Catastrophysics

1.4 GHz line Using a radio telescope we can detect the H 21 cm line which is produced by the spin-flip transition in atomic hydrogen. As this line is not attenuated by interstellar dust or by the earth's atmosphere, we can detect it using ground-based telescopes.   The probability of the spin-flip transition is low (lifetime of an excited state t~ 3.5 x 10^14 seconds), meaning that a single hydrogen atom might undergo this transition only once in 11 million years!  But if we point our radio telescope at the sky we do see this transition. Why? Well, because of the vast abundance of hydrogen in the galaxy. If a chance of transition is 10^(-14) but you got around 10^57 hydrogen atoms, then a simple multiplication might give you an idea of how many HI atoms are emitting this transition. This makes it possible to detect the 1.4 GHz line and use it for astronomy. Scanning the galaxy in steps of 5 degrees of galactic latitude you can see the structure of the galaxy pretty easily. Here I