Nebula

The interstellar medium manifests itself to the astronomer in various phenomena. Many of these are amorphous, diffuse structures known as Nebula. This comes from the Latin word for "cloud." Its plural is nebulae.

A beautiful example of a nebula is an emission nebula. Emission occurs when a cloud of gas is warmed up by some source of continuum radiation, say from a nearby star. The various atomic energy levels are excited by this radiation, and as the electrons jump back down to their lower energy states they emit photons at distinct spectral wavelengths. Since hydrogen gas is the most common form of gas, hydrogen emission lines are most often observed. In particular, a specific transition in the hydrogen atom corresponds to red light and color pictures of these emission regions appear red. This is the so called H-alpha emission line of hydrogen.

Figure: The Trifid Neubla (M20). This nebula exhibits many of the features of nebulae: the red light comes from hydrogen Balmer emission, excited by the ultraviolet radiation from a hot star embedded in the nebula. The dark regions criss crossing the nebula are obscuring dust. The blue region to the left of the main body is a "reflection nebula" so named because it consists of dust that is scattering starlight into our line of sight. Dust scatters short wavelength (blue) light preferentially, hence the blue appearance.

Before their true nature was understood, distant galaxies were called nebula, e.g. the great "spiral nebula" in Andromeda.