21cm radiation.

Hydrogen is the most abundant element in the universe. Atomic neutral hydrogen, has two states, one where the proton and electron spins are aligned and another where they are opposite as shown in Figure 9.2. The separation between these two energy states is known as hyperfine splitting in hydrogen. A transition between these two states causes radiation at $1.42 {\rm GHz}$ or $21 \rm cm$ to be emitted. This is a very important source of information about our Galaxy and external galaxies, neutral hydrogen being found in many galaxies including our own. The Giant Meterwave Radio Telescope (GMRT ,Figure 9.3) located in Narayangaon near Pune is currently the worlds largest low frequency radio telescope operating in several frequency bands from $1.42 \, {\rm GHz}$ to $50 \,{\rm MHz}$. Figure 9.3 also shows the image of a dwarf galaxy DDO210 made with the GMRT using the $21 {\rm cm}$ radiation from neutral hydrogen. The contours show how the neutral hydrogen is distributed while the distribution of stars is shown in black. It is clear that the hydrogen gas which is referred to as the interstellar medium is spread out over a much larger region compared to the stars.

Figure 9.2: Hyperfine splitting in neurtral hydorgen

Figure 9.3: GMRT and radio image of a dwarf galaxy