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. Credit ESO.]]

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.]]

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.]]

Open clusters range from very sparse clusters with only a few members to large agglomerations containing thousands of stars. They usually consist of quite a distinct dense core, surrounded by a more diffuse 'corona' of cluster members. The core is typically about 3–4 light years across, with the corona extending to about 20 light years from the cluster center. Typical star densities in the center of a cluster are about 1.5 stars per cubic light year. For comparison, stellar density near the Sun is about 0.003 stars per cubic light year.

The frequency of binary star systems has been observed to be higher within open clusters than outside open clusters. This is seen as evidence that single stars get ejected from open clusters due to dynamical interactions.

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These blue stragglers are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. Instead, it is thought that most of them probably originate when dynamical interactions with other stars cause a binary system to coalesce into one star. For a typical cluster with 1,000 stars with a 0.5 parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. 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.

Because the stars in an open cluster are all at roughly the same distance from Earth, and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members are due only to their mass. RR Lyrae variables are too old to be associated with open clusters, and are instead found in globular clusters.

Planets

The stars in open clusters can host exoplanets, just like stars outside open clusters. For example, the open cluster NGC 6811 contains two known planetary systems, Kepler-66 and Kepler-67. Additionally, several hot Jupiters are known to exist in the Beehive Cluster.

See also

  • List of open clusters
  • Moving groups
  • Open cluster family
  • Open cluster remnant

References

Further reading

  • The Jewel Box (also known as NGC 4755 or Kappa Crucis Cluster) – open cluster in the Crux constellation @ SKY-MAP.ORG
  • Open Star Clusters @ SEDS Messier pages
  • A general overview of open clusters
  • The moving cluster method
  • Open Clusters – Information and amateur observations