[1] Each one is loosely bound by mutual gravitational attraction and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center.

This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy.

In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer.

Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

[3] Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region.

[11] In his 1610 treatise Sidereus Nuncius, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters.

[16] Between 1774 and 1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets.

[9] In the 1790s, English astronomer William Herschel began an extensive study of nebulous celestial objects.

[9] Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the center of the Milky Way.

[23] However, in 1918 the Dutch–American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times.

[24] As astrometry became more accurate, cluster stars were found to share a common proper motion through space.

[25] Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars bound together as a group.

He demonstrated a relationship between the star colors and their magnitudes, and in 1929 noticed that the Hyades and Praesepe clusters had different stellar populations than the Pleiades.

[27] Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence and rotation.

[28] Many factors may disrupt the equilibrium of a giant molecular cloud, triggering a collapse and initiating the burst of star formation that can result in an open cluster.

This star formation begins enshrouded in the collapsing cloud, blocking the protostars from sight but allowing infrared observation.

In most cases these processes will strip the cluster of gas within ten million years, and no further star formation will take place.

At this point, the formation of an open cluster will depend on whether the newly formed stars are gravitationally bound to each other; otherwise an unbound stellar association will result.

Even when a cluster such as the Pleiades does form, it may hold on to only a third of the original stars, with the remainder becoming unbound once the gas is expelled.

The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in the morphological and kinematical structures of galaxies.

In the Large Magellanic Cloud, both Hodge 301 and R136 have formed from the gases of the Tarantula Nebula, while in our own galaxy, tracing back the motion through space of the Hyades and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.

The Trumpler scheme gives a cluster a three-part designation, with a Roman numeral from I-IV for little to very disparate, an Arabic numeral from 1 to 3 for the range in brightness of members (from small to large range), and p, m or r to indication whether the cluster is poor, medium or rich in stars.

[47] Once they have exhausted their supply of hydrogen through nuclear fusion, medium- to low-mass stars shed their outer layers to form a planetary nebula and evolve into white dwarfs.

One possible explanation for the lack of white dwarfs is that when a red giant expels its outer layers to become a planetary nebula, a slight asymmetry in the loss of material could give the star a 'kick' of a few kilometres per second, enough to eject it from the cluster.

These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars.

Tidal perturbations of large disks may result in the formation of massive planets and brown dwarfs, producing companions at distances of 100 AU or more from the host star.

[50] Clusters that have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but, over time, internal and external processes tend also to disperse them.

[55] This makes open clusters very useful in the study of stellar evolution, because when comparing one star with another, many of the variable parameters are fixed.

[57] Studies have shown that the abundances of these light elements are much lower than models of stellar evolution predict.

While the reason for this underabundance is not yet fully understood, one possibility is that convection in stellar interiors can 'overshoot' into regions where radiation is normally the dominant mode of energy transport.