Sky Object of the Month – March 2015

Sky Object of the Month – March 2015
Messier 47 (NGC 2422) – Open Cluster in Puppis
by Glenn Chaple

Last March, we explored the open star cluster Messier 46 in Puppis. This time around, we turn our telescopes 1 ½ degrees westward to another Puppis cluster – M47. For nearly two centuries, this was one of Messier’s “missing” objects. It was originally discovered by the Italian astronomer Giovanni Battista Hodierna – a forerunner of Messier. Hodierna reported it in a treatise on comets and “admirable objects of the sky,” published in 1654. Hodierna’s work was unknown to Messier who independently discovered the cluster on February 19, 1771. Messier made an error in plotting its position, and the mistake wasn’t rectified until the mid 1900s.
At magnitude 4.4 and having the apparent width of a full moon, M47 is visible to the unaided eye under dark-sky conditions. It’s a fine sight in binoculars and rich-field scopes, which capture M46 as well. While the latter appears as a hazy patch, M47 has an almost Pleiades-like look. Even a small scope will pick up several dozen cluster members.
Like M46, which houses the planetary nebula NGC 2438, M47 is home to a deep-sky delight of its own. Near its center is the pretty double star Struve (STF) 1121 – a pair of white magnitude 7.0 and 7.3 stars separated by 7.4 arc-seconds. STF 1121 is well resolved when M47 is viewed with magnifications of 50X and up.
M 47 is approximately 1600 light-years away and has a true diameter of 12 light-years.

image001 (IAU and Sky and Telescope)




Messier 47 (4.5-inch f/8 reflector at 33X) North is up in this 1.3 degree field. Arrow shows location of STF 1121. Sketch by author

Comments are closed.

Copyright © 2018 North Shore Amateur Astronomy Club.
A non-profit organization under Section 501(c)(3) of the IRS code.