Open clusters are regions a few tens of light years across which contain hundreds or thousands of stars, all of which formed at about the same time, usually within the last few hundred million years. As they orbit around the Galaxy, other stars, which are not members of the cluster, can pass through the cluster, slightly disturbing the motions of the cluster members. Occasionally, this results in one or more cluster stars being ejected from the cluster; and over long periods of time, the clusters fall apart, their stars scattered all around the path they followed, while they were orbiting as a group.
Since most open clusters are young, they are found primarily in the disk of the Galaxy, where they form out of the gas and dust in the spiral arms. Because of this distribution, they are often called galactic clusters. The Sun must have formed in such a cluster, 4.5 billion years ago; but since such clusters only last a few hundred million years at most, the Sun must have been going around the Galaxy on its own for more than 90% of its life.
The ages of clusters are estimated by measuring the brightness and color/temperature of the individual stars, and using the results to plot a Hertzsprung-Russell Diagram. On such a Diagram, called a cluster diagram in this case, the Main Sequence will have a top, where stars which have lifetimes approximately equal to the current age of the cluster are getting old, and moving from the Main Sequence, toward the Red Giant region of the Diagram, which represents the last days of the stars' lives. The position of this "turnoff" point tells us the age of the cluster. The higher it is, and the brighter the stars at the top of the Main Sequence, the younger the cluster must be, since bright stars don't last very long. The lower the turnoff point is, and the fainter the stars at the top of the Main Sequence are, the older the cluster must be, since fainter stars last far longer than brighter ones. In the globular clusters discussed below, the brightest stars are fainter than the Sun, meaning that the clusters are older than the Sun's lifetime; but no open cluster can last that long.
In contrast to open clusters, Globular Star Clusters are very old, and contain few if any stars as hot and bright as the Sun, as stars like the Sun or larger and brighter than the Sun have already died. If such clusters were, like open clusters, found in the disk of the galaxy, and contained only a few thousands or tens of thousands of stars, they would have been tidally disrupted and torn apart by stars passing through them, billions of years ago. But unlike the open clusters, the globulars are scattered around the outside of the galaxy, in the halo, where there are very few stars to pass through them, and a typical globular contains hundreds of thousands of stars, and some even contain millions of stars, so their own gravity is very large, and quite capable of holding them together, even were they to pass right through the galaxy.
SOME OLDER DATA HAVE BEEN SUPERSEDED BY NEWER INFORMATION: USE THE NGC LINKS
Above, a 1.2 degree wide view of the double star cluster, h (or NGC 869, on the right) and c (or NGC 884, on the left) Persei, a pair of relatively young "open" or "galactic" star clusters located about 7,000 light years from Earth, in the Perseus arm of our galaxy (outwards, relative to the center of the galaxy). The clusters are only separated by a few hundred light years, and because of their relative closeness in comparison to their distance from the Sun, appear very close together in our sky. Although thousands of times younger than the Sun (age estimates are just over 5 million years for h, and 3 million years for c), the hottest, most massive stars in these clusters are near the end of their lives, and the brightest stars are already evolving into red giants, the last stage of stellar life. Despite their distance from the Earth, the clusters are visible to the unaided eye as a faint, apparently nebulous object in a dark sky, were probably known in prehistoric times, and are listed in the oldest existing star catalog -- that of Hipparchus, written more than two millennia ago.
The a Persei (= Mirfak) moving cluster. The cluster is a group of several dozen bright stars near Mirfak, which are moving through space in the same direction at nearly the same speed, and therefore represent a true physical cluster of stars. The cluster was first noted by Giovanni Hodierna, sometime before 1654. Its inclusion in his list I (#4) indicates that he observed it as a group of individual stars; but its status as an actual cluster was not confirmed until the early 1900's. It was first described as a stellar group by Eddington in 1910, and cataloged by Mellotte (hence its designation as Mellotte 20) in 1915. The cluster is about 600 light years away, and contains a large number of O and B type Main Sequence stars, giving it the designation "an OB association". That also means that it is very young, as such very hot stars have very short Main Sequence lifetimes. The age of the cluster is estimated at only 50 million years, but its most massive member -- aPersei itself -- has already started to die, and is now an F5 supergiant. Most of the stars in the cluster are contained within a region a little over 3 degrees across, which corresponds to about 30 light years at the distance of the association; but to show the entire cluster would require an even wider field of view than the 5 degree wide image above. Aside from Mirfak, other bright members of the cluster shown in this image are d and y Persei. But e Persei, which is also a member of the cluster, lies several degrees down and to the left of d, and well outside this field of view.
(ESA, AURA/Caltech, NASA)
Another young cluster of stars (its age is estimated at 100 million years), the Pleiades, in Taurus. Even in bright city lights, the Seven Sisters, as they have been called since Hellenic times, are faintly visible as a little "dipper" of half a dozen or so stars. In reality, the dozen or so light years that the cluster spans contains over three thousand stars. Despite the small size of the cluster (most of the other clusters shown on this page are a hundred or two light years across), its relative nearness to the Earth (at four hundred forty light years distance, it is second only to the Hyades, in terms of closeness) makes the cluster over twice as wide as the Moon (see the image below, for a direct comparison), and a difficult object to image in most telescopes, because their field of view is much smaller than the cluster, and only a portion of it can be seen at any given time. The best way to observe the cluster is with a four to six inch telescope set to a very low power (preferably, not much more than twenty times magnification), by using an eyepiece of long focal length or, when taking a photograph, using a reducing lens to reduce the intrinsic magnification of the telescope, and increase the field of view.
