Nebulae

A nebula is a huge cloud of gas and dust. The early Universe was a cloud of particles from which gases formed. The gas, mostly hydrogen, clumped together into nebulae from which all stars formed.

One can classify the different types of nebula by the nature of the light they emit.

Emission Nebulae
These emit light producing emission spectra. The radiation from stars within the nebula excite the gas atoms causing them to emit light.

Large nebulae may contain regions where star formation is occurring. The young stars are often very massive and hot producing lots of radiation. These are often red in colour due to emission by the large amounts of hydrogen they contain.

Reflection Nebulae
Dust particles within the nebula reflect the light of nearby stars. These are often blue in colour as blue light is scattered and reflected more than red.

Absorption or Dark Nebulae
They absorb the light of background stars and produce absorption spectra. They appear dark in colour.

Nebulae may be the result of emission, in the form of a gaseous shell, from a dying star. These are known as planetary nebulae (although they have nothing to do with planets).

Stars with about the same mass as our sun end their lives this way.

Radiation from the central star may excite the gases in this shell causing them to emit radiation themselves.

The Eagle nebula

The Pleiades

The Horse Head nebula

The Cat's Eye nebula

all pics courtesy of NASA

Nebulae may also be the result of a supernova explosion. These types of nebulae often have a distinctive shape.

For many many excellent high resolution pictures and more information about nebulae
look at http://www.seds.org/messier/nebula.html and http://antwrp.gsfc.nasa.gov/apod/lib/aptree.html