H II Regions 🪐🌌⋆⭐💫

The subject name H II means “ionized hydrogen”, meaning hydrogen without electrons. Within the universe, which spans different galaxies, there are regions inside nebulae where hydrogen gas atoms become ionized due to the nearby, very hot, newly formed protostars that emit a high amount of UV radiation. Such regions inside the nebulae are called H II Regions.
 
Also in general, regions where new stars formation takes place, the hot protostars emit enormous amount of UV radiation which eventually results in the ionization of the neutral hydrogen gas atoms in the nebula by shedding the lone electron from the hydrogen atoms outer shell once the hot protostars UV rays interacts with these neutral hydrogen atoms, thus transitioning them into an ionized state. With the increase in the number of ionized hydrogen & free electrons within an H II region, visible light gets emitted across varying wavelengths of the electromagnetic spectrum, making the region glow in different colours within the nebula. Such glowing nebulas are called emission nebulas as they emit their own generated light in specific colours depending upon the elements present across the nebula & the ionized hydrogen gas energy levels. A planetary nebula formed from the ejection of a dying star’s outer shells, leaving behind a white dwarf, is an example of an emission nebula. Also, a type 1a, type 2 supernova explosion creates an emission nebula due to the shockwave that flows outward after the explosion, which heats & ionizes the neutral hydrogen gas atoms in the surrounding Interstellar medium, making them glow. Also, as described above, H II regions associated with star formation, having enormous ionized hydrogen & free electrons, are examples of emission nebulae.

In the Orion Nebula, the 1st H II region associated with star formation was discovered by French astronomer “Nicolas-Claude Fabri de Peiresc” way back in the year 1610. The H II region in the Orion Nebula is about 30 light-years across & about 1500 light-years away from Earth. Also it’s the lone nebula that is visible from Earth with naked eye, meaning under clear sky if one looks at the Orion constellation, in the middle of the Orion’s Sword that hangs exactly below the 3 star’s of Orion’s Belt we’ll be able to see a fuzzy blurry patch of light which is nothing but the Orion’s Nebula, also classified as M42 Messier object under the Messier catalogue named after the French astronomer “Charles Messier”.

A few specifications regarding H II regions include,
H II regions mainly exist in the spiral arms of a galaxy away from the galactic centre. Note: A few H II regions have also been discovered just outside the spiral arms of the galaxy, but mainly within the galaxy disc.
The highest concentration of such H II regions within a galaxy is observed at about 10000 light years away from the galactic centre & then subsequently the concentration of these H II regions starts to diminish as we move further away to the tip of the spiral galactic arms.
H II regions in nebulae are discovered mainly using Radio Telescopes, with which astronomers try to detect specific radio signals to detect such H II regions.
A few of these H II regions that have been detected & discovered are observed to have temperatures as high as 10000 Kelvin, & at such high temperatures matter starts to emit visible light, & accordingly the H II regions within the nebula appear to glow very bright. In here one needs to know that this bright emitting glow comes from specific wavelengths of visible light that are generated & emitted due to the jumping of the electrons of the neutral molecular hydrogen atoms in the region when they interact with the energetic photons of the UV radiation coming from closeby active protostars causing the electron-proton bonds within the hydrogen atoms to break & in process ionizing these hydrogen atoms by liberating the lone electron from their atomic orbits.
H II regions as emission nebulae can be as luminous as star clusters, and the typical size of these regions can range from 50 light-years to 1000 light-years across.
Apart from ionized hydrogen as the dominant element, traces of Helium (also ionized due to stellar UV radiation) & traces of heavier elements like Oxygen, Nitrogen, Sulfur, & Carbon are also found in these H II regions. The abundance of these heavier elements is approximated by analysing & studying the emission line spectrum of these elements in the nebula spectra.
 
In the “M51 Whirlpool” galaxy, seen in the constellation “Canes Venatici” & located 31 million light-years away from Earth, there are many such H II regions emitting a bright glow of Red in its spiral arms. This Red glow mainly highlights the H II regions of active star formation & dust clouds, which absorb & then re-emit longer red visible wavelengths.