Open clusters occur in the flat disk of our own Milky Way Galaxy, and are
far more numerous than globular clusters. About 1,000 open clusters are
visible through telescopes and there may be as many as 18,000 within our own
galaxy. One of the nearest star clusters, the Hyades (in the constellation
Taurus) is plainly visible to the unaided eye on a clear winter night. Other
relatively close open star clusters, such as the Pleiades (also in Taurus)
and the Praesepe (in Cancer) are just visible to the naked eye as fuzzy
patches but become beautiful sights in binoculars. More distant open clusters
are fine sights though small and medium size telescopes. The average open
cluster ranges between 10 or 20 light years in diameter.
Although globular clusters are highly visible through small and medium
size telescopes, a larger aperture is usually needed to resolve their stars.
The two largest globulars are Omega Centauri and 47 Tucanae, both too far
south to be seen from mid-northern latitudes. The brightest globular visible
in the northern hemisphere is M13, the Great Hercules Cluster.
The distribution of galaxies in space is not uniform, rather, they tend to cluster into groups of galaxies. Galaxies are extremely distant objects, even the nearest large galaxy, M31, the Andromeda Galaxy, is 2.2 million light years from us, hence, they appear faint and small and relatively few of them offer much visual detail even in larger telescopes. In the eyepiece most galaxies appear merely as a stellar nucleus within a tiny, low-surface-brightness, ambiguously-edged halo. This halo is the combined light of billions of stars so far away that they can not be resolved. Even so, knowing what they are and how far away they are presents a mindboggling scenario.
Galaxies take on different shapes:
Elliptical Galaxies are spheroidal in shape and display no spiral pattern of other structure. There is usually a brighter central concentration thinning outward. They are classed from E0 to E7 with E0 being perfectly round to E7 the most elongated.
Lenticular Galaxies (S0 or SB0)) are a sort of transitional type resembling ellipticals showing a flattened central plane or disc but no spiral pattern.
Spiral Galaxies are beautiful objects containing spiral arms surrounding a central bulge or hub. Their main structural classifications are Sa, Sb, Sc. Sa galaxies have tightly wound spiral arms, Sb have moderately open spiral arms, and Sc have the more defined or open spiral pattern. Transitional types are indicated as Sab, Sbc, etc. When a central bar surrounding the nucleus is present a capital letter "B" is added, such as: SBa, SBb, SBc.
Irregular Galaxies (I or Irr) are amorphous intergalactic patches of gas and
dust within which stars are presently being formed, these objects lack
symmetry or smooth distrbution.
Dark nebulae are simply cold clouds of interstellar dust silhouetted against bright nebulae or against the Milky Way itself. They contain hydrogen and light-absorbing grains of graphites, silicates, ices, and possibly metals.
Bright nebulae are divided into four types based upon the process by which they shine: reflection, emission, planetary nebulae, and supernova remnants.
Reflection nebulae are bright nebulae that shine by light reflected from stars within them or nearby. They contain basically the same matter as dark nebulae but usually exhibit the same spectrum as that of the illuminating star. Because blue light is scattered more than red light, the reflection nebula appears bluer than the star. The stars in M45, The Pleiades Star Cluster, display a fine example of reflection nebulae.
Emission Nebulae are interstellar clouds of ionized gas glowing by what amounts to simple fluorescence. These nebulae are always near or around hot O and early-B stars because it is the ultraviloet radiation from these stars that ionizes the atoms in the gas. Regions of this kind are known as H-II Regions, an outstanding example is M42, the Great Nebula in Orion.
Planetary Nebulae are the remaining gaseous shells ejected from nova (dying stars). Their resemblance to planets (especially Uranus) when viewed through a small telescope was noted by Sir William Herschel who gave them this term in 1785. These objects are the death shrouds of red giant stars near the end stage of stellar evolution. As the star ages and becomes unstable the mechanism of atmospheric pulsations in the star's outer layers reaches escape velocity. The multiple shells or rings of most planetaries suggest that the ejection of the red giant's entire envelope usually requires several episodes. Having ejected sufficent material the remaining central star, no longer generating any energy by nucleosynthesis, shrinks to become a white dwarf. Well know examples of planetary nebulae are the "Ring" (M57) in Lyra and the "Dumbbell" (M27) in Vulpecula.
Supernova Remnants (SNRs) are the expanding debris clouds of supernova explosions from massive stars that eject most of their mass at very high volocites. The energy released from a supernova is a million times that of a Nova. The explosion can become as bright as an entire galaxy for a few days. After the explosion the star is either totally destroyed, or its central core collapses into either a very dense Neutron Star or a Black Hole. The most famous supernova remnant is the Crab Nebula (M1) in Taurus, seen in 1054 A.D. by Chinese astronomers.