The most important Interstellar medium line from the neutral Hydrogen is 21 cm (radio) line. It comes from the hyperfine splitting of the ground state of the Hydrogen atom. Hyperfine structure can be seen because of the coupling (or interaction) between the nuclear and the electronic spins. The energy difference between ortho (higher) and para (lower) state is This energy difference due to the spin flip transition of ortho and para state produces emission of rest frequency,  = 1420.41 MHz and rest wavelength

Hydrogen atom are excited into the upper state through collisions. The oscillator strength of this transition is so small that it takes an average time of for a photon to be emitted from one hydrogen atom. Such a rare transition can only be observed if there is an enormous abundance of neutral hydrogen. It has no dipole moment. Thus, no rotational and vibrational transition. No electronic transition upto 5000 K.

Ionized Hydrogen (HII)

Ionized gases produces ’emission lines’ from electronic transitions. [UV radiation emitted from the young stars ionize and heat up the neutral or molecular hydrogen forming HII region] The strongest hydrogen emission line at 656.3 nm gives HII regions their characteristic red color. This wavelength corresponds to energies > 13.6 eV, the ionization energy from the ground state of hydrogen. HII region are regions of fully or partially ionized hydrogen surrounding very hot young stars of O and B spectral types Hot stars produce UV photons (). These energetic photons photo ionize hydrogen producing a region of HII (ie H+).

PN is like a compact HII region, where population inversion becomes possible. The over populated excited state then decays by stimulated emission i.e. it becomes a maser. When electrons in HII region gyrate relativistically in a magnetic field, a synchrotron radiation can be observed in optical and radio. Supernova remnants have strong emission lines. They mix with low density component of Interstellar medium forming HII region.

The molecular hydrogen exhibit ‘spin isomers’ two protons have parallel spin (ortho hydrogen, higher energy state) and anti parallel spin (para hydrogen, lower energy state). Due to ‘spin isomer’ shows quadrupole moment. So, molecular hydrogen has a weak rotational quadrupole transition, not detectable using today’s technology. Thus, the direct detection of  must be based on the weak quadrupole spectrum. The para dominates at low temperatures (approximately 99.8% at 20k). At room temperature and thermal equilibrium, para is found to be only 25% (ortho 75%).

The far ultraviolet radiation is a very efficient dissociation of . Thus absorption line (dark lines) in the far UV spectrum can be observed. There are two problems: first there must be UV source (O and B star, new born star) nearby molecular cloud, second far UV spectrum can not be observed from the Earth’s surface. So, space observatory is needed to observe it.

Indirect measurement of : CO molecule is most abundant after in the molecular cloud. Interestingly, the densities of CO and are found to be in equal proportion in all molecular clouds. The ratio is about :CO = . Thus the amount of CO in molecular cloud is useful as a tracer of . Mapping CO density is therefore used to determine the distribution of cold gas in the Interstellar medium. CO has strong lines at 2.6 mm from transition between rotational states.

(a) Molecular clouds (): extremely cold

(b) Neutral hydrogen (H): cold / warm

(c) Ionized hydrogen (HII): HII region / coronal gas

Mckee and Ostriker proposed three phase interstellar medium in the year 1977. This classification is based on the states of Hydrogen (molecular form, neutral form and ionized form). These gases emits detectable radiation:

: CO emission lines / UV absorption lines

HI: 21 cm radio emission and absorption lines

HII: optical emission lines, Bremsstrahlung soft x rays

The composition of these gases in the Interstellar medium are as follows:

State of Hydrogen	Fractional volume	Temperature (K)	Density atoms/cm cube	Classification	Obervational technique
Molecular	< 1%	< 50 K		Molecular cloud	Molecular emission / absorption lines
Neutral (H)	1% – 5%	50 – 100		cold neutral medium	21 cm absorption
Neutral (H)	10% – 20%	1000 – 5000	0.1 – 10	warm neutral medium	21 cm emission
Ionized (HII)	20% – 50%		0.01	warm ionized medium	H alpha emission
HII Region	~ 1%			ionized	H alpha emission
Hot ionized (HII)	30% – 70%			coronal gas	x-ray emission

Interstellar dust

Interstellar dust consists of particles of silicates or carbon compounds, ice or carbon compounds. These dust particles are extremely small (the largest ~ atoms size or 0.5 micrometer; the smallest < atoms) just a fraction of micron across which happens to be approximately the wavelength of blue light. The particles are irregularly shaped.

When light from other stars passes through the dusk, the light will be completely blocked leading to dark area called dark nebula. The Horsehead nebula is an example of this. When the dust reflects the star light, the region visible to us is said to be reflection nebula (example: NGC1999). In general , light passing through a duct cloud may not be completely blocked causing extinction. The extinction depends upon the density of the dust as well as wavelength of light.

Cosmic rays

Cosmic rays are energetic charged sub atomic particles originating from outer space. It includes protons, nuclei, relativistic electrons etc. Cosmic rays can have energies of over far higher than that man made particle accelerator can produce. Because of the accelerated charged particle, Zeeman splitting of 21 cm line takes place.