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A protoplanetary nebula or preplanetary nebula [27] (PPN, plural PPNe) is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase.
Reflection nebula are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets). Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. Some 500 reflection nebulae are known.
An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star .
This is a list of diffuse nebulae. Most nebulae are diffuse , meaning that they do not have well-defined boundaries. Types of diffuse nebulae include emission nebulae and reflection nebulae .
Box Nebula NGC 6445: 1786 4.5 11.2 Sagittarius: Eye of Sauron Nebula M 1-42: 10 14 Sagittarius
The following articles contain lists of nebulae: List of dark nebulae; List of diffuse nebulae; List of planetary nebulae; List of protoplanetary nebulae;
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Nebulae become visible if the gas glows, or if the cloud reflects starlight or obscures light from more distant objects. The catalogues that it may refer to: Catalogue des nébuleuses et des amas d'étoiles (Messier "M" catalogue) first published 1771; Catalogue of Nebulae and Clusters of Stars (William Herschel 'CN'/"H" catalogue) first ...