Stars are often form in groups called star clusters.
There are two general types of star clusters - open clusters (also called
galactic clusters) and globular clusters.
Open Star Cluster
Open star clusters are groups of
stars which formed from the same huge cloud of gas and dust. They are usually
made up of groups of tens to hundreds of younger stars and are found in
and in the spiral arms of spiral galaxies.
A typical open cluster is about 20 parsecs
or less in diameter.
The stars in an open cluster will gradually move apart from each other with time.
It is estimated that there are 20,000 open clusters within our galaxy.
Globular Star Cluster
Globular clusters are large spherical groups of older stars bound together by
gravity. These clusters contain from ten thousand to one million stars and
are distributed in a spherical "halo" around the galaxy.
A typical globular cluster is 20 parsecs
or more in diameter.
There are about 200 globular clusters in our galaxy.
Many new star clusters are still partially hidden by the dust and gas leftover
from star formation, making them more difficult to view in visible light.
Infrared light, however, can penetrate this dust and provide us with
a deeper view of star clusters. The image to the right is an infrared
view of the Quintuplet Star Cluster, which lies near the center of our
Milky Way Galaxy. Since the center of our galaxy is a very dusty place,
infrared observations are often the best way to view objects embedded in
and hidden by this dust. This image, taken with the NICMOS instrument on the
Hubble Space Telescope, is the clearest view yet of this cluster.
Don Figer (STScI) et al., NASA
The Spitzer Space Telescope's
observations will focus primarily on open (or galactic)
clusters, gravitationally bound systems of thousands of young stars typically
found within the plane of our Galaxy. Clusters are thought to be a natural
location for discovering faint brown dwarfs.
Spitzer will conduct detailed
imaging studies of the Pleiades and Hyades clusters, in part to search for
previously unseen cluster members as small as ten Jupiter masses.