The following article was written for our astronomy club newsletter, September 2020. I am a member of The Royal Astronomical Association (Canada), Windsor branch.
Lying very close to Stephan’s Quintet in Pegasus (which is more like a quartet, but I quibble), the eg 7331 quintet is much more rewarding in moderate sized telescopes, thanks to the overwhelming presence of one of the great NGC galaxies. Four of its visual neighbours can be located in dark skies with a 12” scope, a few of them quite easily. Anything larger, including our club’s 14” instrument, should provide satisfying views. A sixth NGC member is a very faint double star, and a seventh was also incorrectly labelled, actually being a star.
I first came across eg 7331 on the night of October 16th, 1991. Back in those days I could see galaxies from my back deck in Anderdon with my 8” Edmund reflector. I logged it as a major surprise of the night, since I was also competing with an 8-day old moon! That is one bright galaxy. It was easily swept up at 36x. 112X showed the elongated shape, with a much fuller size seen at 169x. The central area remained bright, but the outer areas were pretty faint. None of my maps at the time showed the other galaxies.
It wasn’t until I returned to Pegasus with the 12” reflector, undergoing my NGC project in that constellation (there are 298 NGC objects here) that I made a new visit to 7331, and went in search of the smaller objects alongside it. This was in October 2015, and I was observing from Hallam. I had just finished up with observations of Stephan’s Quintet, and moved my scope the short distance to eg 7331. It was remarkably bright at 60x, and very elongated. It is a large galaxy, 10’.5 x 3’.5, and visually bright at mag. 9.5, though its surface brightness drops to 13.2 because of its size. It is still a good object even in a 6” scope, but in a 12” mirror it is an unforgettable sight. It bears magnification easily, so use the highest possible one that still gives good images. 250X showed me a thin, dark line just preceding the bright center.
The four smaller galaxies we are concerned with consist of eg 7335, 7336, 7337, and 7340, all located just following the main galaxy. Simply by allowing 7331 to drift to the edge of the field brings the others into view. Taking them in order, eg 7335 is the brightest and most obvious of the smaller objects, and can be seen with an 8” if one knows where to look. At 12” I first glimpsed it using 100x, just north following the central core of 7331. Views improve with high power, and it eventually appears like a smaller, fainter version of 7331 itself, even in the same position angle. This galaxy is 1’.3 x 0’.6 in size. Visual brightness is 13.4, and its surface brightness is 13.
The next number is eg 7336. This one is pretty faint, but located just north of 7335, so it’s not difficult to locate. I viewed it at 187x and 200x, noting its elongation and slightly brighter center. It’s size is a tiny 0.’8 x 0.’4. Visually it clocks in at mag. 14.5, but due do its tiny size, the surface brightness ramps it up to a 13.1. Expect to see something halfway between those two magnitudes, thus about 13.8. It is not difficult to see in a 12”, and may be visible in a 10”.
Eg 7337 is a challenge, and it took me some time to see it. Not only is it rather faint, but it sits close to a star that is brighter than it, hampering the view. Milky Way stars can be a real nuisance sometimes, and this is one of those time. The galaxy’s size is 1.’1 x 0.’7. Visually it is mag. 14.4, with a surface brightness of only mag. 14. This one is challenging with a 12” scope, but quite doable if you are patient. It lies south of 7335, and aligned with the south end of 7331.
Last but not least comes eg 7340. After 7335, it is the easiest of the little galaxies to find. Though only a tiny 0’.9 x 0’.6, it is not difficult to see at 136x and 187x. It shows an oval shape at higher powers.
Stephan’s Quintet is easy to find from 7331, too, if you are in the mood for another quintet. Just sweep south preceding until you arrive. It’s fun to sweep back and forth between quintets, getting to know this area of the sky. In a future article I will detail my difficult search for Copeland’s Septet, seven very small and faint galaxies in Leo. In the meantime, enjoy clear skies!
Messier of the Month: Messier 92
Globular star clusters make a fascinating study. There are only 170 listed in Uranametria, including ones too faint to see with modest scopes, and those visible in the Magellanic systems. They are rarer than one might think. I remember my Tasco 4.5” days very well (1971-1978), trying to imagine what a resolved cluster might look like at the eyepiece. The first one I viewed with an 8” scope (1978) was M 56 in Lyra, followed by M 13 and M 92 in Hercules. What a revelation it was! Even today, despite the incredible resolved photos from Hubble of these mini-galaxies of stars, I prefer visual observation to photos of globulars and open clusters. Some sights can still easily take my breath away.
I logged M 92 officially in late July, 1992, from Lake Penage, west of Sudbury, with my Edmund 8” reflector. I was expecting a bright ball of haze, perhaps beginning to resolve at higher power. Instead, I saw one of the finest globular clusters in the north sky. Not only is M 92 big and bright, but it can be observed directly overhead, usually the best part of the sky in which to observe. I was surprised, to say the least, when it was located at 36x and stars were already resolving to near the center. This is what I had dreamed about during those Tasco 4.5” days! At 59x stars resolved to the core, and I observed three distinct sections: the bright central core, surrounded by a bright collar, or ring, and surrounding all was the outer region of individual stars. M 92 is magnificent at 112x, revealing a blindingly bright core surrounded by a rich background of stars made up of lines, curves, and various patterns. At 169x the area behind the core’s many resolved stars was blazing, revealing too many stars to draw or even count. At 200x the effect was like observing a giant explosion of stars hurtling outward. The full cluster goes beyond the field of view. 254X showed good core detail, as it began to break up into chunks of brightness and stars.
In July of 2016 I observed it from Hallam (and many times since!) with my 12” reflector. Though about one third smaller than M 13, and certainly not as rich, it is still one of the most awe-inspiring sights I have ever seen (and I’ve been to the Rockies 40 times!). At 187x and 200x I noted that to the north and south of the center, the rich outliers reminded me of the wings of a moth, with a very bright head between them. At 374x (I took my time to get a really, really good focus) the core has broken into two halves, and is mostly all resolved. Breathtaking!
M 92 in Hercules can be easily viewed throughout September, offering up a wonderful contrast to M 13. Use the 14” at moderate and high powers, and I guarantee a sight worth seeing.
M 92 (gc 6341): Size 14’; Visual Mag. 6.5; Brightest star Mag. 12.1
Mapman Mike
