Galactic Astronomy


What are 21 cm line observations ? (Sumit)

The spin-flip transition of atomic hydrogen in its ground electronic state leads to the emission of a photon with a wavelength of 21cm.

The 21cm. spin-flip transition of atomic hydrogen cannot be observed in terrestrial laboratories because quantum-mechanically, it is a highly forbidden transition. However, the sum total of rare occurrances over very long paths (typically hundreds of light-years) does allow the astronomical detection of the 21 cm. line from interstellar gas clouds.

The vast bulk of the interstellar medium is in relatively cold gas clouds (T of the order of 10-100 K) where hydrogen exists in either atomic or molecular form. The hydrogen gas in the cold clouds is too cold to have any measurable emission at optical wavelengths. The astronomical study of the interstellar medium at the radio wavelength of 21 cm. is the only method by which atomic hydrogen gas can be directly observed, from one end of the Galaxy to the other. These observations are also important, because a measurement of the Doppler shift of the 21-cm. line radiation also yields the component velocity of the atomic hydrogen along the line of sight.

Historically, in 1945 Van de Hulst announced that it should be possible to observe atomic hydrogen at a radio wavelength of 21 cm. Van de Hulst's theoretical prediction bore fruit in 1951 with the almost simultaneous detection of the 21-cm. line by Ewen and Purcell and by Muller and Oort. The discovery of the 21-cm. line is considered as probably the most important achievement in the astronomical study of the interstellar medium.

FORBIDDEN TRANSITIONS : Under normal circumstances, not all possible transitions are equally likely. Quantum mechanics provides certain"selection rules" which predict the more commonly observed transitions between different energy levels. These "allowed" transitions generally represent energy that one photon can carry off easily (in the emission case) and still conserve the total angular momentum of the system.Other transitions are mathematically possible, but are considered "forbidden"because the chances are good under terrestrial conditions that before the atom can radiate by such a "forbidden transition", a collison with another atom or molecule will deexcite the atom collisionally. Collisions are much more rare in the interstellar medium because atoms are spaced much further apart on the average there; so "forbidden" transitions play an important role in the radiation produced from gas clouds in the spaces between and surrounding stars.

SPIN-FLIP TRANSITIONS : The lowest orbital energy state (ground state) of atomic hydrogen is actually sub-divided into two levels by a quantum-mechanical effect known as hyperfine structure. A spinning proton and a spinning electron act like two bar magnets. The magnetic polarity of the proton is the same as its spin direction; that of the electron is opposite to its spin-direction. The higher energy level is due to parallel spins of the proton and electron (equivalent to bar magnets with opposite polarities in a single line) while the lower energy level is due to anti-parallel spins of the proton and electron (equivalent to bar magnets with same polarities on a single line). A hydrogen atom which finds itself in the parallel state will spontaneously make a "spin-flip"transition to the lower-energy anti-parallel state, with the energy difference being carried off by a photon with a wavelength of 21-cm.   Top


Does the interstellar material have the same density and uniformly spread throughout space ? (Sumit)

No. The density varies in the space. The mean interstellar density is 1 particle per cc. However, there are certain regions such as starforming regions like orion, taurus etc., where density goes upto ten to the power 6 particles per cc. The spatial distribution of interstellar medium is not uniform but clumpsy. Infact there are different types of clouds existing in space.

  • Diffuse interstellar clouds.
  • Dark clouds.
  • Molecular clouds.   Top


Last updated on: February 20, 2024