Answer to Question #89655 in Astronomy | Astrophysics for Sakshi

Question

What is the difference between the HII region and HI region of the interstellar medium? Explain the process involved in the detection of HI region in the ISM.

Answer 1

HII and HI regions are the regions of hydrogen. Scientists use roman numerals to indicate the degree of ionization of elements: a one, "I", means "not ionized at all", a two, "II", means "one electron has been removed," a three, "III", means "two electrons have been removed," and so forth. The HI clouds are the regions of neutral Hydrogen atoms, whereas HII are the regions of of ionized H⁺ ions (protons). Ionized atomic Hydrogen regions, broadly termed “HII Regions”, are composed of gas ionized by photons with energies above the Hydrogen ionization energy of 13.6eV. Ionized Hydrogen is found throughout the Universe in a wide variety of forms. The term “HII Region”, however, specifically refers to a bright diffuse nebulae photoionized by young O and B stars in regions of recent massive star formation.

Our Milky Way Galaxy contains more than 10 billion sun's worth of material floating between the stars in what astronomers call the Inter Stellar Medium, or ISM for short. Astronomers studying the ISM have found that about 90% of its atoms are hydrogen. Detecting HI clouds directly was not possible until astronomers began using radio telescopes in the mid-1950s. Both the electron and the proton in an H atom have a property that is akin to the spin of a top. Electromagnetic interactions between the two particles can flip the spinning electron causing it to emit or absorb a photon with a wavelength 21 cm (which is in the radio part of the spectrum). These photons have such a long wavelength they can penetrate most intervening material. The prevalence of hydrogen gas along with the penetrating power of the 21-centimeter radiation it emits in neutral form have allowed astronomers to detect HI clouds across great distances and use them to map out the spiral structure of the Milky Way.