Open cluster

An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. 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. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. 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. Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away. Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the Pleiades, the Hyades and the Alpha Persei Cluster, are visible with the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example.

Historical observations

The prominent open cluster the Pleiades, in the constellation Taurus, has been recognized as a group of stars since antiquity, while the Hyades (which also form part of Taurus) is one of the oldest open clusters. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light. In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe Cluster, the Double Cluster in Perseus, the Coma Star Cluster and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster. However, it would require the invention of the telescope to resolve these "nebulae" into their constituent stars. Indeed, in 1603 Johann Bayer gave three of these clusters designations as if they were single stars.

The first person to use a telescope to observe the night sky and record his observations was the Italian scientist Galileo Galilei in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where previously observers had noted only 6–7 stars in the Pleiades, he found almost 50. In his 1610 treatise Sidereus Nuncius, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters." Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became possibly the first astronomer to use a telescope to find previously undiscovered open clusters. In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451. It was realized as early as 1767 that the stars in a cluster were physically related, when English naturalist Reverend John Michell calculated that the probability of even just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000. Between 1774 and 1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets. This catalogue included 26 open clusters. In the 1790s, English astronomer William Herschel began an extensive study of nebulous celestial objects. He discovered that many of these features could be resolved into groupings of individual stars. Herschel conceived the idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction. He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.

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