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A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by. [6] 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. [7]
The Solar System has its own interplanetary dust cloud, as do extrasolar systems. There are different types of nebulae with different physical causes and processes: diffuse nebula, infrared (IR) reflection nebula, supernova remnant, molecular cloud, HII regions, photodissociation regions, and dark nebula.
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.
Interstellar dust is found between the stars, and high concentrations produce diffuse nebulae and reflection nebulae. Dust is widely present in the galaxy. Ambient radiation heats dust and re-emits radiation into the microwave band, which may distort the cosmic microwave background power spectrum.
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 .
The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae. [2] They possess diameters of 0.01–0.1 pc (2,000–20,000 AU) and a particle number density of roughly 10,000 to 100,000 cm −3. [a] [35] [37] The initial collapse of a solar-mass protostellar nebula takes around 100,000 ...
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 ...
Cometary knots are one type of ionized photoevaporation flow, which is characteristically associated with planetary nebulae, but several other types of photoevaporation flows (proplyds, cometary globules, elephant trunks, and champagne flows) are known from H II regions such as the Orion Nebula.