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High power UV photons ionise the hydrogen in the interstellar gas cloud. As the hydrogen recombines and also returns to its neutral state, optical photons are emitted.

You are watching: In the milky way, the most abundant gas in emission nebulae is


Emission nebulae space clouds the ionised gas that, together the surname suggests, emit their own light at optical wavelengths. Your mass typically ranges indigenous 100 come 10,000 solar masses and also this material deserve to be spread over a volume of much less than irradiate year to several hundred light years. Because that this reason, their densities are highly varied, varying from countless atoms/cm3 to only a few atoms/cm3 relying on the compactness the the nebula. Commonly they have densities of the stimulate of a thousands atoms/cm3, i beg your pardon is still extremely rarified compared to the air we breathe on planet (2.5×1019 particles/cm3), and their average temperature is around 10,000 Kelvin.

One of the many common varieties of emissions nebula occurs when an interstellar gas cloud dominated by neutral hydrogen atoms is ionised by nearby O and B form stars. These exceptionally hot and also luminous stars provide off huge quantities of high-energy ultraviolet (UV) photons which break the neutral hydrogen atoms right into hydrogen nuclei and electrons. These later recombine to kind neutral hydrogen again, yet this time in one excited state. As the neutral hydrogen atom returns to that is lowest power state, that emits photons in ~ wavelengths indistinguishable to the power differences in between the allowed energy claims of hydrogen. At optical wavelengths, the most essential of this transitions coincides to a wavelength the 656.3nm in the red finish of the spectrum. This is the wavelength of Hα, and also it is this change that gives emission nebulae your distinctive red colour.

This type of emission nebula is usually referred to as a HII region (pronounced H-two region), since it is typical practice for astronomers to refer to neutral hydrogen as HI (H-one) and ionised hydrogen as HII. These nebulae are solid indicators of current star formation since the O and also B stars that ionise the gas live for only a really short time and also were most likely born in ~ the cloud they are currently irradiating. One of the most well known emission nebulae is the Orion Nebula (M42) located just listed below Orion’s belt.

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Another common kind of emissions nebula is a planetary nebula. This objects covers a central white dwarf star surrounded by clouds of gas released as the initial star developed to the white dwarf phase. In this case, the excited gas is no necessarily conquered by HII, however can also contain far-reaching amounts of ionised helium (HeII; blue emission) and doubly-ionised oxygen (OIII; green emission). Because much an ext energy is required to ionise helium 보다 hydrogen, the bluest areas of planetary nebulae space the hottest and also indicate the areas of highest possible excitation.


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The Orion Nebula (M42) is perhaps the most famed emission nebula. Huge stars located in the love of the nebula are bombarding the gas through UV radiation resulting in it come glow.Credit: AAO/David Malin