https://padlet.com/539142/w63qyks43tx8 davontay mosby 4th hour en-us 2018-03-05 17:25:02 UTC 2025-10-22 05:59:19 UTC hello@padlet.com 539142 https://padlet.com/539142/w63qyks43tx8/wish/238220106 emission nebula ]]> 2018-03-05 17:27:25 UTC https://padlet.com/539142/w63qyks43tx8/wish/238220106 539142 https://padlet.com/539142/w63qyks43tx8/wish/238220894 Emission nebulae are clouds of ionised gas that, as the name suggests, emit their own light at optical wavelengths. ]]> 2018-03-05 17:28:35 UTC https://padlet.com/539142/w63qyks43tx8/wish/238220894 539142 https://padlet.com/539142/w63qyks43tx8/wish/238221445 Their mass generally ranges from 100 to 10,000 solar masses and this material can be spread over a volume of less than light year to several hundred light years. ]]> 2018-03-05 17:29:26 UTC https://padlet.com/539142/w63qyks43tx8/wish/238221445 539142 https://padlet.com/539142/w63qyks43tx8/wish/238221654 For this reason, their densities are highly varied, ranging from millions of atoms/cm³ to only a few atoms/cm³ depending on the compactness of the nebula.]]> 2018-03-05 17:29:41 UTC https://padlet.com/539142/w63qyks43tx8/wish/238221654 539142 https://padlet.com/539142/w63qyks43tx8/wish/238221938
Typically they have densities of the order of a thousand atoms/cm³, which is still extremely rarified compared to the air we breathe on Earth (2.5×10¹⁹particles/cm³), and their average temperature is around 10,000 Kelvin.

]]> 2018-03-05 17:30:05 UTC https://padlet.com/539142/w63qyks43tx8/wish/238221938 539142 https://padlet.com/539142/w63qyks43tx8/wish/238222828 One of the most common types of emission nebula occurs when an interstellar gas cloud dominated by neutral hydrogen atoms is ionised by nearby O and B type stars. ]]> 2018-03-05 17:31:22 UTC https://padlet.com/539142/w63qyks43tx8/wish/238222828 539142 https://padlet.com/539142/w63qyks43tx8/wish/238223137 These extremely hot and luminous stars give off vast quantities of high-energy ultraviolet (UV) photons which break the neutral hydrogen atoms into hydrogen nuclei and electrons.]]> 2018-03-05 17:31:48 UTC https://padlet.com/539142/w63qyks43tx8/wish/238223137 539142 https://padlet.com/539142/w63qyks43tx8/wish/238224079 These later recombine to form neutral hydrogen again, but this time in an excited state. As the neutral hydrogen atom returns to its lowest energy state, it emits photons at wavelengths equivalent to the energy differences between the allowed energy states of hydrogen.]]> 2018-03-05 17:33:12 UTC https://padlet.com/539142/w63qyks43tx8/wish/238224079 539142 https://padlet.com/539142/w63qyks43tx8/wish/238224271 At optical wavelengths, the most important of these transitions corresponds to a wavelength of 656.3nm in the red end of the spectrum. This is the wavelength of H_α, and it is this transition that gives emission nebulae their distinctive red colour.]]> 2018-03-05 17:33:27 UTC https://padlet.com/539142/w63qyks43tx8/wish/238224271 539142 https://padlet.com/539142/w63qyks43tx8/wish/238224530 This type of emission nebula is normally referred to as a HII region (pronounced H-two region), since it is common practice for astronomers to refer to neutral hydrogen as HI (H-one) and ionised hydrogen as HII. ]]> 2018-03-05 17:33:53 UTC https://padlet.com/539142/w63qyks43tx8/wish/238224530 539142 https://padlet.com/539142/w63qyks43tx8/wish/238224968 these nebulae are strong indicators of current star formation since the O and B stars that ionise the gas live for only a very short time and were most likely born within the cloud they are now irradiating. One of the most famous emission nebulae is the Orion Nebual]]> 2018-03-05 17:34:28 UTC https://padlet.com/539142/w63qyks43tx8/wish/238224968