Nebulae: a cloud of dust and gas that we see in light
- Emission Nebulae or Bright Nebulae: a glowing gas (hydrogen); e.g. Great Nebula in Orion, heated by the Trapezium
- Absorption Nebulae or Dark Nebulae: dark dust clouds; e.g. Horsehead Nebula
- Reflecting Nebulae: reflecting dust cloud; e.g. Pleiades in Taurus
- Planetary Nebulae: excited by central star; e.g. Dumbbell Nebula
Stars form in Giant Molecular Clouds about 100,000 to 1 million solar masses. A few thousand in the Milky Way Galaxy, Giant Molecular Clouds break into denser bits, contract, and eventually form stars. The Orion Molecular Cloud has about 500 stars. The Trapezium and the Orion Nebula have solar masses of matter with young stars.
- Non-stellar galactic objects reside in HII regions with molecular clouds of pre-main sequence stars and dense clumps of dust.
- Protostars and newborn stars about 1/2 to 1 solar mass reside in Molecular Clouds.