Back to Article List
Late tonight, moonless skies offer great deep-sky observer. Try for open cluster NGC 6939, neighbor to the famous Fireworks Galaxy.
Open cluster NGC 6939 (upper left) in Cepheus lies close to the Fireworks Galaxy, a lovely face-on spiral galaxy known for its many supernovae. Credit: Ram Viswanathan (Flickr, CC BY-NC 2.0)
Looking for a sky event this week? Check out our full Sky This Week column.
August 17: Mars at Gemini’s feet
Wait until after about 10 P.M. tonight to begin your deep-sky observing and the Moon won’t be a bother. An excellent target is NGC 6939, an open cluster in Cepheus the King. This cluster is quite compact, about 8’ across, and shines at 8th magnitude. You can locate it in the north late tonight — by 10 P.M. local daylight time, it is some 60° high, to the upper left of Alderamin, Cepheus’ alpha star. From this magnitude 2.5 sun, slide just under 6° southwest to land on the cluster.
Although NGC 6939 is an open cluster and these groups are young, it’s one of our galaxy’s older such clusters, at some 1 billion to 2 billion years old. It can be viewed in binoculars, but larger apertures will pull out its stars as points, rather than simply showing a dim, amorphous glow.
NGC 6939 also has a famous neighbor — some ⅔° to its southeast (and 2.1° southwest of magnitude 3.4 Eta [η] Cephei) is NGC 6946, a face-on spiral known as the Fireworks Galaxy. Glowing at magnitude 8.9, this galaxy has had at least 10 supernovae pop off within it over the last century! Compare that with our own galaxy, which sees about one every century.
Spanning roughly 10’, NGC 6946 can be a bit challenging because its light is spread out over a relatively large area, making it appear dim. It’s also located behind a swath of light-absorbing dust within our own galaxy. Even so, it can be found in binoculars and it will show up as a faint, relatively detail-less glow in smaller scopes. In larger telescopes (11 inches), the brighter core and fainter arms become distinct, and grow even more detailed as you scale up your aperture.
Both objects show up well in astrophotos, making this a popular pair for imagers to target.
Sunrise: 6:14 A.M.
Sunset: 7:53 P.M.
Moonrise: 12:47 P.M.
Moonset: 10:38 P.M.
Moon Phase: Waxing crescent (41%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
If you’re willing to stay up late, you’re in luck: NGC 7293, the Helix Nebula, is highest around 1 A.M. local daylight time on August 19, when it stands some 30° above the southern horizon. It is also cataloged as Caldwell 63.
The Helix Nebula is a lovely ring-shaped planetary in the constellation Aquarius, visible after moonset on August 19. Credit: Stephen Rahn (Flickr, CC0 1.0)
Located in Aquarius, the Helix is a lovely circular planetary nebula generated as well as lit up by the dying star at its center. Although it is 8th magnitude — quite bright for a planetary — its inner portion covers some 16’ of sky, making it one of the larger planetary nebulae we know. It also means that (like the Fireworks Galaxy yesterday) its light is spread out over a larger area, making its overall surface brightness low. But that doesn’t mean it can’t be seen — in fact, it can still be caught in binoculars or a small scope. On a very dark night (and if you have very good eyesight), you may even be able to see it without any optical aid! It’s in a patch of sky in southwestern Aquarius devoid of bright stars, but you can hop some 7.8° southwest of 3rd-magnitude Skat (Delta [δ] Aquarii) to find it. For a fainter but closer signpost, use 5th-magnitude Upsilon (υ) Aqr — it’s just 1.2° west of this star.
If you’ve got a large scope, screw in a nebula filter to bring out more detail, including brightness variations along the edges of its ring.
Alison Klesman is senior editor of Astronomy magazine. She holds a Ph.D. in astronomy and has studied a variety of topics, from minor planets to supermassive black holes.