NGC 7331 with 7x35 binoculars. My first attempt to spot NGC 7331 with my binoculars was under a full moon and I was not able to pick out the galaxy. My next attempt was a few nights after full moon. The sky was clouded over at sundown but cleared after dark. A ground fog developed while I was observing. If I didn’t know NGC 7331 was there somewhere nothing would have caught my attention. As it was, I believe I did catch fleeting glimpses of a faint low contrast glow of light near the location marked on the map. I couldn’t say definitively that I spotted it with 7x35 binoculars but I will keep trying. If you are a seasoned observer and can readily locate the Andromeda Galaxy consider this: NGC 7331 is comparable in size to the Andromeda Galaxy. Andromeda’s light has traveled for 2.3 million years before reaching your eyes. NGC 7331’s has traveled 47 million years. What a trip!

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