In an effort to help out the non-astronomer viewing these images, we hope that the following tidbits of information will help to provide some basics of understanding the content of the images shown in our various web pages and give at least an introductory answer to the question: What are these things?
Diffuse nebulae are clouds of gas ( mostly hydrogen ) which are distributed throughout our galaxy. They may be broadly classed into two types: Bright and Dark nebulae.
Bright nebulae are clouds that are being illuminated or excited into emission of light by nearby stars. Generally, one can determine the type of anebula from its color when one see a color image. Blue nebulae are generally those that are being illuminated by nearby stars while red nebulae are those that are being stimulated by intense energetic photons from nearby stars into a state of emission and the reddish color is generally that of excited Hydrogen ( known as H alpha emission at 656.3nm ). Examples of well known bright nebulae: M8 in Sagittarius, M42 in Orion
Dark nebulae are seen as dark obscuring clouds of gas that block the light from the stars or nebulae lying behind the dark gas cloud. Examples of dark nebulae: Horsehead in Orion, Snake Nebula in Ophiuchus
In many cases these clouds of gas are the birth places of stars.
Planetary nebulae are generally roundish clouds of gas and they are the remnants of stars that have either exploded in a grand catastrophic ending or that have thrown off outer layers of gas in less cataclysmic explosions. These nebulae represent the death cycle of stars that are somewhat larger than our own sun. In color images of these nebulae, one sees the red of H-alpha, the blue of reflection, and a blue green color that is characteristic of the light emitted from excited Oxygen (OIII at 500.7 nm). All planetary nebulae that we can view are located within our own galaxy. Examples of Planetary Nebulae: M57 in Lyra , NGC 7293 in Capricornus
Open clusters are loose associations of stars that are generally gravitationally bound into the cluster. Open Clusters may contain anywhere from a few stars to several thousands of stars. They are distributed within our galaxy. Examples of Open Clusters: The Double Cluster in Perseus, M35 in Gemini
Globular Clusters are dense clusters of tens of thousands of stars into a nearly spherical cluster. Globular Clusters seem to lie in a large shperical distribution just outside the main body of our own galaxy. Examples of GLobular Clusters: M13 in Hercules, M92
Galaxies are gravitationally bound groupings of stars that may contain something on the order of 100 to 200 million stars. These are the only objects shown here that lie beyond the limits of our own galaxy. They exist in a variety of shapes ranging from spherical to spiral to irregular. The dark bands seen in some galaxies are actually clouds of obscuring gas. Examples of Galaxies: M51 in Canes Venatici, M101 in Ursa Major, NGC 2903 in Leo