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Earth's air has a density of approximately 10 19 molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 10 4 molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters.
The nebula measures around two light years long from end to end, and contains a central white dwarf with an apparent magnitude of 11. The inner layers of the nebula were formed some 1,500 years ago. [4] The two ends of the nebula are marked by FLIERs, lobes of fast moving gas often tinted red in false-color pictures. [5]
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.
The closest and largest dark nebulae are visible to the naked eye, since they are the least obscured by stars in between Earth and the nebula, and because they have the largest angular size, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift.
Jewel Bug Nebula, [citation needed] Gummy Bear Nebula [5] See also: Lists of nebulae NGC 7027, also known as the Jewel Bug Nebula or the Magic Carpet Nebula , [ 6 ] is a very young and dense planetary nebula [ 7 ] located around 3,000 light-years (920 parsecs ) from Earth in the constellation Cygnus .
A 6" telescope with a magnification around 100x will reveal a slightly bluish disk, while telescopes with a primary mirror at least 16" in diameter may reveal slight color and brightness variations in the interior. [4] This nebula has an elliptical shape with a triple-shell structure. The brightest is the main shell, which spans 12″ × 18 ...
A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm 3. [40] (The Earth's atmosphere, by comparison, contains 2.5 × 10 19 particles per cm 3.) Young planetary nebulae have the highest densities, sometimes as high as 10 6 particles per ...
The Trifid Nebula was the subject of an investigation by astronomers using the Hubble Space Telescope in 1997, using filters that isolate emission from hydrogen atoms, ionized sulfur atoms, and doubly ionized oxygen atoms. The images were combined into a false-color composite picture to suggest how the nebula might look to the eye.