, an
open cluster dominated by hot
blue stars surrounded by
reflection nebulosity Open clusters are confined to the
galactic plane, and are almost always found within
spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as
Messier 67 (the closest and most observed old open cluster) for example. They form in
H II regions such as the
Orion Nebula. Open clusters typically have a few hundred members and are located in an area up to 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of
giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation". The most prominent open clusters are the
Pleiades and
Hyades in
Taurus. The
Double Cluster of
h+
Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die. A subset of open clusters constitute a binary or aggregate cluster. New research indicates
Messier 25 may constitute a ternary star cluster together with NGC 6716 and Collinder 394. Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by
Cepheids variable stars, which are then used as
standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe (
Hubble constant). Indeed, the open cluster NGC 7790 hosts three
classical Cepheids which are critical for such efforts.
Embedded cluster Trapezium cluster seen in
X-rays which penetrate the surrounding clouds Embedded clusters are groups of very young stars that are partially or fully encased in
interstellar dust or gas which is often impervious to optical observations. Embedded clusters form in
molecular clouds, when the clouds begin to collapse and
form stars. There is often ongoing
star formation in these clusters, so embedded clusters may be home to various types of
young stellar objects including
protostars and
pre-main-sequence stars. An example of an embedded cluster is the
Trapezium Cluster in the
Orion Nebula. In
ρ Ophiuchi cloud (L1688) core region there is an embedded cluster. The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through
radiation pressure,
stellar winds and
outflows, or
supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in particularly dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered, often leading to the disruption of a star cluster. Most young embedded clusters disperse shortly after the end of star formation. The open clusters found in the Galaxy are former embedded clusters that were able to survive early cluster evolution. However, nearly all freely floating stars, including the
Sun, were originally born into embedded clusters that disintegrated. ==Globular cluster==