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Hydrogen line
The hydrogen line, 21 centimetre 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 1420.405751786 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 the Planck-Einstein relation.
==Cause==
The ground state of neutral hydrogen consists of a spherically symmetric electron cloud bound to a proton. Both the electron and the proton have intrinsic magnetic dipole moments ascribed to their spin, whose interaction results in a slight increase in energy when the spins are parallel, and a decrease when antiparallel. The fact that only parallel and antiparallel states are allowed is a result of the quantum mechanical discretization of the total angular momentum of the system. When the spins are parallel, the magnetic dipole moments are antiparallel (because the electron and proton have opposite charge), thus one would naively expect this configuration to actually have ''lower energy'' just as two magnets will align so that the north pole of one is closest to the south pole of the other. This logic fails here because the electron is not spatially displaced from the proton, but encompasses it, and the magnetic dipole moments are best thought of as tiny current loops. As parallel currents attract, it is clear why the parallel magnetic dipole moments (i.e. antiparallel spins) have lower energy.〔D. J. Griffiths, "Hyperfine Splitting in the Ground State of Hydrogen", ''Am. J. Physics'' 50, 8 (1982)〕
This transition is highly forbidden with an extremely small rate of 2.9×10−15 s−1, and a lifetime of around 10 million (107) years. A spontaneous occurrence of the transition is unlikely be seen in a laboratory on Earth, but it can be artificially induced using a hydrogen maser. It is commonly observed in astronomical settings such as hydrogen clouds in our galaxy and others. Owing to its long lifetime, the line has an extremely small natural width, so most broadening is due to Doppler shifts caused by bulk motion or nonzero temperature of the emitting regions.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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