what is a nebula?

Answer 1

http://en.wikipedia.org/wiki/H_II_region

http://en.wikipedia.org/wiki/Emission_nebula
http://en.wikipedia.org/wiki/Reflection_nebula
http://en.wikipedia.org/wiki/Dark_nebula

a nebula is an interstellar cloud of gas and/or plasma and dust. there are three kinds: emission nebulae, reflection nebulae, and dark nebulae. the three kinds often occur within the same cloud complex.
http://antwrp.gsfc.nasa.gov/apod/image/0702/ngc2170_croman_big.jpg
http://imgsrc.hubblesite.org/hu/db/2006/01/images/a/formats/xlarge_web.jpg

emission nebulae are clouds that fluoresce. they are frequently large regions of star formation, but they can also be small regions that are the remnants of normal stars. the more massive stars form more quickly and are hotter. because they are hotter, they emit most of their radiation at higher frequencies, such as ultra-violet, so they appear blue or blue-white. this high-energy light excites or ionizes atoms and molecules in the cloud when they absorb specific frequencies of light. later, the atoms and molecules emit light when the electrons fall to lower energy shells or recombine to form a neutral atom or molecule again. the energy of the is equal to the energy lost by the electron. each atom and molecule has its own set of emission frequencies so astronomers can identify the composition of the cloud spectroscopically.

reflection nebulae are clouds that do not fluoresce. they only reflect the light of nearby stars.
http://antwrp.gsfc.nasa.gov/apod/image/0601/pleiades_gendler_big.jpg

dark nebulae are clouds that do not fluoresce or reflect light. they only obscure the light of more distant sources.
http://en.wikipedia.org/wiki/Image:ENebula.jpg
http://en.wikipedia.org/wiki/Image:Bok_globules_in_IC2944.jpg

Answer 2

The word Nebula means mist.A nebula is a giant cloud of gas and dust in the sapce, of which many still exist in the universe. Examples --
The Rosette Nebula
Horse Head Nebula
Ring Nebula
The Great Orion Nebula

Answer 3

A Nebula is the gas left over after a star blows up in the solar system.

Answer 4

A nebula is an interstellar cloud of dust, gas and plasma.

Answer 5

A nebula (Latin: "mist"; pl. nebulae or nebulæ, with ligature; from Greek nephele, "cloud") is an interstellar cloud of dust, gas and plasma.

Answer 6

Per big Bang theory it would be the dusty remains of the big explosion which is still a visible cloud in space.