IC 4996 and NGC 6888 in Cygnus, and IC 4997 in Sagitta

September 2020 - Nebula and Cluster of the Month

This month we’ll be looking at three different objects. Two because they’re great objects, and the third as a bit of a challenge and a talking-point.

We’ll start with this month’s open cluster – IC 4996 in Cygnus. Although apparently observed by William Herschel in 1786, IC 4996 did not make its way into his catalogue, nor into the NGC. It had to wait until 1908 to be included in the Second Index Catalogue of Nebulae Found in the Years 1895 to 1907. The IC credits the discovery to Frank Bellamy (1863 – 1936). The description given is accurate, if brief: Cluster, stars magnitude 8–13.

IC 4996 is a young open cluster, about 8 to 10 million years old. Its distance is not known precisely, but lies somewhere around 1915 ± 110 pc, or between 5900 and 6600 light-years. It lies deep within the Milky Way, with extinction values from galactic dust ranging between 1.3 and 2.4 magnitudes across the cluster. This would be a spectacular object if it were located closer to us. It appears to be connected with the Cygnus-OB3 association, which, according to Gaia data, lies at the same distance.

Archinal & Hynes give a Trumpler classification of II3p n (II = detached, slight concentration, 3 = both bright and faint member stars, p = poor (fewer than 50 stars), n = involved in nebulosity) for the object and yet give the number of member stars as 56. If this is true, then the type should be II3m n, though it should be noted that the Deep-Sky Field Guide to Uranometria gives the number of member stars as 15!

Visually, despite its distance and extinction, IC 4996 still appears a bright little cluster. The centre is marked by a tight little grouping of seven stars, the brightest of which form a trapezium approximately 1’ across. Three of the four stars that make up the trapezium are the brightest stars in the cluster, with magnitudes of 8 or 9. The other stars in the cluster are fainter but there is a good mix of brightnesses. The visual impression is much closer to 50 stars than 15.

All in all, IC 4996 is a nice, bright, compact little open cluster. Definitely worth a look.

This month’s nebula is much more of a challenge; NGC 6888 in Cygnus. Something like 300,000 years ago, the massive star HD 192163 evolved into a red giant and ejected some of its mass in a slow-moving stellar wind. The red giant phase was brief, and since then the star has contracted and become a Wolf-Rayet star with a much more vigorous and fast-moving stellar wind. The more recently ejected material has now caught up with the slower-moving, previously ejected material. This has energized both winds and also caused a rebound of some of the faster-moving material, resulting in two shock waves, one expanding and one contracting. The result is this delicate-looking shell of nebulosity which nevertheless is energetic enough to emit X-Rays.

The object was discovered my William Herschel on 15 September 1792. He describes it as A double star of 8th magnitude, with a faint south-preceding milky ray joining to it, 8’ long and 1½’ broad. He placed it in his fourth class, planetary nebulae, as 72 H.IV.

I have to confess that whilst writing this article, I read Herschel’s description for the first time, and was struck by how close to my own observation of the object it is. The double star Herschel mentions is ΟΣ 401 (HD 192182). The components are magnitudes 7.2 and 10.2 and are separated by about 13”. The position for NGC 6888 given below is for ΟΣ 401.

My observation (reproduced here) was made with a 12” (305mm) reflector under sub-optimal conditions, using a power of x83 (36’ field) and an OIII filter. This is an object that responds well to UHC or OIII filters, and except under very good skies would most likely be unseen without them. The notes that accompany this drawing read Just a very small segment of this huge nebula was seen. It appeared faint, though use of the OIII filter helped. Wisps of nebulosity stretch away SW from a 7th mag star with small traces around another. A little difficult but quite certain.

The magnificent images of this object which can be seen in many books, magazines and online give no hint as to the actual visual appearance of this ephemeral object. It’s a challenge, but it’s not an impossible one. I would be very interested in observations (successful or otherwise) of this object under different conditions, apertures and filters.

The Wolf-Rayet star which produced this nebula is located 5.5’ south of ΟΣ 401 and is magnitude 7.5, making it the second-brightest star in the field, and the southernmost of an obvious ‘lozenge’ of 7th and 8th magnitude stars.

Finally, as if the last object wasn’t challenge enough, let’s try something very different. IC 4997 (only coincidentally the next entry in the IC after our cluster of the month) is a tiny, stellar planetary nebula in Sagitta. These stellar planetary nebulae are fascinating objects, though it could be argued that they are less than spectacular to look at. The same could be said for quasars, of course.

IC 4997 appears stellar at all powers. Its true nature is revealed by the use of an OIII filter (a UHC will serve). It is bright, its magnitude normally given as 10.5. 1’ to the south-west of the planetary is a 10th magnitude star. Without an OIII filter, the star is the brighter of the pair. With the filter in place, the planetary is brighter.

So far, so so-so. What I find interesting – and it was the subject of a discussion of the Webb Society Forum a while ago – is that as soon as some observers have the field of a stellar planetary nebula in sight, they can almost immediately spot which ‘star’ is the nebula. I am one of those observers, though I know I’m not alone. There is something about the object. Maybe it’s just not quite stellar or maybe there’s something about the quality of the light itself. Stars emit a continuum, planetary nebulae do not.

Here, then, is the challenge. Direct your telescope to the field of IC 4997 and see if you can spot which star is the nebula without use of a filter. If you can, try to determine what it is about the object that makes it different from the actual stars. I’d be fascinated to hear your thoughts (click here to email me).

Object	RA	Dec	Type	Magnitude
IC 4996	20h 16m 31s	+37° 38’ 44”	Open cluster	7.3
NGC 6888	20h 12m 13s	+38° 26’ 34”	Emission nebula
IC 4997	20h 20m 09s	+16° 43’ 54”	Planetary nebula	10.5

Patrick Maloney