Question: What will happen to most hydrogen in HII regions?
About the article "Nebula", section "Planetary nebulae".

Candidate passages:
Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs.
They are emission nebulae with spectra similar to those of emission nebulae found in star formation regions.
Technically they are HII regions, because most hydrogen will be ionized, but they are denser and more compact than the nebulae in star formation regions.
Planetary nebulae were given their name by the first astronomical observers who became able to distinguish them from planets, who tended to confuse them with planets, of more interest to them.
Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation.
Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.
In visible light these nebulae may be divided into emission and reflection nebulae.
Emission nebulae emit spectral line radiation from ionized gas (mostly ionized hydrogen); they are often called HII regions (the term "HII" is used in professional astronomy to refer to ionized hydrogen).
Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.
Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called "dark nebulae".
Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae. 
  Fine structure cooling : The process of fine structure cooling is dominant in most regions of the Interstellar Medium, except regions of hot gas and regions deep in molecular clouds.
It occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: CII and OI in the neutral medium and OII, OIII, NII, NIII, NeII and NeIII in HII regions.
Collisions will excite these atoms to higher levels, and they will eventually de-excite through photon emission, which will carry the energy out of the region.
Cooling by permitted lines : At lower temperatures, more levels than fine structure levels can be populated via collisions.
For example, collisional excitation of the n=2 level of hydrogen will release a Lyformula_11 photon upon de-excitation.
In molecular clouds, excitation of rotational lines of CO is important.
Once a molecule is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud.  
A few of the brightest H II regions are visible to the naked eye.
However, none seem to have been noticed before the advent of the telescope in the early 17th century.
Even Galileo did not notice the Orion Nebula when he first observed the star cluster within it (previously cataloged as a single star, θ Orionis, by Johann Bayer).
The French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610.
Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.
William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns".
Confirmation of this hypothesis had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, but turned out to be galaxies consisting of hundreds of millions of individual stars.
Others looked very different.
Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines.
In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical element.
At first it was hypothesized that the line might be due to an unknown element, which was named Nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.
However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, Nebulium was not.
In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.
Interstellar matter, considered dense in an astronomical context, is at high vacuum by laboratory standards.
Physicists showed in the 1920s that in gas at extremely low density, electrons can populate excited metastable energy levels in atoms and ions, which at higher densities are rapidly de-excited by collisions.
Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line.
These spectral lines, which can only be seen in very low density gases, are called forbidden lines.
Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).
In HII regions, however, the dominant spectral line has a wavelength of 656.3 nm.
This is the well-known H-alpha line emitted by atomic hydrogen.
Specifically, a photon of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state from n=3 to n=2.
Such state-changes happen very frequently when an electron is captured by an ionised hydrogen atom (a proton), and the electron cascades down from some higher excitation state to n=1.
Thus, it was concluded that HII regions consist of a mix of electrons and ionised hydrogen that are constantly recombining into hydrogen atoms.
During the 20th century, observations showed that H II regions often contained hot, bright stars.
These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years).
Therefore it was surmised that H II regions must be regions in which new stars were forming.
Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse.
The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it was formed.
Only a trace of reflection nebulosity remains. 
One of the important aims of the telescope is to search for the highly redshifted 21-cm line radiation from primordial neutral hydrogen clouds in order to determine the epoch of galaxy formation in the universe.
Pulsar research is another major area for GMRT study.
Astronomers from all over the world regularly use this telescope to observe many different astronomical objects such as HII regions, galaxies, pulsars, supernovae, and sun and solar winds.