Orion Nebula is most easily seen in the Northern Hemisphere during the winter months. Orion nebula is only three hundred thousand years old and has a temperature of 10,000 Kelvin.
Dark nebulae are the only nebulae dense and cool enough to allow stars to form.
Reflection nebulea reflects starlight off of dust, mixed in with the gas, of the nebula and appears blue. Once the protostars form into O and B stars, these stars emit UV light that ionizes the surrounding hydrogen and creates an H II region. H II regions are small, bright, and hot areas of ionized hydrogen. Orion nebula is one example of this type of emission nebula, that was formed after the O and B stars were formed. An emission nebula has the lowest density of all the nebulae.