What is an Open Cluster?

I have mentioned open clusters several times when discussing certain Messier objects.  An open cluster is a loose distribution of stars that are bound together gravitationally.  Unlike globular clusters they are contained within the disk of the galaxy and not in the halo.  Open clusters typically contain less stars than a globular cluster and are considered relatively new.  Stars in an open cluster are formed from the same molecular cloud of gas.  Some of these clusters have shed their nebula through the stars' combined radiation pressure dispelling the gas from their gravitational hold.  An obvious example of an open cluster that still has a cloud of gas is the Trapezium in the heart of the Orion Nebula.

Messier 42 - Orion Nebula

The interesting thing about open clusters is they typically contain stars of approximately the same age.  Some stars are ejected from the cluster by interacting with other matter as they orbit the center of the galaxy.  An example of  cluster with thousands of stars is shown below in an image I took back in November.  This is the Wild Duck Cluster which is supposed to resemble a flock of flying ducks.  Honestly, I don't quite see that.  Any cluster could be construed to resemble a flock of birds.

Messier 11 - Wild Duck Cluster

An example of an open cluster with very few stars is shown below.  It is Messier 39, located in the rich constellation of Cygnus.  It has around 30 stars.

Messier 39

The trick to observing open clusters is to use a telescope with a relatively low focal length using a low power eyepiece.  Open clusters tend to be quite expansive as viewed from Earth and therefore lose their beauty when viewed under high magnification.  The Pleiades cluster expands nearly 2° across the sky and could not be viewed in its entirety under high powers.  The Hyades stretches nearly 6° and your best bet is probably just your own eye.  Binoculars are a wonderful instrument that can be used to view them.  .  I would recommend a moderate sized refractor with a wide field eyepiece.  In order to see the nebulosity associated with open clusters you are best off using a telescope to take a long exposure photograph.  Narrow band emission line filters sometimes help if it is an emission nebula but if it is a reflection nebula, a basic light pollution filter might be your best bet.

Messier 45 - Pleiades 

If it is clear, use Orion to fine the red eye of Taurus the bull to see the naked eye Hyades open cluster.  Above the Hyades you should be able to see the Pleiades.  See the viewing chart below!

Hyades and Pleiades Star Chart