### aggregation | cluster:

Galaxy cluster
* are essentially stable following initial star formation, says a Canadian researcher
  - great labs for cosmology
  - powerful tools for studying the evolution of our universe
  - simply large numbers of galaxies that are all orbiting around one another
  - strong sources of neutrinos
* are the largest gravitationally bound groupings in the universe
  - most massive gravitationally bound objects in the universe
  - traditionally strong X-ray emitters
* attract each other to produce superclusters of tens to hundreds of clusters.
* can also be gravitational lenses.
* represent the most classical target for multiobject spectroscopy.

### aggregation | cluster:

Globular cluster
* Many globular clusters are strong X-ray sources, perhaps from many neutron stars
  - resolve into myriad stars
* appear to contain the oldest stars in the universe.
* are also present, so they contain both old and young stars
  - unique because they contain thousands and sometimes millions of stars
* are among the oldest components of our galaxy
  - objects known
  - astronomical objects
  - bright and obvious, although they plainly suffer from lack of resolution
  - collections of hundreds of thousands of old stars
  - densely packed collections of ancient stars
  - extremely luminous objects
  - gold starbursts
  - gravitationally unstable
* are huge balls of stars which usually contain hundreds of thousands of stars
  - collections of up to millions of stars, packed into dense balls
  - ideal laboratories for studying the formation and evolution of stars
  - leftover relics of the early days of the formation of the Milky Way
  - located near galaxy
  - massive assemblies of stars
* are much older, more compact, objects existing on the edges of our galaxy
  - smaller and much less massive than galaxies
  - old star groups that formed early in our galaxy's life, long before the Earth
  - older aggregates of stars, formed early in the history of galaxies
* are spherical gatherings of tens to hundreds of thousands of stars
  - swarms of hundreds of thousands of old stars
  - star clusters
  - stars grouped together in the shape of a ball
* are the old metal poor clusters that are found throughout the halo of the galaxy
  - oldest easily identifiable remnants from the early evolution of galaxies
  - oldest, densest, and most populous star clusters in our galaxy
  - thought to contain some of the oldest stars in our galaxy
  - tight groups of hundreds and thousands of very old stars
  - too far away to measure their distances by parallax
* are very old and their stars seem to have been formed at about the same time
  - clusters, almost as old as our galaxy
  - well known as the oldest conglomerations of stars in the Milky Way
* are, lets face it, big globs of stars.
* can contain up to more than a million stars
  - have hundreds of stars, open clusters at most a few
* consist of an extremely tightly packed group of stars in a spherical ball.
* contain almost no gas or dust and show few signs of recent star formation.
* contain hundreds of thousands, and sometimes millions of stars
  - or even millions of stars
  - no dust, no gas, and no massive main sequence stars
  - only old stars
  - stars which are very old
* contain thousands of stars and sometimes hundreds of thousands of stars
  - stars, maybe up to a million
* encircle spiral galaxies.
* get smaller by removing stars.
* have no young stars and very little dust.
* orbit most galaxies, including our Milky Way.
* orbit the center of the Milky Way in very elliptical orbits
* show little evidence of dark matter.
* start to pull in to center of galaxy.
* surround the main galaxy in the form of a halo.
+ Star cluster
  * Globular clusters are tight groups of hundreds and thousands of very old stars. Open clusters are more loose, and they have less than a few hundred stars, which are often very young. A famous star cluster is the Pleiades, which is an open cluster.