The Different Types of Nebulae

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In general, a nebula is characterized as a cloud of ionized gas in space. Originally, the term was used to describe any diffused astronomical object such as galaxies, but the term has changed meaning over time. Many nebulae are often star forming regions- gas, dust, and other materials clump together to form denser regions which attract further matter, and eventually these clumps of matter will become dense enough to perform hydrogen fusion.

Emission Nebulae:
Emission nebulae are the most common type of nebula, and are frequently displayed when talking about the beauty of space. These nebulae are large clouds of interstellar ionized gases that emit light of various wavelengths as a result from surrounding stars' radiation. The energy emitted from nearby stars ionizes large amounts of the gases in the nebula, causing a beautiful display of color and wonder.

Reflection Nebulae:
Reflection nebulae are clouds of interstellar dust that reflect the light of a nearby star or stars. In the case of a reflection nebula, the energy from the nearby star(s) is insufficient to ionize the gas of the dust to create an emission nebula (see above), but is enough to give sufficient scattering to make the dust visible.

Dark Nebulae:
A dark nebula is a type of interstellar cloud that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Like other interstellar dust or material, things it obscures are only visible using radio or infrared waves.

Planetary Nebulae:
Planetary nebulae are the remnants of the final stages of stellar evolution for lower-mass stars. These low mass stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells, while leaving behind the star's core in the form of a white dwarf. Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions. They are denser and more compact than nebulae found in star formation regions.

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