Galaxy Clusters
Given a group of galaxies which are gravitationally interacting, the zero-velocity surface is used to determine which objects are gravitationally bound (i.e. not overcome by the Hubble expansion) and thus part of a galaxy cluster, such as the Local Group.
Read more about this topic: Zero-velocity Surface
Famous quotes containing the words galaxy and/or clusters:
“for it is not so much to know the self
as to know it as it is known
by galaxy and cedar cone,
as if birth had never found it
and death could never end it:”
—Archie Randolph Ammons (b. 1926)
“Where we such clusters had,
As made us nobly wild, not mad;
And yet each verse of thine
Outdid the meat, outdid the frolic wine.”
—Robert Herrick (1591–1674)