Open clusters are extended families of a few hundred comparatively young stars bound together by gravity. They originate from vast clouds of dust and gas – nature’s star-factories – spread all across the Milky Way.
Many open clusters such as the Pleiades and the Hyades are large enough to be seen with the naked eye, and hundreds more become visible when using a telescope.
Because open clusters are strongly concentrated close to the galactic plane the best place to look for them is near the Milky Way. Most open clusters have large angular diameters, so when observing use a wide-field eyepiece and low power.
Try to examine each cluster for a few minutes and write in your logbook all that you see. Are there any double stars within the cluster? Is the cluster completely resolved into stars, or are there any unresolved stars causing the cluster to appear nebulous? What is its shape and how many stars can you see?
Open clusters observed with a 150-mm telescope
M48 is a quite conspicuous object and under good conditions it can be seen with the naked eye. The cluster is large and dense, in the center I’ve seen a grouping of brighter stars.
M6 is a fairly difficult object due to its southern declination, but it can be easily seen if you have a clear southern horizon. It is composed of approximately ten bright stars disposed in the form of a rectangle, and other 40 stars of medium brightness.
Trumpler 2 is a beautiful cluster, elongated west-east. About 20 stars magnitude 8 to 12 are visible.
NGC 7160 is a small cluster in Cepheus, composed of ten faint stars that surround a brighter one. To the south I’ve noticed two bright stars.
NGC 1857 is a large, dense cluster in Auriga. It is mostly composed of 7th-magnitude stars.
Collinder 62 is an unimpressive cluster, consisting of only three bright stars surrounded by fainter ones in the background.
Stock 2 is a very large cluster, it doesn’t fit in my one-degree field of view. Over 100 stars are visible, scattered in the field.
Open clusters observed with a 114-mm telescope
Upgren 1 is a very little known cluster in Canes Venatici. It is not plotted on Sky Atlas 2000, but it’s included in Uranometria. It is the only open cluster in this constellation, it consists of approximately ten stars, scattered in the visual field of the eyepiece.
Collinder 394 is composed of 15 stars, three brighter ones arranged in the form of an isosceles triangle. The two eastern stars that form the base of the triangle are double stars, easily separated at 53x.
NGC 6716 is an interesting cluster, composed of 15 stars of medium brightness which form a pattern similar to that of the letter “U”. The cluster is not completely resolved, with averted vision I have seen traces of granulation in the background.
NGC 7686 is composed of 10 bright stars, the other stars of the cluster are pretty faint. At a magnification of 53x I observed faint traces of nebulosity (the unresolved stars of the cluster). NGC 7686 is easily discernible from the background.
M52 is a small cluster, with a shape similar to that of a bean. East from the cluster I’ve found a bright star that contrasts with the other faint stars of which the star cluster is composed. M52 is extremely dense, composed of over 50 easily observed stars.
Trumpler 28 is a small cluster in Sagittarius, composed of a few brighter stars surrounded by nebulosity.
NGC 7235 is composed of three moderately bright stars disposed in the form of a triangle. At 36x faint granulation and nebulosity surrounds the cluster.
NGC 6383 is composed of 15 bright stars, of which a very bright one in the est.
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