nebula

A generic term for a fuzzy, diffuse astronomical object. Astronomers have observed four different types of nebulae: H II regions, reflection nebulae, planetary nebulae, and supernova remnants.

HII regions

A "stellar womb" composed of hydrogen, helium, interstellar gas and dust that is illuminated with the light and energy of new born stars. An example is the Orion Nebula (M 42), located in the constellation Orion.

reflection nebulae

A nebula that is mainly composed of cool interstellar dust that reflects and scatters light from nearby stars. These nebulae are mainly bluish in color. The blue color provides a clue to the size of the dust grains, because in order to scatter the blue light, the dust grain size must be close to the wavelength of blue light.

planetary nebulae

Contrary to their name, these nebulae have nothing to do with planets. A planetary nebula is created when a star in the last stage of its life puffs off its outer atmosphere. The nebula usually looks like a donut, sometimes with the small, hot, rapidly evolving star visible in the center. The Ring Nebula (M 57) in the constellation Lyra is an example.

supernova remnant

This type of nebula is the result of a supernova explosion ripping apart and scattering the stellar atmosphere of its progenitor. The radiation and shock wave produced in the explosion illuminate the remnants of the stellar atmosphere. The shock wave, which plunges through the gas like a snowplow, causes the gas to glow all across the electromagnetic spectrum. An example is the Crab Nebula (M 1), which is located between the horns of Taurus the Bull.