Observations of The Small Magellanic Cloud (SMC)

NGC 371

Andrew Murrell

At the northern end of the central bar of the SMC lies the bright cluster NGC 371.

It is located 20 arc minutes north east of NGC 346. NGC 371 is very large, covering an area in excess of 7 arc minutes. This corresponds to a real size of approximately 350 light years.

Discovered by Sir John Herschell during his observation of the SMC, he described it as, a faint large cluster of 14^th to 16^th magnitude stars round in shape.

The objects high surface brightness makes it easily visible through a 6 x 30 finder. A good pair of binoculars shows NGC 371 as a bright knot on the end of the main bar of the SMC, due south of the bright globular NGC 362 hectors 90 x 20 finder shows the cluster complete with some of the surrounding bright stars. Employing a UHC filter in this telescope reveals the faint glow of the surrounding nebulae as well. NGC 371 and NGC 346 make a marvelous pair of objects for rich field scopes.

Through Hector NGC 371 appears as a very large very diffuse cluster with stars ranging from 9.5 magnitude to well below 16^th. These is only a slight hint of concentration in the centre of the cluster. The brightest stars in the centre range from 12^th to 13^th magnitude and form the shape of a chair, facing north west, with the backrest orientated north east to south west.

The 13^th magnitude star that marks the centre of the backrest has a chain of 13^th magnitude stars trailing away to the north 3½ arc minutes, were it meets a double star. From this double star the chain then loops west and leads back toward the centre of the cluster. Several other 12^th and 13^th magnitude stars are scattered about. Peppered through these brighter stars are a multitude of 14^th to 16^th magnitude stars. Through Hector it still appears to be resolved with these fainter stars. This grouping increases the overall size of the cluster to 5 arc minutes across, still round in shape.

Through out the cluster a ghostly glow can be glimpsed. This is the nebulosity associated with the cluster. This glow is roughly round in shape corresponding to the size of the cluster. By placing a UHC filter in line changes the view through the eyepiece dramatically.

The nebulae is roughly round in shape 5 arc minutes in diameter with a slightly flattened edge on the southwestern edge. The faint members of the cluster are not visible, but the bright stars that form the chair shape are still visible. The nebula has a fairly even surface brightness with a slight darkening toward the centre.<

When I consulted the NGC catalogue, NGC 371 was listed as a cluster with no mention of any nebulosity. Even the Mati Morel detailed charts did not show the nebulosity. Only having checked the Hodge atlas of the SMC was I able to identify the nebulosity as Heinze 1997. It seems strange to me that no mentions of nebula is made even though it is visible in a 10" scope with a UHC filter. In fact it may even be visible in even smaller scopes.

Removing the filter and lowering the magnification revealed a faint outer halo of unresolved stars. This halo became most obvious around the Southwest corner of the cluster. This may be the glow of the Stellar Association or star cloud. Due to the clusters location in the SMC the sky background is quite bright. The outer halo has no distinct edge as it blends in with the general sky background. I would estimate though that the cluster would appear to increase in size to 7 arc minutes by including the halo.

The view of NGC 371 without the filter reminds me of the Helix Nebula viewed through a 4" telescope at low power. This object is magnificent viewed with any size instrument.

The brightest stars of NGC 371 approach -7 magnitude making them some of the brightest individual stars in the entire SMC.

Timo Karhula

Observed with 20x80 binoculars and described as bright, large and diffuse.