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X-Rays - Another Form of Light
Emission and Absorption Lines
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Chandra High Energy Transmission Grating Spectrum
of NGC 4151
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When a free-ranging electron is accelerated by the electric field of a proton or
charged atom (ion), the photons emitted can have a wide range of energies that
depends on how fast the electrons are moving and how much they are accelerated.
The distribution of photon energies due to this process is called a continuous
spectrum, and can be graphed as a smooth curve.
In contrast, if the electron is in orbit around the nucleus of a neutral or
charged atom, the spectrum is a series of sharp peaks, or lines. This happens
because the orbits of electrons in an atom are strictly regulated by the rules of
quantum theory. These orbits, or more accurately, energy states, are separated by
a specific amount of energy, just as stairs are separated by a specific height.
Just as you cannot move to a position between stair steps, an electron in an atom
cannot move to a position between energy states. The atoms for each element, such
as oxygen, carbons, etc., have their own unique sets of energy states.
Normally the electrons in atoms are in the lowest energy state, at the bottom of
the stairs. But if the atom has been excited by a collision with a free electron,
another atom, or a photon, the lowest energy level will be unoccupied. One of the
orbiting electrons will quickly jump down to this level, releasing energy in the
form of a photon of a specific energy. These photons give rise to an emission
line in the spectrum. A hot gas composed of many atoms will give off a spectrum
composed of many emission lines due to the various elements that are present in
the gas.
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Atomic Emission
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The opposite process can also occur. If a stream of photons encounters a gas,
those photons whose energy corresponds to energy levels in an atom a step will be
absorbed by the atom. This process gives rise to a series of absorption lines in
the gas.
Careful studies of the energies of the photons emitted or absorbed by an atom of
a particular element give a blueprint for the energy states of that atom. Knowing
this blueprint, or energy spectrum, astronomers can look for it in the radiation
from stars and gas, and determine the amount of each element present. In this
way, astronomers have determined that stars are mostly made of hydrogen, with a
mixture of helium and traces of heavier elements such as carbon, nitrogen,
oxygen, and so on.
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