*


*

*

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

You are watching: In which of these galaxies would you be least likely to find an ionization nebula?


Emission nebulae are clouds that ionised gas that, together the surname suggests, emit their own light at optical wavelengths. Their mass generally ranges indigenous 100 come 10,000 solar masses and also this material can be spread over a volume of less than irradiate year to numerous hundred light years. Because that this reason, their densities are extremely varied, ranging from millions of atoms/cm3 to just a couple of atoms/cm3 depending on the compactness the the nebula. Commonly they have actually densities of the stimulate of a thousand atoms/cm3, i m sorry is still very rarified compared to the air us breathe on planet (2.5×1019 particles/cm3), and also their typical temperature is roughly 10,000 Kelvin.

One that the most common species of emission nebula occurs once an interstellar gas cloud overcame by neutral hydrogen atoms is ionised by surrounding O and also B form stars. These incredibly hot and also luminous stars give off substantial quantities of high-energy ultraviolet (UV) photons which break the neutral hydrogen atoms into hydrogen nuclei and electrons. These later on recombine to form neutral hydrogen again, however this time in one excited state. As the neutral hydrogen atom returns to the lowest energy state, the emits photons in ~ wavelengths tantamount to the energy differences in between the permitted energy says of hydrogen. At optical wavelengths, the most crucial of this transitions synchronizes to a wavelength that 656.3nm in the red end of the spectrum. This is the wavelength that Hα, and it is this transition that gives emission nebulae your distinctive red colour.

This form of emission nebula is normally referred to as a HII region (pronounced H-two region), due to the fact that it is usual practice because that astronomers to refer to neutral hydrogen together HI (H-one) and ionised hydrogen together HII. These nebulae are solid indicators of current star formation since the O and also B stars the ionise the gas live for just a really short time and also were most likely born in ~ the cloud they are now irradiating. Among the most well known emission nebulae is the Orion Nebula (M42) situated just below Orion’s belt.

See more: Who Was Headmaster Before Albus Dumbledore Appointed As Headmaster?

Another common type of emissions nebula is a planetary nebula. This objects consist of a main white dwarf star surrounded by clouds of gas released together the initial star developed to the white dwarf phase. In this case, the excited gas is no necessarily dominated by HII, but can likewise contain significant amounts that ionised helium (HeII; blue emission) and also doubly-ionised oxygen (OIII; environment-friendly emission). Due to the fact that much an ext energy is compelled to ionise helium than hydrogen, the bluest locations of planetary nebulae are the hottest and indicate the locations of highest excitation.


*

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