By Brian Cuthbertson
In the right conditions, the night sky is an amazing experience. Once, when assisting Dr. James Wray with some research at McDonald Observatory, I remember stepping out of the McDonald sleeping quarters into a moonless night sky dominated by the Milky Way. It was so dark I couldn’t see my hand in front of my face, but the Milky Way was there – totally awe inspiring! Observers who make the effort to seek out truly dark skies have of course had similar experiences. But in this day and age, the vast majority of folks, at least here in the U.S., have likely never seen a truly dark sky – what a shame. So count yourself lucky if you have, and try to share it with those less fortunate when you have the chance. Enjoy!
NGC 6939 rating EASY
open cluster in Cepheus
RA 20h 31.4m Dec +60d 38.2′ (2000)
Magnitude 7.8
Discovered by William Herschel in 1798, NGC 6939 is a modest star cluster about 2 degrees SW of 3rd magnitude Eta Cephei in far eastern Cepheus. It’s the only deepsky object of any note in a small section of Cepheus almost cut off from the rest of the constellation by a finger of Cygnus
running north just to the east of it.
About 4100 light years away, NGC 6939 is visible as a dim glow in binoculars in moonless dark skies away from city lights. It looks much the same as its close neighbor, galaxy NGC 6946, located about 1/2 degree SE near the base of the Cygnus finger mentioned above. In fact, since the two objects form a duo in the same 1-degree field, they are often mentioned together in observing notes and shown together in deep sky images.
There are no stars brighter than 12 in the cluster. Six- to 8-inch scopes will pick up only 4 to 10 stars (depending on your seeing) set against a background glow of fainter stars. The glow has the general shape of a fan about 150 degrees wide. Larger scopes in the 10- to 13-inch range can pull in 50-100 stars in a 12-15′ diameter.
NGC 6946 rating: MEDIUM
spiral galaxy in Cygnus
RA 20h 34.9m Dec +60d 10′ (2000)
Magnitude 8.8
Also known as the Fireworks Galaxy and Arp 29, NGC 6946 highlights an interesting case of celestial gerrymandering: If you look at the constellation borders adopted by the IAU in 1930, you’ll immediately see that galaxy NGC 6946 should by all rights be in Cepheus, not Cygnus. But for some obscure reason, a thin 30′ finger of Cygnus points about 1 degree north and snares enough of NGC 6946 to put it in the Swan. The rationale for this Cygnus finger would be interesting to know, if it could be dug up. Any celestial archaeologists out there?
To find NGC 6946, start in western Cepheus at 3rd magnitude Eta Cephei, and move your field 2 degrees southwest. There you’ll find it, lying in the same binocular field as neighboring open star cluster NGC 6939, which is half a degree to the northwest. In fact, the two neighbors look similar in binoculars and small refractors, appearing as uniform unconcentrated glows. In moderate and larger sized reflectors (a 12-inch will do nicely) NGC 6946 presents a 30″ core with visible knots in a broader 3×2′ low surface brightness glow. If you insist on seeing spiral arms, use a 16-inch or larger scope at around 200x. With luck you may see a ragged backward lazy-S pattern.
Deep images of NGC 6946 show an open, nearly face-on Sc spiral reminiscent of Triangulum galaxy M33. In fact, its spiral structure would be obvious in smaller telescopes if the galaxy weren’t buried behind obscuring gas and dust close to the plane of our Milky Way.
NGC 6946 is notable on at least two accounts. One is that the galaxy has produced a significant number of supernovae – 9 during the last 100 years. This makes it the most prolific known galaxy for this type of event, hence its “Fireworks” nickname. The first outburst was caught in 1917, followed
by others in 1939, 1948, 1968, 1969, 1980, 2002, 2004 and 2008. Consequently, it wouldn’t hurt to periodically check the galaxy when you’re wandering the neighborhood.
Secondly, NGC 6946 is very close to the Local Group. Close enough, in fact, that historically there was some controversy over whether it is a member of our Local Group of galaxies. These days, its
redshift suggests it’s a bit too distant to be a voting member. Instead, it’s one of the nearest galaxies just beyond the Local Group, at a distance of 10 to 20 million light-years. Contrast this with the 4000 light-year distance of star cluster NGC 6939 only half a degree away: a distance difference factor of roughly 5000!
Jones 1 rating: HARD
planetary nebula in Pegasus
RA 23h 35.9m Dec +30d 28′ (2000)
Magnitude 12.1
Jones 1, aka PK104-29.1, is a large dim planetary located in northern Pegasus, just a bit south of the border with Cassiopeia. If you decide to chase it on the web, do NOT confuse it with another planetary having a similar name: 14th magnitude Jones-Emberson 1 in Lynx. To locate its approximate position visually, draw a line between Alpheratz and Scheat (Alpha and Beta Pegasi), the two 2nd-magnitude stars marking the northern side of the Great Square of Pegasus. At the mid-point of this line, move about a degree north, and you’re in the immediate neighborhood of Jones 1.
Jones 1 has an apparent diameter of just over 5′, and includes a 16th magnitude central star. It is not an easy find for smaller scopes or beginners. Although visible to experienced observers as a round haze in scopes as small as 6 inches in very dark skies, only the availability of large aperture Dobsonians and narrow band filters has made this sort of low surface brightness object more generally accessible.
As is often the case for large planetaries, an OIII filter can make all the difference, in this case turning a slight brightening in the sky into a more clearly defined broken ring having two opposing lobes to the NW and SE, with the NE lobe being the brighter.
Jones 1 is about 2300 light years distant and has an actual diameter of about 3.5 light-years.
