An Etymological Dictionary of Astronomy and Astrophysics
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A group of hundreds to thousands of very young stars packed into an unbelievably small volume of a few parsecs in size. These objects represent the youngest stage of → massive star cluster evolution yet observed. The most massive and dense SSCs, with ages less than 10⁶ years, may be proto globular clusters. SSCs are thought to dissolve within 10 million years and merge into the field star population.

→ super; → star; → cluster.

1) An aggregation of clusters of galaxies (→ galaxy cluster). Superclusters are typically about one hundred million (10⁸) → light-years in diameter and contain tens of thousands of galaxies. Some examples are the → Local Supercluster, → Centaurus supercluster, → Laniakea supercluster, → Perseus-Pisces supercluster→ Shapley supercluster→ Virgo supercluster.
2) For stellar aggregations, → dynamical stream.

→ super-; → cluster

Grouping of galaxies in supercluster structure.

→ super-; → clustering

A cluster of stars in which members are closely situated so that high resolution observations are required to distinguish them individually.

→ tight; → star cluster.

A star cluster located in the center of the → Orion Nebula whose four brightest stars form the vertices of a trapezoid. They are known as θ¹ Ori A, B, C, and D and are of magnitude 6.7, 8.0, 5.1, and 6.7 respectively. They are packed in an area 22 arcseconds across (10,000 A.U.). The Trapezium stars are responsible for the illumination of the entire Orion Nebula. The principal and the most massive star of the group is θ¹Ori C, a young main sequence → O star of type O6. The three others are → B stars. Almost all of the Trapezium stars are multiple: the most massive star θ¹Ori C is double, the next massive star θ¹Ori A is triple, θ¹Ori B is at least quadruple, and θ¹Ori D is apparently single.

→ trapezium; → cluster.

A cluster of objects, such as stars or galaxies, in which the members are not tied together gravitationally. In such a cluster the members scatter through space following different directions. → bound cluster.

→ unbound; → cluster.

A moving star cluster containing over 100 stars that are scattered over an area of sky more than 1,000 minutes of arc in diameter. The five brightest stars of the → Big Dipper as well as → Sirius belong to this → dynamical stream.

→ Ursa Major; → star; → cluster.

The largest and nearest galaxy cluster to the Local Group. The Virgo cluster spans 120 square degrees on the sky and contains on the order of 2,000 galaxies. It is located at a distance of about 60 million light-years. It is an irregular cluster with no central concentration. The giant elliptical galaxy M87 is the most massive in the cluster.

→ Virgo; → cluster.

The irregular supercluster that contains the Virgo cluster and the Local Group. At least 100 galaxy groups and clusters are located within its diameter of 110 million light-years. It is one of millions of superclusters in the observable Universe.

→ Virgo; → supercluster.