Globular clusters are gravitationally bound concentrations of stars, which form a nearly spherical system around our galaxy. They orbit the galactic center along highly elliptical paths, and on average one revolution takes 300 million years. These wondrous swarms of ancient suns are impressive sights in almost any telescope. The greatest of the globular star clusters, and one of the nearest to the Earth, is the magnificent Omega Centauri, some 17,000 light years distant and visible to the naked eye. Unfortunately, it can only be observed from the Southern Hemisphere.

However,there is also a beautiful star cluster for us,Northern Hemisphere types.M13, the Great Cluster in Hercules, teeters on the edge of naked eye visibility and appears in binoculars as a small, circular, hazy glow. It contains over 300,000 stars, and its total luminosity is 300 thousand times the Sun. The cluster is located in the "Keystone" of Hercules, about a third of the way along a line drawn from Eta to Zeta Herculis, and any optical aid will reveal it. Small refractors or large binoculars show a large, bright smudge, but resolution of individual stars is difficult. A 4.5-inch telescope starts to show stars at the cluster's edge as individual points, and with a 6-inch scope their images are strong and steady. With 8- and 10-inch instruments the size of the cluster swells a bit, and the clarity of individual stars closer to the center increases. Nevertheless, resolving the bright core into stars is nearly impossible. Careful observers with 6-inch or larger instruments might also notice dark patches against the disk of the cluster. The cause is probably intervening interstellar matter. After studying M13 through your telescope, imagine now the sky as from deep within the cluster. We would see uncountable numbers of blazing stars, many as bright or brighter than our naked eye planets. Inhabitants on a planet inside M13 would probably know nothing of the Galaxy and other galaxies, as their view would be completely blocked by the brilliance of their own skies. To them, the Great Cluster in Hercules would be "the Universe".

The Double Cluster in Perseus NGC869(mag4.3) and NGC884(mag4.4). These twin gems lie side by side separated by a mere 1/2°. They easily fit in the same telescopic field of view in all but the longest focal length instruments. In binoculars, they are beautiful. In a small telescope, the are beautiful. In a large telescope...words can not describe the beauty. But... I'm gonna try :) In the 25", the Double Cluster appears like diamonds strewn across a black velvet background. They seem to sparkle and shimmer as if electrified...like nothing you've ever seen. 

The Double Cluster is pretty easy to find. They sit midway between Perseus and Cassiopeia. Under moderately dark skies, they are clearly visible to the naked eye as a "fuzzy" patch of light between the two constellations.

This image was taken by me on the 6th December 2009 with a Canon 40D attached to a Stellarview 102ED Appo Refractor. One image of 2 mins.@ ISO 400.

The Pleiades, also known as Messier 45 (M45), are among those objects which are known since the earliest times. At least 6 member stars are visible to the naked eye, while under moderate conditions this number increases to 9, and under clear dark skies jumps up to more than a dozen, well before the invention of the telescope, astronomer Moestlin has correctly drawn 11 Pleiades stars, while Kepler quotes observations of up to 14.

Modern observing methods have revealed that at least about 500 mostly faint stars belong to the Pleiades star cluster, spread over a 2 degree (four times the diameter of the Moon) field. Their density is pretty low, compared to other open clusters. This is one reason why the life expectation of the Pleiades cluster is also pretty low (see below).

The Pleiades also carry the name "Seven Sisters"; according to Greek mythology, seven daughters and their parents.

Longer exposure photographs (and also short focal ratio, i.e. short focal length compared to their aperture, "rich field" telescopes of considerably good quality, especially good binoculars) have revealed that the Pleiades are apparently imbedded in nebulous material, obvious in my image taken with an 80mm ED Refractor and Canon 300D. 

The Pleiades nebulae are blue-colored, which indicates that they are reflection nebulae, reflecting the light of the bright stars situated near (or within) them. The brightest of these nebulae, that around Merope.