Perhaps the most majestic entities in space, nebulae are interstellar clouds of gases (hydrogen, helium, oxygen, nitrogen, carbon, and sulfur) and particles formed from exhausted stars. There are a few different classes of nebulae including: emission, reflection, dark, and planetary nebulae. These different types exhibit unique traits that are easily identifiable.
The various nebula types all form from an exploding star but do so slighlty differently. Planetary nebula form when a medium sized star (about the size of our sun) runs out nuclear fuel. The star explodes and crates a shell of debris. The stars that form this nebula are not large enough to become supernovae so they simply shed their materials in symmetrical formations. Emmision and reflective nebulae are also formed from exploding stars, but could also contain ancient material (dust and gasses formed early after the creation of the universe). The stars inside nebulae are formed by gravity pulling particles together and they will eventually undergo fusion.
Planetary nebulae consist of a shell created by an exploding star at the end of its existence. Contrary to their name, planetary nebulae are irrelevant to planets. They were originally discovered in 1764 by William Herschel. This name was acquired because they sometimes look like planets through low-powered telescopes. This is actually at the opposite end of a star's lifetime from planet fromation. The planetary nebula pictured below is know as the cat's eye nebula. At the center is a binary star system. It is suspected that it's apperance comes from jets of material that sprayed from the stars' equator. It is estimated to be 1000 years old. Each different color represents a different wavelength of light emmited by a different element. The red is hydrogen, blue is oxygen, and green is nitrogen.
After being energized, they lose their energy and it is shown when they give off radiation. This type of nebula is a site for star formation. Generally larger than Planetary but smaller than Dark nebulae, they may simply be older Planetary nebulae that have expanded and thinned so that they no longer have much of their earlier shape. This type of nebula usually emits the greatest vareity of color because it is composed of the greated vareity of elements. Below is the veil nebula. The entire nebula is quite large (spanning three degrees in our sky), but this is only a segment. It is a supernova remnant that consists of of charged, glowing gasses. These gasses are quite hot and range from temperatures between 7,000 and 20,000 K.
Dark (or Reflective) nebulae are large (able to contain many star systems) clouds of dust. The 2 names are a simple matter of how they look from Earth's point of view. If there is a strong light source behind it, it will appear as a dark nebula. If there is a bright star between it and the Earth, it will be lit up from "this side" so we call it a Reflective nebula. These tend to be blue because of the same light scattering effect that makes our sky blue. All of these nebulae tend to be large (hundreds of l.y. across or more) and could be harboring many young stars. Their temerature is relatively cold (around 10,000 K).
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