STAR CLUSTERS: Star clusters are groups of stars that are physically associated with each other. Stars are gaseous bodies that emit radiation generated by nuclear fusion. Stars are primarily composed of hydrogen and helium. They are balanced by the dynamics of their own intense heat and explosive nuclear reactions and the gravetational forces that keep them contained.
Star clusters are generally described by their associative attributes and fall into the categories open clusters and globular clusters.
Open Clusters: Are loose irregularly shaped star clusters that usually contain from a few thousand to tens of thousands of members. The stars not only share proximity in space, but freqently also a common origen. The individual stars sometimes shift in relation to each other within the group, but more frequently travel through space together in relative formation. They tend to be considerably smaller than their cousins the globular cluster, ranging from a few to about 10 light years. Good examples of well known and easily viewed open clusters are The Pleiades (M45), NGC 869 & 864 (The Double Cluster) and M35.
Globular Clusters: Much like their name implies, are roughly spherical and relatively compact clusters of stars that are gravetationally bound. They frequently contain several thousand and up to several million stars. Some extend over 300 light years. Our own Milky Way galaxy has about 150 globular clusters. These are very old congregations of stars that have been dated back as far as 13 billion years. In general the stars contained within them are old Population II stars that are "poor" in heavy elements. Some of the best examples of globular clusters are NGC 5139 "Omega Centauri" and M13, M3 and M4.
COMETS: Comets are essentially dirty cosmic snowballs. They are made up of frozen gas and dust. It is generally believed that comets are infant planets (planetesimals) formed about 4.6 billion years ago with our solar system, but never quite grew up due to a lack of mass.
The word Comet comes from the Greek kometes translated as "long-haired star", which is an apt description of their appearance.
The core (nucleus) of a comet is usually only a few kilometers in diameter. As the comet approaches our sun, the solar winds dissolve the gasses, which form a coma around the nucleus and many times a tail (or tails). The visible tails are usually a straight ion tail, and a more curved dust and gas tail.
Many comets are periodic, which means they revisit us with a parabolic sweep around the sun on a regular, albeit sometimes very prolonged schedule.