NGC 957 and IC 351 in Perseus
November 2022 - Nebula and Cluster of the Month
Our cluster for this month lies close to the double cluster in Perseus (NGCs 869 and 884, or h and χ Persei). In fact, it’s the closest open cluster to the pair, lying just 1.5° to the ENE.
It was discovered by John Herschel in 1831. He entered it in his catalogue as JH 227, and it subsequently entered the General Catalogue as GC 555, and finally in 1881 in the New General Catalogue as NGC 957. The description in the NGC is fairly basic: Cluster, pretty large, pretty rich, stars mag 9–15.
Although the NGC description is accurate, as far as it goes, it’s always worth having a look for yourself. Pretty large? It’s usually quoted at 11–12’ across. Visually I estimated 10’ through my 12” (300mm) Newtonian at x81. Deeper images tend to show a slightly wider spread of faint stars than can be seen easily visually. Pretty rich? Descriptions vary. The Trumpler classification usually quoted for the cluster is III2m, which translates as ‘no discernible central condensation, detached, moderate range of brightnesses, medium rich (50-100 stars)’. Visually I counted about 40, with no determination of membership, of course. Steve Coe, in his published observations, describes it as ‘not rich’. I described it as ‘moderately rich’. This is the joy of visual deep-sky observing. You make of it what you will, and no two observations are ever quite the same.
The Deep Sky Field Guide to Uranometria 2000.0 gives the number of stars as 30 with the magnitude of the brightest star being 9.5. Archinal & Hynes in their Star Clusters give the star count as 112 with the brightest star being of magnitude 11.9. No two textbooks are ever quite the same, either. A paper1 by A. Giménez and J. García-Pelayo of the Instituto de Astrofisica de Andalucia in Spain lists photometric measurements for 212 stars in NGC 957. 91 of these stars are listed as ‘probable members’. This presumably leaves 121 certain members. Again, it’s always worth having a look yourself.
The cluster lies at the end of a little curl of bright, eighth-magnitude stars, which I guess are not members. It’s quite an attractive cluster, elongated more or less east-west and well populated with fairly bright stars. HD 15690, on the southern edge of the cluster, is a bright, easy double. With magnitudes of 8.0 and 9.7, this is presumably not a true member of the cluster. The Spanish measurements place NGC 957 at a distance of 1,150 parsecs (about 3700 light-years) so well within the Perseus Arm of the Galaxy.
This month’s nebula is a little planetary nebula, IC 351, still in Perseus. Discovered by Barnard in 1890, it is described in the Index Catalogue as appearing like a tenth-magnitude star. Like other brief catalogue descriptions, this is true as far as it goes. It’s always worth investigating these ‘stellar’ planetary nebulae because quite often they are not truly stellar.
Magnitudes cited for this object are around 11.9 to 12.0. The diameter is something between 7 and 12” (sources vary). The central star, when mentioned at all, is suggested to be a very difficult magnitude 15.8.
Observations quoted in the Night Sky Observer’s Guide suggest that in telescopes as small as 8” (200mm), a tiny disc can be perceived, whilst with objectives of 16–18” (400-450mm), the nebula can be clearly seen to be elongated. My own observation, made with a 12” (300mm) reflector tends to confirm this.
I observed IC 351 in December 2013 under very poor skies. Especially at lower altitudes, the sky was a sickly mustard colour. The whole sky was covered by thin, hazy, high cloud and I was under the impression that most of the time I was observing through it. At the time of the observation, IC 351 was at a healthy altitude of 72°, so minimising the effects of the poor sky.
I observed a tiny disc that looked quite bright. A brighter section was possibly detected in the southwest quadrant of the object. Using the SP10mm eyepiece + 3x Barlow lens (giving a magnification of x450) it looked non-homogeneous. An outer envelope was suspected, as was a slight elongation. I also noted that the object did not seem to respond to the OIII filter.
Altogether a much more satisfying view than would be expected from the terse IC description.
It’s always worth looking.
|NGC 957||02h 33’ 17”||+57° 33’ 14”||Open cluster||7.6|
|IC 351||03h 47’ 33”||+35° 02’ 47”||Planetary nebula||11.9|
- Astron. Astrophys. Suppl. Serv. 41, 9-12 (1980)