Hydrogen is the most common material in the Universe, and can be found in virtually every environment investigated by astronomers, from the regions surrounding black holes, in stars and galaxies, as an important component in clusters of galaxies, and spread on large scales through the universe itself. Three states of hydrogen, HI, HII and H2, are often referred to by astronomers, and mean:

• HI - unionized form of hydrogen
• HII - ionized form of hydrogen
• H2 - molecular hydogen

Much of the Hydrogen in galaxies can be detected via the 21-cm spin-flip which is studied by radio telescopes. Using radio telescopes and maping the spin-flip transitions, we are able to map the shape of our galaxy.

HII regions exist most commonly in the disk of a spiral galaxy. For an HII region to exist, there must be a source to proved the ionizing heat required to strip the electron, so HII regions are common near very hot stars. As a result, HII regions can be very hot, around ~10,000K. Such areas are emission nebula. These areas of starbirth are strong in UV radiation. Newly formed stars within the spiral of a galaxy are literally blown away from its association with other stars formed in the same "nursury." A great example of this is the Orion Nebula.

As for molecular clouds, 99% of their composition is in the form of molecular hydrogen (H2). The rest include over 60 other molecules such as H20, SO2, HCN, CS, H2CO, CO, CN, SiO, OH, H2S, and many other variations of H, C, N and O.

False color image of the Rosette nebula (NGC2237) in the constellation Monoceros (the Unicorn) taken in the light of hydrogen a, [O III], and [S II], (red, green and blue). The Rosette is a prominent star formation region, glowing due to ultraviolet light from the young, hot, blue stars whose winds also cleared the central hole.   Image Credit.

Star forming regions like the Orion Nebula are best viewed in the infrared:

In this case, the HII clouds (blue) are surrounding hot, young stars (yellow).

Old galaxies - like elliptical type galaxies - will have little HII regions as star birth and supernova would have either used up or cleared away these areas.

For ionization of hydrogen to occur, ultraviolet photons (with a temperature from 10,000 K to 60,000 K) are needed. White dwarf stars (the stellar remnant of normal stars) have a temperature near 200,000 K and are also very good at ionizing hydrogen. Examples of these are planetary nebula:

This montage of Hubble Space Telescope images demonstrate the wide variety of planetary nebulae. In addition to ionized hydrogen, other debris surrounding a planetary are oxygen, nitrogen, silicone, and helium.

Ionization and Emission Lines (C. Flynn, 2005):

H II regions have a certain size around an ionizing source which is a balance between the flux of ionizing photons on one hand, and the rate at which the plasma can cool and the ionized electrons can re-combine with the protons. If the ionization rate is greater than the recombination rate, then the plasma will become almost fully ionized. Studies indicate that less than a per cent of the hydrogen remains unionized in typical regions.

Three spiral galaxies imaged in a broad band red filter (top panels) and the light of Ha (bottom panels). HII regions are remarkably tightly correlated with the spiral and ring like structures in the galaxies. The red filter is more sensitive to the general distribution of old stars, while the Ha filter highlights the sites of young stars in star forming regions.

Ultra-violet photons below a wavelength of l = 912 Å, have sufficient energy to ionize hydrogen, removing the electron from the ground shell. The electron is eventually recaptured, but because interstellar gas is of such low density, this can take some time.

Hot stars are the best sources of the UV photons needed to ionize the gas. In practice this means very early type stars, such as O and B stars, and white dwarfs. The surfaces of the O and B stars range in temperature from 10,000 to 60,000 K, while white dwarfs can reach surface temperature of up to 200,000 K.

Recombination leads to emission lines, as shown in figure 8. Important ones are Ha at 6563 Å, [N II] at 6583 Å, [O II] at 3726 and 3729 Å and [O III] at 4959 and 5007 Å.

A special notation has been used here, the square brackets, "[ ]''. This indicates that the line is normally "forbidden'', meaning it is only seen in very low density conditions. In the laboratory, plasmas are generally of such high density that these line are not seen. The lines originate from energy states just above the ground state, and are meta-stable with long lifetime.

The emission lines form very useful diagnostics of the physical conditions in the the gas. For example, the [O III] and [N III] lines are temperature sensitive, while [O II] and [S II] (at 6716 and 6731 Å) are more sensitive to the electron density. This is because the latter lines are emitted at different levels but with nearly the same excitation energy, so that their relative ratio is a diagnostic of the collisional de-excitation, or the density, of the gas. The plasmas are found to be very thin by terrestrial standards, with particle densities of order 10 to 10⁶ cm^-3.

Observations at radio wavelengths are very interesting, because these regions typically emit bremsstrahlung radiation at these frequencies, giving an excellent independent probe of the temperatures and electron densities in the clouds.

Examples of Bremsstrahlung:

(Image Credit)

(Image Credit)

Bremsstrahlung is the German word for "braking radiation." As a free electron interacts with a proton, the electron is slowed (but not captured) releasing a photon.

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