The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength or frequency falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.
The microwaves of the hydrogen line come from the atomic transition between the two hyperfine levels of the hydrogen 1s ground state with an energy difference of 5.87433 µeV. The frequency of the quanta that are emitted by this transition between two different energy levels is given by Planck's equation.
Famous quotes containing the words hydrogen and/or line:
“All you of Earth are idiots!... First was your firecracker, a harmless explosive. Then your hand grenade. They begin to kill your own people a few at a time. Then the bomb. Then a larger bomb, many people are killed at one time. Then your scientists stumbled upon the atom bomb—split the atom. Then the hydrogen bomb, where you actually explode the air itself.”
—Edward D. Wood, Jr. (1922–1978)
“A route differs from a road not only because it is solely intended for vehicles, but also because it is merely a line that connects one point with another. A route has no meaning in itself; its meaning derives entirely from the two points that it connects. A road is a tribute to space. Every stretch of road has meaning in itself and invites us to stop. A route is the triumphant devaluation of space, which thanks to it has been reduced to a mere obstacle to human movement and a waste of time.”
—Milan Kundera (b. 1929)