Astronomy — The 21-centimeter radiation
Radio telescopes are important because they can detect the 21-cm wavelength radiation emitted by clouds of cool hydrogen in space. This radiation is emitted when a hydrogen atom’s electron changes its energy by changing the direction of its spin.
Quantum mechanics tell us that particles like protons and electrons have a property akin to the spinning of a top. This spin creates a magnetic field around the particle like that produced when electricity flows through a coil of wire in an electromagnet. These magnetic fields can alter the binding energy of the hydrogen atom, and this is what produces the 21-cm radiation.
The proton and electron in a hydrogen atom can spin in one of two ways. They either spin in the same direction or they spin in the the opposite direction. If they are spinning such that their magnetic fields help hold the atom together, then the electron is more tightly bound. If they spin the other way, the magnetic field lightly opposed the bonding of the atom.
This means that he ground state of the hydrogen atom is really two states that differ from each other according to the spin of the proton and electron. If we could look at the energy levels of a hydrogen atom with a powerful magnifying glass, we could find the ground state split into two levels that differ by a very small amount of energy. If the electron is in the higher level, it can spontaneously flip over and spin the other way, changing its magnetic field to the lower energy state and thus dropping to the lower energy level. The atom gives up this extra energy by emitting a photon. The two energy levels are so close together that the proton must have a very low energy –corresponding, in fact, to a wavelength of 21 centimeters.
Only cold, low-density clouds of hydrogen produce 21-cm radiation. If the gas is warm and dense, the atoms collide so often that the electrons are never in the ground state long enough to flip their direction of spin and emit a 21-cm photon. Thus radio telescopes are not confused by hot hydrogen in stars and can use the 21-cm radiation to map the clouds of cold hydrogen between the stars.
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