The name Open Clusters is used because the stars are placed relatively far away from each other in comparison with the stars in a Globular Cluster. More than 1,000 such clusters have been observed, each containing from ten to one thousand stars. They are concentrated in the spiral arms, and are thus also called Galactic Clusters.
Their stars are young, surrounded by nebulosity
Pleiades : Seven Sisters, although usually only six main stars are observed with the naked eye.
Jewel Box : in the Southern Cross, gets its name because its stars flash with many colors.
Hyades : Prominent as a V-shape around, but not including, Aldebaran. The cluster actually consists of about 350 stars with half the mas concentrated in a sphere of about 12 parsecs diameter. The Hyades has no remaing nebulosity indicating an old cluster.
Praesepe (the Beehive) : in Cancer
Double Cluster, twin clusters in the sword of Pegasus. These are 7000 light years away, meaning that constituent stars are very bright intrinsically.
Spherical cluster of many stars (hundreds of thousands of stars). Towards the center, the stars seem to merge into a single glowing mass.
A hundred and fifty or so globulars have been detected orbiting independently in a spherical region around the halo, most close to the Bulge.
Since the center of the Galaxy is in Sagittarius, this is the general region of the sky where they are seen.
Their stars are predominantly older Population II stars. There is little gas and dust between the stars
Their distances are determined with the help of RR Lyrae stars. Their distribution was an important factor brought to bear in the 'Great Debate'.
M13 in Hercules is the only one visible with the naked eye from Britain.
Omega Centauri So bright that Johann Bayer gave it a stellar designation. Oval shape rather than the usual spherical shape.
47 Tucanae. Magnitude 4.5 Adjacent to the Small Magellanic Cloud.
HII regions (sometimes called emission nebulae) are regions of starbirth. Usually they are composed of a gas cloud surrounding new stars in the process of 'switching on'. The clouds absorb radiation from these stars and re-emit the energy in its own characteristic spectrum. Occasionally, as in the Orion Nebula, the stars themselves can actually be seen - these stars in the Orion Nebula are known as the Trapezium.
ExamplesOrion Nebula (M42 or NGC 1976) - diameter of 25 light years. In fact, it is a bright blister on the side of a larger object called the Orion Molecular Cloud.
A dense cloud (molecular cloud) which blocks out starlight.
ExamplesCoal Sack in Southern Cross
They reflect light from nearby stars without becoming hot enough to actually glow itself. They appear bluish because the dust reflects the blue better than the red. The Triffid Nebula is a good example.