NGC1981: The often-overlooked open cluster in Orion
12/22/2013 2:37 PM
If one was looking with a telescope in an star-poor, somewhat sparse region of the galaxy, NGC1981 would be a fantastic sight upon which to stumble. Let's start with where to find this cluster. Look in Orion at the "sword region" of stars and nebulosity (see graphic below):
The three stars within the "sword" region of Orion from north to south are 45, Theta and Iota Orionis. Iota is the brightest on the bottom of the sword, and careful observers may note that the Theta area is not actually stellar, or point like. Rather, it looks like a "fuzzy" star. That is the Orion Nebula, or M42 that can be seen naked eye.
Above those three stars is NGC 1981.
A collection of about 20 or so stars, largely magnitudes 7 through 10, this cluster is about 40 light years closer to Earth than the Orion Nebula is. And it's worth viewing after one has already perused the nebula region to it's south.
Some observers have noted that the cluster looks somewhat like a crocodile or alligator, as viewed from above, with the eastern star forming the snout and the western star forming the tail. Two lines of three stars form the "legs" and "body" in the middle. Sue French of Sky and Telescope sees this area as a line from a bouncing ball. Whatever you may see or not see, let's look at a bit of the data from this area.
Object: NGC 1981
Open Cluster, type III2p, in Orion
Right Ascension (2000.0): 05:35:09.0 (h:m:s)
Declination (2000.0): -04:25:54 (deg:m:s)
m_v: 4.2 (mag)
Dimension: 28.00 (arcmin)
Distance: 400pc / 1300 light years
The star cluster also contains a double star, Struve 750. If we go with the "alligator" shape, look at the "front" legs near the "head" of the alligator, to the east. The leg towards the north, or top left (in the Northern Hemisphere) is Struve 750. This is a fairly tight double star; just 4 and a half arc seconds apart. So steady skies, a good telescope, and somewhere north of 100x magnification are likely needed to split these stars cleanly, though depending on conditions and optical quality, less or more magnification may be needed.
For more on this cluster and other objects in the Orion Sword region of the sky, check out the video below. And don't forget about the double stars and nebulae in the same area of this constellation too.