ATOMIC HYDROGEN

Clouds of hydrogen surrounding young, hot stars appear as EMISSION NEBULAE (or "H II regions"). The energetic ultraviolet light from nearby stars strips electrons off the atoms ("ionization"), and when the electrons eventually recombine with the protons, they emit photons of special colors (especially the red, H-alpha transition) that produce the optical light. Some examples include:

• Orion Nebula
• Eagle Nebula
• Lagoon Nebula

Clouds of hydrogen also emit 21 centimeter radio-frequency radiation (through a spin flip transition). Radio maps of 21 cm provide a means to map hydrogen throughout the disk of the Milky Way.

DUST

Dust comes in the form of microscopic particles made of silicon, carbon, and oxygen.
Heated dust emits infrared light.
Clouds of dust can block the optical light of background stars ("dark nebula").
Dust surrounding hot stars can scatter their radiation in our direction (to produce a blue "reflection nebula").

An example is the Horsehead Nebula.