A NEBULA IS A CLOUD OF DUST AND GAS inside a galaxy. Nebulae becomes visible if the gas glows, or if the cloud reflects starlight or obscures light from more distant objects. Emission nebule shine because their gas emits light when it is stimulated by radiation or around the nebula. Dark nebulae shine because of their dust reflects light from the stars in or around the nebula. Dark nebulae appear as silhouettes because they block out light from shining nebulae and or stars behind them. Two types of nebula are associated with dying stars
Planetary Nebulae
Supernova Remnant
Planetary nebulae and Supernova remnants. Both consists of Expanding shells of gas that were once the outer layers of a star. A Planetary nebula is a gas shell drifting away from a dying stellar core. A supernova remnant is a gass shell moving away from a stellar core at a great speed following a violent explosion called a supernova. Stars are often found in groups known as a clusters. Open clusters are loose groups of a few thousand young stars that were born in the same cloud and are drifting apart. Globular clusters are densely packed, roughly spherical groups of hundreds of thousands of older stars.
