21-centimetre radiation,
electromagnetic radiation
of radio wavelength emitted by cold, neutral, interstellar hydrogen
atoms. The hydrogen atom is composed of a positively charged
particle, the proton, and a negatively charged particle, the
electron. These particles can be considered as spinning around
their own axes of rotation. When the spins of the two particles
are antiparallel, then the atom is in its lowest energy state.
When the spins are parallel, the atom has a tiny amount of extra
energy. In the very cold space between the stars, the interstellar
hydrogen atoms are at a state of lowest possible energy. Collisions
between particles, however, can at times excite some atoms (which
makes the spin of the particles parallel), giving them a tiny
amount of energy. According to the rules of quantum mechanics,
such atoms radiate their acquired energy in the form of low-energy
photons that correspond to a wavelength of 21 centimetres, or
a frequency of 1,420 megacycles. This radio radiation was theoretically
predicted by the Dutch astronomer H.C. van de
Hulst soon after the
end of World War II and was experimentally detected by Harold
Ewen at Harvard University in 1951. Since that
time, 21-centimetre hydrogen emission has come to play a vital
role in the study of the
Milky Way Galaxy, because
it readily penetrates the clouds of interstellar dust particles
that obstruct optical observations deep into the galactic centre.
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