Open cluster

An open cluster is a type of <a href="/facts/Star_cluster/Crs7Y6me">star cluster</a> made of tens to a few thousand <a href="/facts/Star/PFbd6ICx">stars</a> that were formed from the same <a href="/facts/Giant_molecular_cloud/5jEwCJxs">giant molecular cloud</a> and have roughly the same age. More than 1,100 open clusters have been discovered within the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> galaxy, and many more are thought to exist. Each one is loosely bound by mutual <a href="/facts/Gravity/bqAN7DdQ">gravitational attraction</a> and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the <a href="/facts/Galactic_Center/TI4MwWwM">Galactic Center</a>. 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 <a href="/facts/Globular_cluster/FL0EERJU">globular clusters</a> of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in <a href="/facts/Spiral_galaxy/hRT2dtxJ">spiral</a> and <a href="/facts/Irregular_galaxy/P95rRpYR">irregular galaxies</a>, in which active <a href="/facts/Star_formation/SMyfVDY6">star formation</a> is occurring.
Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an <a href="/facts/H_II_region/oDXMyTE1">H II region</a>. Over time, <a href="/facts/Radiation_pressure/xA6japY0">radiation pressure</a> 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 <a href="/facts/Stellar_evolution/Qa4vAGmn">stellar evolution</a>. Because the cluster members are of similar age and <a href="/facts/Chemical_element/0jducgWD">chemical composition</a>, their properties (such as distance, age, <a href="/facts/Metallicity/nhhPqbYF">metallicity</a>, <a href="/facts/Extinction_(astronomy)/rLNLDfe1">extinction</a>, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the <a href="/facts/Pleiades/tjwtx82z">Pleiades</a>, the <a href="/facts/Hyades_(star_cluster)/XmNqJhqA">Hyades</a> and the <a href="/facts/Alpha_Persei_Cluster/nxfprx0e">Alpha Persei Cluster</a>, are visible with the naked eye. Some others, such as the <a href="/facts/Double_Cluster/FX52hQjB">Double Cluster</a>, are barely perceptible without instruments, while many more can be seen using <a href="/facts/Binoculars/DPHVmKIL">binoculars</a> or <a href="/facts/Optical_telescope/dVfBR10I">telescopes</a>. The <a href="/facts/Wild_Duck_Cluster/LfPu5B8U">Wild Duck Cluster</a>, M11, is an example.

Open cluster open-in-new

Open cluster