ISM AND STELLAR BIRTH

Gas Clouds are Called Nebulae

Stars are born in gas clouds, called nebulae. Nebulae (pronounced "Neb-you-lie") is the plural of nebula. There are threetypes:

• Dark - dense and cold, seen in silhouette
• Reflection - seen by light reflected off the gas; the light comes from nearby stars
• Emission - bright because the gas is heated and excited, emitted light is in emission lines from hydrogen

Interstellar reddening occurs because blue light scatters in the gas clouds and the red light passes through.

Star Formation

Stars usually form when a shock wave compresses the cloud. The shock wave can come from the death explosion of a nearby star. Stars formation takes time, but low mass stars take a very long time to form and massive stars form much more quickly.