universe beyond our Earth is a study in contrasts of temperature.
The center of our sun reaches 40 million degrees, and the fusion
reactions in larger stars, discussed in Chapter 8, can reach 2 billion
degrees or more. At the other end of the scale, temperature has
little meaning in nearly empty space. If we define temperature in
terms of the average kinetic energy of molecules, what does temperature
mean in a region of outer space that has only one or two atoms per
cubic centimeter? Temperature in a vacuum also can be defined in
terms of the radiation passing through it, compared with the radiation
from a perfectly non-refelective black body of measurable temperature.
Interstellar space is filled with microwave radiation in the millimeter
wavelength range, corresponding to a black-body temperature of only
3K by this
definition. Theoreticians have proposed that this radiation is the
last remnant of the primeval "big bang" fireball with which the
universe began 15 billion years ago.
The chemistry that we know on Earth is confined to a minute span
within this broad range of temperature. At sufficiently high temperatures,
electrons are stripped from atoms, and matter exists only as an
ionized plasma of electrons and nuclei. In gases the temperature
is high enough to overcome the attractions between molecules. In
plasmas the temperature is so high that even the attractions between
nuclei and electrons are overcome, and neutral atoms no longer exist.