A star cluster is a group of stars held together by their mutual gravitational attraction Although stars may appear to be randomly distributed throughout the night sky, they are actually organized into distinct groupings referred to as star clusters. There are several types of star clusters, including open clusters, globular clusters, and stellar associations.
Open Clusters
Open clusters are some of the most easily observable star clusters in the night sky. They are relatively young, having formed relatively recently in galactic terms. These clusters usually contain anywhere between a few dozen to a few thousand stars and are generally gravitationally unbound, meaning that the individual stars within will eventually drift apart. Open clusters are typically composed of stars that are similar in age, composition, and motion. Examples of open clusters include the Pleiades and Hyades in the constellation Taurus, the Double Cluster in Perseus, and the Beehive Cluster in Cancer.
Globular Clusters
Globular clusters are the most populous type of star clusters and can contain millions of stars within an extremely dense core. They are typically very old, having formed several billion years ago and contain stars that are ancient and metal-poor. Globular clusters are highly gravitationally bound and exhibit an almost spherical formation, which is why they are referred to as “globular”. Examples of globular clusters include Omega Centauri, M13, and Messier 3.
Stellar Associations
Stellar associations are large groupings of young, massive stars that are gravitationally unbound and often located in or near molecular clouds. These clusters tend to be scattered and contain anywhere from a few tens to a few thousands stars. Stellar associations are thought to form when a molecular cloud is compressed enough to form stars, with the stars then gravitationally interacting with one another and forming small associations. Examples of stellar associations include the Scorpius-Centaurus OB association and the Orion OB1 association.
Five Examples of Star Clusters
1. Pleiades: Also known as the “Seven Sisters”, the Pleiades is one of the most recognizable star clusters in the night sky. It is an open cluster located in the constellation Taurus and contains around 500 stars.
2. Orion Nebula Cluster: The Orion Nebula Cluster is a large open cluster located in the Orion molecular cloud complex. It contains around 3,000 stars, the majority of which are low-mass stars.
3. Omega Centauri: Omega Centauri is the largest and brightest globular cluster in the night sky and is located in the constellation Centaurus. It contains up to 10 million stars.
4. Messier 7: Messier 7 is an open cluster located in the constellation Scorpius. It is one of the closest open clusters to Earth and contains around 80 stars.
5. M80: M80 is a globular cluster located in the constellation Scorpius. It contains over 100,000 stars, making it one of the most densely populated globular clusters in the Milky Way.