Treasures Of The SMC Part III
By Andrew Murrell
In this instalment I continue my trek through the deep sky objects in the SMC. In response to demands for a good finder for these objects I have included a Treasures Of The SMC - Table Of Objects described thus far in this and previous instalments.
This open cluster with associated nebula can be located 20 arc minutes North of NGC 248 (see previous issue for description). The Nebula is 2 arc minutes across and almost round in shape. A star of 12th magnitude can be found just North of the centre of the nebula. It has a low surface brightness and no central condensation. Several low brightness stars are scattered across the face of the nebula, these being the cluster members. The Northern edge of the cluster appears well defined against the background even though it has a low surface brightness, owners of 6" telescopes may wish to try for NGC 249 at medium to high power with a UHC filter. When a filter is used with the 20" scope the nebula changes dramatically. A faint outer envelope becomes visible with the added contrast making the object about 4 arc minutes in diameter, and changing shape to be more oval in appearance. The star near the centre is drowned out and a distinct brightening can be seen toward the middle. A very faint extension of the nebula seems to project East toward NGC 261 an 11.5 magnitude star lies 2½ arc minutes to the South West.
This bright nebula is almost a twin with NGC 249 and can be found only 5 arc minutes East from it's border. It is similar in size being 2 arc minutes in diameter and also contains a 12th magnitude central star. No other stars are visible across the face of the nebula. The North Eastern edge of the nebula appeared to be flattened, otherwise it appeared oval in shape. The nebula had a higher surface brightness than NGC 249 and no central brightening could be seen. Immediately East is a large star cloud so the sky background is quite bright and peppered with 15-16 mag stars. NGC 261 was well defined from this background. This slightly higher surface brightness will make this an easier object for the 6" scope owner. The UHC filter increased the contrast on this nebula making it appear 4 arc minutes in diameter. A faint halo could be seen surrounding the object and a faint extension could be glimpsed projecting to the West toward NGC 249. The central star remained visible, this made the nebula appear like a planetary nebula.
When I consulted the Hodge Atlas of the SMC, I discovered that the surrounding area is filled with emission nebula. The large star cloud to the East of NGC 261 is a complete nebula complex with the designation of Henize 19. In what size scope this complex could be seen I do not know, why not write about your observation and tell.
Moving East from NGC 261, through the star cloud, about 45 arc minutes is this fairly bright open cluster. This cluster is compact being about 1½ arc minutes in diameter. It is roughly round in shape and the outer regions of the cluster seem to partly resolve. A slight condensation could be seen toward the nucleus. The individual stars glimpsed would be around 15th magnitude. The cluster is sharply defined against the background sky, which is quite bright.
No nebulosity is mentioned in association with this cluster but observations show that the cluster has a brightening in the OIII on the Northern edge. It is very small being only l0 arc seconds in diameter (if that) and appears brighter than the entire cluster. Due to it's compact size and high surface brightness this cluster should be visible at high power in a 6" telescope. The RNGC lists this object at 12th magnitude, but gives no description.
L40 is a large open cluster about 4 arc minutes across positioned 20 arc minutes North of NGC 290. The cluster is primarily made up of 15th -16th magnitude stars, with a scattering of 13th -14th magnitude stars thrown in. The brightest star on the Northern edge is 12.8 magnitude. The cluster is predominantly square shaped due to the distribution of the brighter stars. An absence of the stars in the centre of the cluster make it look like there is a dark nebula there. The Northern edge of the cluster is the brightest and appears sharply defined, otherwise the cluster is not as well detached from the background sky. The large size and low surface brightness will make this cluster a challenge with a 10" telescope. No nebulosity is associated with this cluster at all, in fact it became invisible when observed with a filter.
This compact open cluster is 30 arc seconds in diameter with a bright central condensation. It is round in shape and has no stars scattered across the surface. It is located 8 arc minutes East of L40. The surface brightness is higher than L40 so it should be visible in a 10" scope. It was discovered in 1956 during a photographic survey of the SMC. It's position in the central bar of the SMC insures that the sky background is higher but it is still well defined. A 10th magnitude star lies 7 arc minutes to the South-South-West of the cluster, with an 11th magnitude star also 7 arc minutes to the North-North-East. No nebula is associated with this cluster either.
Kron 31 is a large diffuse open cluster and has no central brightening. The low surface brightness of this cluster and it's 3 arc minutes diameter will make this object a challenge for an 8" scope. It can he found 10 arc minutes North East of Kron 29. The brightest stars of the cluster appear to be about 14th magnitude with a slight concentration towards the North Eastern edge. The cluster appears to lie on the Eastern edge of the bar of the SMC. The cluster does not stand out from the background sky as easily as some of the previous clusters. Large scopes may reveal a chain of stars headed West from the clusters Southern edge.
A compact cluster about 20 arc minutes East-North-East of Kron 31. It is about 1 arc minute in diameter and has a slight concentration toward the core. The surface brightness is otherwise even. The cluster is sharply defined from the background sky and is round in shape. The cluster should be visible in an 8" telescope at high power. When the cluster was observed with a UHC filter no nebulosity could be seen. A triangle of 12th magnitude stars were visible just off the North Western edge of the cluster. No stars are across the clusters surface.
This open cluster can be found 40 arc minutes South-South-East of NGC 290. It is quite compact being about 35 arc seconds in diameter. The cluster has a very bright central core with a high overall surface brightness. No stars are visible across the face of the object. This cluster is situated out of the main bar of the SMC so the background sky has darkened considerably. It is sharply defined against this background. The cluster should be visible in an 8" scope at high power.
Kron 38 is located 30 arc minutes East of Lindsay 47. The main bar of the SMC is almost 1 degree to the West, so the sky background here is low, which for this cluster is good. The cluster is 3½ arc minutes in diameter. It has no central condensation and seems to be primarily made up from 15th -16th magnitude stars. There are several 12th magnitude stars scattered across the face of the cluster. The surface brightness of the cluster is very homogenous with a very slight brightening to the North The cluster has an ill-defined edge and is not separated very well from the background sky. The very low surface brightness and large size will make this cluster a difficult object for a 12" telescope from a good dark sky. No nebula is associated with this object when it was checked with the OIII filter, in fact the cluster was not visible at all.
The star Lambda Hydrus lies almost 1 degree South of the Southern edge of the SMC. This 5.1 mag star is the starting point to find the next two objects. NGC 339 is a globular cluster 40 arc minutes to the North East of that star. The cluster is bright and large being 2 arc minutes in diameter. There is a slight condensation toward the core. No individual stars are visible in the cluster but it does have the grainy look about it that suggests it will resolve in a slightly larger telescope. The surface brightness is even in the centre of the cluster and starts to drop about ½ the diameter out. The edge of the cluster is diffuse and no bright stars are visible near.
10 arc minutes North of NGC 339 is the compact open cluster Kron 37. The cluster is 3 arc mins in diameter with an even surface brightness. The cluster resists attempts to resolve even with the 20" scope. The cluster is quite faint and should be visible through a 10"- 12" telescope. No brightening towards the nucleus was visible and no stars were scattered across the face. The cluster was detached from the sky background with no stars brighter than 12th magnitude in the area.
Next month we will look at the brightest objects in the SMC: The amazing NGC 346 and it's companion NGC 371.