The stars embedded in the nebula went unnoticed as a true cluster until 1931, when Per Collinder published his seminal paper, "On Structural Properties of Open Galactic Clusters
and their Spatial Distribution." Today the cluster carries the name Collinder 72 (Cr 72), but don't bother looking for it on any popular star atlas.
One additional physical quantity we know now is the total baryonic angular momentum of each galactic cluster
. This angular momentum value determines all the quantization states of the system in which the gas and the individual galaxies (i.e., particles) can occupy.
I like to star-hop to them from the sprawling galactic cluster
An open or galactic cluster
is one that has less than several thousand members and lives only in the disk; more populous ones are called globular clusters and are found in the disk, bulge, and halo.
But there's another way that galaxies such as C153, which are hurtling through a galactic cluster
, can be torn apart.
Ellis of Durham University in England and his colleagues weren't searching for a gravitational lens when they began analyzing a Hubble image, taken last November, of a remote galactic cluster
It is important to notice that the meticulous observations of galactic clusters
stand out among the most promising strategies for understanding dark matter.
For example, small differences in temperature across the sky show where parts of the universe were denser, eventually condensing into galaxies and galactic clusters
(Schekochihin and Cowley, 2008) have proposed a model for the fast growth of magnetic field in galactic clusters
using a self-accelerating dynamo.
Later chapters turn to the life-cycle of stars, the Milky Way galaxy, the varieties of galaxies beyond our own, and galactic clusters
. The last chapter reflects on "universal questions" and the search for life.
In a universe that never sits still, other galaxies orbit ours, and the Milky Way is on the move, too; our so-called Local Group of galaxies orbits its gravitational center, and also casts gravitational glances out to other galactic clusters
So some unseen mass must be at work, conventional wisdom declares, holding galactic clusters
together and governing the rotation rate of matter at the outer edges of individual galaxies.