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Some nebulae are variably illuminated by T Tauri variable stars.
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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 air we breathe has a density of approximately 10 to the power 19 molecules per cubic centimeter, by contrast the densest nebulae can have densities of 10,000 molecules per cubic centimeter. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms.
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The Orion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by. Most nebulae are of vast size some are hundreds of light-years in diameter. The remaining material is then believed to form planets and other planetary system objects. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars. Nebulae are often star-forming regions, such as in the " Pillars of Creation" in the Eagle Nebula. nebulae, nebulæ or nebulas ) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Evidence from the Spitzer Space Telescope suggests that the pillars may already have been destroyed by a supernova explosion, but the light showing us the destruction will not reach the Earth for another millennium. The " Pillars of Creation" from the Eagle Nebula.