The blue reflection nebulae surrounding the brighter stars used to be thought clouds of dust left over from the stars' formation, but it is now thought that the cluster is simply passing through a relatively dusty region. Regardless of its source, interstellar dust scatters blue light more than red light, in a way similar to our atmosphere's scattering of sunlight, and since the stars are themselves very hot, blue objects, the light scattered by the dust is exceptionally bluish in color. Unfortunately, only long exposures reveal the beauty of the dust clouds, as they are far fainter than the stars.
Linguistic tidbit: the Japanese name of the Pleiades is "Subaru".
Hence the name of the Subaru telescope, and the logo (above) of Subaru automobiles.
The Moon passing by the Pleiades, showing how large the cluster appears in our sky. Since the Pleiades lie near the Ecliptic, the path followed by the Sun, Moon and planets, the Moon occasionally passes close by, or even in front of the cluster. In this view, the crescent Moon is considerably overexposed, revealing features on the dark side of the Moon, by reflected Earthshine. When the Moon is in crescent phase, the Earth appears nearly full, as seen from the Moon, and because of its larger size and reflective clouds, shines nearly a hundred times more brightly on the Moon, than the full Moon shines on the Earth. Since the Moon always keeps one face to the Earth, the dark side of the Moon shown in this view is the same side we see when it is in sunlight, at fuller phases. The usage of "dark side of the Moon" to describe the side facing away from us (which is technically called the far side of the Moon) is incorrect, unless by "dark", one means "unknown and unexplored", as in the days when explorers risked their lives in "darkest Africa". (Jerry Lodriguss (Catching the Light), apod050414)
M44, also known as the Beehive, or Praesepe, an open cluster of stars about 580 light years away, in the constellation of Cancer. About the same size as the Pleiades, it spans about ten light years, and contains a few hundred stars, but being older (400 million years), its individual stars are fainter than those in the Pleiades, and being about twice as far away makes them look another four times fainter. So while several of the Pleiades are visible as individual stars, to the unaided eye M44 appears as only a faint smudge, even in a dark sky. It was only when Galileo turned his telescope to the cluster that its true nature was finally revealed. (Wil Milan, apod980803)
The left half of this image is nearly filled by M35, an open cluster of stars about 3000 light years away. Like the Pleiades, this cluster is about 100 million years away, and contains a few hundred stars, but even though physically larger (about 25 light years across), being ten times further away makes it look only a little larger than the full moon. On the lower right, NGC2158, which looks much smaller, because it's about 16000 light years away, contains thousands stars, and at more than a billion years age, is one of the oldest open clusters known. Clusters which only have a few hundred stars, such as M35 and the Pleiades, are disrupted by the gravity of stars passing through them, without a few hundred million years, at most; but clusters with thousands of stars, such as NGC2158, can hold together for a few billion years; and clusters with hundreds of thousands or millions of stars, such as globular clusters (discussed below), can hold together practically forever. (N. A. Sharp, NOAO, AURA, NSF, apod021129)
NGC 4755, or The Jewel Box, is an open cluster 7500 light-years away, in the southern constellation of Crux (the Southern Cross). A little over a hundred stars, formed about 10 million years ago, span an area 20 light-years across -- such a small region, in comparison to its distance, that the cluster appears almost the same as a star, to the unaided eye; and hence, leads its also being labeled as k (kappa) Crucis, a Beyer designation usually assigned only to stars. Small clusters such as NGC 4755 fall to pieces as they move through the galaxy, and other stars pass between the cluster stars, disturbing their motion relative to each other, over periods of just a few tens of millions of years. (Michael Bessell, RSAA, ANU, MSO, apod010618)
About two hundred thousand light-years distant, in the Small Magellanic Cloud, are the open clusters NGC 290 (above) and NGC 265 (below). Each cluster spans about 65 light years, and contains hundreds or thousands of brilliant young stars. Most of the stars in these images are not cluster members, but lie in front of (in our own galaxy, or the Small Magellanic Cloud) or behind (in the Small Magellanic Cloud) the clusters. (E. Olszewski (U. Arizona), HST, ESA, NASA; NGC 290 above, apod060501; NGC 265 below, Hubble Site)
The open star cluster R136, in the Large Magellanic Cloud (about 160 thousand light years distant) is one of the youngest, most massive star clusters known. Most of the radiation which lights up the Tarantula Nebula comes from the nearly half million solar mass cluster. The cluster contains dozens of extremely massive, hot stars (50 to 80 solar mass type O3 blue giants), which have formed within the last million or two years. The central portion of the cluster is so thickly populated that until recently, it was thought to be a single, supermassive star, but is now know to consist of a dozen or more giant stars. Given the stars and mass it contains, R136, though currently classified as an open supercluster, may eventually form a small globular cluster. Click here or on the image for a larger view of the cluster and the area surrounding it, and of the Tarantula Nebula, itself. (NASA, ESA, & F. Paresce (INAF-IASF), R. O'Connell (U. Virginia), & the HST WFC3 Science Oversight Committee, apod091221)