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  Nebulae: Shapes in the Clouds:  


A nebula is a cloud in interstellar space – a cloud of dust particles and gases that occurs either at the start or end of a star’s evolution. Its name comes from the Latin term for “cloud” and the cataloguing of nebulous forms dates back to Ptolemy (AD 90-168).

The Two Main Types
Most nebulae are one of two main types: diffuse nebulae or planetary nebulae.

Diffuse nebulae are the type most commonly found. These nebulae are large and have no well-defined boundaries. Along with bright nebulae and reflection nebulae, diffuse nebulae comprise H II regions, where star formation takes place.

Planetary nebulae on the other hand occur at the end of a star’s evolution. When a star enters its red giant phase, it ejects shells of ionized gas and plasma. This volatile combination emerges as a nebula. Since they are dependent on such a singular event, planetary nebulae tend to have shorter lifetimes. They burn bright and then fade (in a few tens of thousands of years).

Other Nebula Types
In addition, there are more specific types of nebulae, such as emission nebulae and dark nebulae. These nebulae are all defined by a certain characteristic, either in their appearance or formation:

• Emission nebulae: Clouds of high-temperature ionized gas that gives off light and color. Young stars are the source for the ionized charge. An example is the Omega Nebula (Messier 17).

• Reflection nebulae: These clouds give off light as well, but rather than be charged with a glow, they simply reflect the light of a nearby star. In general, these nebulae often appear “blue.”

• Dark nebulae: Dense and cold, these clouds of dust block light from the stellar objects behind it. An example is the Horsehead Nebulae (Barnard 33).

• Pulsar Wind Nebulae: When a star goes “supernova,” it blasts out much of its stellar material. The result is a supernova remnant (SNR); pulsar wind nebulae are often found inside SNR shells.
  Did you know:
  Did you know that the term “nebula” was once used to describe any extended astronomical object? It’s true. For instance, the galaxies found outside of the Milky Way were at a time referred to as “nebulae.”

It wasn’t until Edwin Hubble proved the existence of other galaxies (1925) that the term “nebula” began to take on its stricter definition.