Astronomic Nebulae

Why are images of astronomical nebulae mostly colored?

How about the stars?

The Hubble color palette

Types of astronomical nebulae

Link to Wikipedia: Bayer Matrix / Bayer Filter
Link to Hubble Space Telescope site
Link to Wikipedia: Emission Nebula
Link to Wikipedia: Reflection Nebula
Link to Wikipedia: Dark Nebula
Link to Wikipedia: Molecular Cloud
Link to List of planetary nebulae
Link to esa site: Planetary Nebulae.