The early universe, about 13 billion years ago, consisted of a nearly homogeneous mass of hydrogen gas with a tiny amount of helium. Something, possibly the helium, caused density variations to occur. The denser areas shrunk by self-gravitation which increased their gravitational pull on neighboring, less dense, material. These early mass concentrations MassCons continued to differentiate internally into the earliest galactic clouds.
Eventually smaller MassCons within the galactic clouds contracted sufficiently to raise their core temperature and pressure high enough for hydrogen to begin to fuse into helium creating the first Population I stars. Many of these stars were very large, had short lifetimes and became supernovae which seeded the galactic clouds with heavier elements. Shock waves from supernova blasts and the presence of heavier elements increased the rate of star formation.
As gravity contracts a MassCon, the entire object spins-up like a like a spinning skater when they pull in their arms. The faster a fluid body spins and the greater its mass, the more oblate (egg shaped) it becomes. Jupiter is a huge rapidly spinning gas ball and its equatorial diameter is larger than its polar diameter. This explanation is intuitive; a rigorous one involves complex mathematics.
The early universe was expanding rapidly so all MassCons acquired considerable angular momentum (definition below). The larger ones would have condensed into rotating disks while smaller ones would have assumed a more spherical shape. It is conjectured that galaxies evolved from the large disks and their gravitation captured smaller nearby MassCons which have evolved into globular star clusters. Observations of these clusters indicates that they contain predominantly Population I stars which supports an early genesis.
Within galactic disks, stars formed from smaller MassCons. The host galaxy's superior gravitation disrupted formation of independent stellar arrays. Instead these arrays acquired a common orbital velocity around the galactic core but no motion around a local center. These arrays are called open star clusters or stellar associations if the stars are widely dispersed. The requirement for membership is genesis from a common MassCon which can be ascertained by analysing a candidate's age and composition.
Galaxies are also clustered due to the same gravitational forces affecting stars but on a larger scale. The local group is an association of nearby galaxies and is itself a member of the Virgo Supercluster of galaxies.
M80 in Scorpius
M45 "Pleiades" in Taurus
Images copied from http://www.wikipedia.com
Consider a distant object and draw a line from any point in space to the object. Determine the object's velocity perpendicular to the line and multiply the velocity by the object's mass. Multiply the result by the distance along the line to the object. The result is the object's angular momentum about the selected point.
For any fluid MassCon there exists a point within where the angular momentum of every consitituent part sums to zero. Simply stated, a MassCon has a center-of-gravity about which all parts orbit.