The Essentials of Thermodynamics and Heat Transfer:

Thermodynamics, as the study of energy, is a rather basic branch of
physics. It was developed mainly from practical studies of steam engines
in the 19th century; yet it applies to matters as esoteric as the
direction of time.


The Laws of Thermodynamics:

0th law: Temperature is defined as the average kinetic energy of the
molecules/atoms/particles making up a body.  (Heat is different: it's the
amount of energy a body has, due to the microscopic motion of its
particles.)

1st law: In an isolated, closed system, with no losses, energy is
conserved.  It cannot be created or destroyed from nothing; it can be
converted into other forms of energy.  (In 1905, Einstein amended this to
conservation of mass-energy, since E = mc^2.)

2nd law: In the absence of other energy inputs, heat by itself transfers
from a hot body to a cooler body.  Another way to say this is that the
amount of disorder, or entropy, in the Universe is always increasing. Of
course it is possible to make a refrigerator, in which it is cooler than
its surroundings, but notice: you have to plug the refrigerator in! 

3rd law: By no finite series of processes is absolute zero possible. 
(Long story.)


There are three methods of heat transfer:

	Conduction, from the motions of molecules/atoms/particles, for 
		example, when you touch a hot radiator;

	Convection, from macroscopic mass motions, for example, in the
		boiling water inside the hot radiator; and

	Radiation, by electromagnetic radiation, for example, when you
		hold your hand 30 cm from a hot radiator.

Conduction, while common in everyday phenomena on Earth, is not so
important in astrophysics: one of the few places it matters is the solar
corona.  This leaves convection and radiation.  In the center of the Sun,
it is too hot and dense for convection to occur.  It does become
convective further out, but becomes mostly radiative again near the
surface, since energy may escape here more easily.  (Again, this is a long
story, requiring calculus for a telling that doesn't feel like I have a
foot nailed to the floor.)