The Interstellar Medium & Nebulae
Nebulae are simply clouds of interstellar gas and dust and appear either as dark regions blotting out background stars - the so-called dark or absorption nebulae or as brighter clouds of gas that emit or reflect light. They are the most visible components of the interstellar medium.
The Interstellar Medium
Despite what you might think, space is not a perfect vacuum. The space between the stars is filled with a tenuous range of material that provides the building blocks of stars. This material is gas and dust and collectively is known as the interstellar medium (ISM). The ISM gas is predominantly hydrogen whilst the dust is about 1% by mass and includes carbon compounds and silicates. Dust is responsible for the interstellar reddening and extinction of starlight. The more of the ISM a star's light travels through on its way to an observer on Earth the more it gets scattered and absorbed, decreasing the star's apparent brightness and reddening its appearance.
Properties of the ISM vary widely depending upon its location within a galaxy. At its most tenuous, in hot regions between denser clouds, it may have a density of only 100 particles per cubic metre, mostly ionised hydrogen atoms. In the inner regions of shells of gas surrounding stars the density can be as high as 1017 particles per m3 although this is still a million times less dense than a normal vacuum on Earth.