The long cold nights of the Northern Hemisphere winter months are populated with some of the finest constellations in the sky. During the months of December, January and February many celestial gems are visible in the evening sky. They include spectacular open clusters, stunning nebulae as well as numerous bright stars. Below is a list of five of the best open clusters visible at this time of year; all of which can be spotted with the naked eye and each one a superb binocular object.
We start the countdown with the faintest and smallest cluster on our list, M35 in Gemini. At mag. +5.2 it appears to the naked eye as a somewhat misty patch of light. Surprisingly it wasn't discovered until 1745-46. When seen through binoculars M35 is a fantastic sight with the brightest dozen or so stars resolvable. Enhancing the view is a hazy glow surrounding the stars. Telescopes reveal many more stars but the glow effect disappears. M35 is 2,800 light years distant and spans 28 arc minutes which is similar to the full Moon diameter. It's estimated to contain up to 200 stars.
Located about 15 arc minutes southwest of M35 is open cluster NGC 2158, which at mag. +8.6 is faintly visible with binoculars.
The Double cluster consists of two bright open clusters, NGC 869 and NGC 884, separated by just half a degree in the constellation of Perseus. They are easily visible to the naked eye and through binoculars and telescopes are a wonderful sight. The Double Cluster has been known since antiquity and early celestial cartographers named them as stars "h Persei" (NGC 869) and "χ Persei" (NGC 884).
To the naked eye it appears like a large detached unresolved part of the Milky Way. Of the two clusters, NGC 869 is marginally the brighter at mag. +4.3 while NGC 884 glows at mag +4.4. Through binoculars the view is superb with many bright blue and white stars visible in a hazy mist. The increased aperture and magnification offered by telescopes reveal dozens more stars that fill the eyepiece field of view. A gem of a cluster.
NGC 884 and NGC 869 are located at distances of 7600 and 6800 light-years from Earth respectively. Each cluster is estimated to contain a few hundred stars.
The Hyades is the perfect binocular open cluster due to its brightness and large apparent size. Spanning 5.5 degrees of sky it's equivalent to 11 times the diameter of the full Moon. When seen through 7x50 or 10x50 binoculars it fills the complete field of view and can even over-spill in some models. Easily found since it surrounds first magnitude orange giant Aldebaran (α Tau - mag. +0.87) in Taurus, at least 20 stars are visible to the naked eye with the number rising to above 100 with binoculars. The cluster is notably "V" shaped. Of interest Aldebaran is only a foreground star and not a true member.
At a distance of 153 light-years the Hyades is the nearest open cluster to us. Located 12 degrees to the northwest is another open cluster on our list, M45 the brilliant Pleiades.
M44 is a sprawling open cluster in Cancer that's also known as the Praesepe or Beehive cluster. It covers 1.5 degrees of sky and like the Double Cluster appears to the naked eye as a large misty cloud. Since it shines with a combined mag. of +3.7, it can be spotted with the naked eye even with moderate amounts of light pollution. Through binoculars it's a magnificent object that bursts into life, neatly filling a good proportion of the field of view with dozens of stars sprinkled throughout a hazy background.
The extra aperture and magnification range afforded by small or medium size telescopes bodes well when observing M44. With a 80mm (3.1-inch) telescope at about 30 to 40x magnification the Praesepe is wonderful with many stars arranged in pairs and triplets in equatorial shape triangle groupings. Some of the magic is lost in larger scopes when it over-spills the field of view.
M44 is 577 light-years distant and estimated to contain up to 350 stars.
No surprise the number one on the list is the finest open cluster in the sky, M45. Commonly known as the Pleiades or Seven Sisters it's been known since antiquity with at least 6 stars easily visible to the naked eye and up to 10 or more under ideal conditions. At the heart of the cluster is a set of stars that form a small dipper shape, similar to the brightest stars of Ursa Major.
The Pleiades covers almost two degrees of sky in Taurus. When viewed through binoculars it's simply stunning with the main stars bright and distinct and dozens more fainter ones visible. A nice trail of 7th magnitude stars extends to the southeast from one corner of the dipper shape. Small scopes of around 80mm (3.1-inch) or 100mm (4-inch) aperture at low magnifications provide a breathtaking view. The cluster fits neatly in the field of view with many bright stars standing out against the crisp black background. The Pleiades loses some of its mystique in larger scopes due to its large size. However, there is a faint reflection nebulosity surrounding the brightest stars that can be glimpsed in larger scopes although it's much easy to image or photograph.
The Pleiades are located 425 light-years from Earth. Around 500 members belong to the cluster.