Hydrogen line

From Wikipedia, the free encyclopedia
Jump to: navigation, search
For hydrogen lines in general, see Hydrogen spectral series.

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,[1] which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength 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 of an electron between the two hyperfine levels of the hydrogen 1s ground state that have an energy difference of 5.87433 µeV.[2] The frequency, ν, of the quanta that are emitted by this transition between two different energy levels is given by the Planck-Einstein relation E = h \nu The constant of proportionality, h, is known as the Planck constant.


Further information: Hyperfine structure

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.[3] The transition has an energy difference of 5.87433 µeV that when applied in the Plank equation gives:

\lambda = \frac {1}{\nu} = \frac {h}{E} \times c =  \frac {4.1357\times 10^-15}{5.87433 \times 10^-6} \times 2.9979 \times 10^8 = .21106 m = 21.106 cm

where h, is known as the Planck constant and c is the speed of light.

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.



The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University,[4] and published after their data was corroborated by Dutch astronomers Muller and Oort,[5] and by Christiansen and Hindman in Australia. 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.


In radio astronomy[edit]



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 the dynamics of individual galaxies.

In cosmology[edit]


However, 21 centimeter observations are very difficult to make. Ground based experiments to observe the faint signal are plagued by interference from television transmitters and the ionosphere, so they must be made from very secluded sites care taken to eliminate interference. Space based experiments, even on the far side of the Moon (where would bed sheltered from interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other effects, such as synchrotron emission and free-free emission on the galaxy. Despite these problems, 21 centimeter observations, along with space-based gravity wave observations, are generally viewed as the next great frontier in observational cosmology, after the cosmic microwave background polarization.

In terrestrial remote sensing[dubious ][edit]

The Soil Moisture & Ocean Salinity (SMOS) satellite's main scientific instrument Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) uses the 1400-1427 MHz frequencies (including 1420.406 MHz) to monitor the ocean surface salinity and the soil moisture of the Earth. The choice of HI band results from: (1) much better radiative signature of salinity and moisture in microwave than in higher frequencies, and (2) no electromagnetic interference from anthropogenic sources, as HI is reserved for radioastronomy.

Possible uses for SETI[edit]



\pi \times 1420.40575177 \text{ MHz} = 4.46233627 \text{ GHz}


2 \pi \times 1420.40575177 \text{ MHz} = 8.92467255 \text{ GHz}

Since pi is a transcendental number, such a frequency couldn't possibly be produced in a natural way as a harmonic, and would clearly signify its artificial origin. Such a signal would not be overwhelmed by the HI line itself, or by any of its harmonics.[7]

See also[edit]


  1. ^ Darling, Jeremy (September 2004). "A Laboratory for Constraining Cosmic Evolution of the Fine‐Structure Constant: Conjugate 18 Centimeter OH Lines toward PKS 1413+135 at z = 0.24671" (PDF). The Astrophysical Journal 612 (1): 59. doi:10.1086/422450. Retrieved 6 October 2015. 
  4. ^ Ewan, H.I.; E.M. Purcell (September 1951). "Observation of a line in the galactic radio spectrum" (PDF). Nature 168 (4270): 356. Bibcode:1951Natur.168..356E. doi:10.1038/168356a0. Retrieved 2008-09-21. 
  5. ^ Muller, C.A.; J.H. Oort (September 1951). "The Interstellar Hydrogen Line at 1,420 Mc./sec., and an Estimate of Galactic Rotation" (PDF). Nature 168 (4270): 357–358. Bibcode:1951Natur.168..357M. doi:10.1038/168357a0. Retrieved 2008-09-21. 
  7. ^ Makovetsky P. Smotri v koren' (in Russian)



External links[edit]