Hydrogen line

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The hydrogen line refers to the spectral line created by changes in the energy state of neutral hydrogen and occurs at 1420.40575 MHz, or a wavelength of around 21 cm. The line is used extensively in astronomy, particularly radio astronomy, as the line falls well within the radio spectrum.

1 Cause of the hydrogen line
2 Discovery
3 Uses in radio astronomy
4 See also

[edit] Cause of the hydrogen line

Neutral hydrogen consists of a single proton orbited by a single electron. As well as their orbital motion, the proton and electron also have spin. Classically, this is analogous to rotational motion (like the Earth rotating as it orbits the Sun), but as they are quantum particles the concept has a slightly different meaning. The spin of the electron and proton can be in either direction - in the classical analogy they are rotating clockwise or anticlockwise around a given axis. They may both have their spin oriented in the same direction or in opposite directions. Because of magnetic interactions between the particles, a hydrogen atom that has the spins of the electron and proton aligned in the same direction (parallel) has slightly more energy than one where the spins of the electron and proton are in opposite directions (anti-parallel). The lowest orbital energy state of atomic hydrogen has hyperfine splitting arising from the spins of the proton and electron changing from a parallel to antiparallel configuration. This transition is highly forbidden with an extremely small probability of 2.9×10⁻¹⁵ s⁻¹. This means that the time for a single atom of neutral hydrogen to undergo this transition is around 10 million (10⁷) years and so is unlikely to be seen in a laboratory on Earth. However, as the total number of atoms of neutral hydrogen in the interstellar medium is very large, this emission line is easily observed by radio telescopes.

The line has an extremely small natural width because of its long lifetime, so most broadening is due to doppler shifts caused by the motion of the emitting regions relative to the observer.

[edit] Discovery

During the 1930s, it was noticed that there was a radio 'hiss' that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to be coming from the centre of the Galaxy. These discoveries were published in 1940 and were seen by Professor J.H. Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Dr Hendrik van de Hulst who, in 1944, discovered that neutral hydrogen could produce radiation at a frequency of 1420.4058 MHz due to two closely spaced energy levels in the ground state of the hydrogen atom.

The 21cm line (1420.4058 MHz) was first observed in 1951 by Ewen and Purcell who were closely followed by Muller and Oort and Christiansen and Hindman. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way.

[edit] Uses in radio astronomy

Luckily, the spectral line appears within the radio spectrum (in the microwave window to be exact). Light in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.

Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a Hydrogen Line. The only difference between each of these lines is the doppler shift that each of these lines have. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm Hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.

Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the universal gravitational constant and to study dynamics of individual galaxies.