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 (http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_allsky1.html)
What is an eV?
An eV (short for electron Volt) is a unit astronomers often use to
measure energy. It corresponds to the energy needed to move an electron through
one Volt of potential. An electron Volt is well-liked by astronomers
because it is about the right amount of energy to affect individual
particles such as those found in space. For example, 13.6 eV of energy
will strip one electron off a Hydrogen atom. Such "singly ionized
Hydrogen" is very common throughout the Universe.
To give a better idea of how much energy an eV really is, it is useful
to know that air molecules at room temperature have an energy of
about 0.024 eV and air molecules at a million degrees have about 100 eV of
energy. Particles in a gas that emits in the X-rays (keV range) have
temperatures of hundreds of millions or even billions of degrees Celsius.
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