Dense matter in compact stars

The purpose and motivation of these lectures can be summarized in the following
two questions:
• Whatis the ground state (and its properties) of dense matter?
• Whatis the matter composition of a compact star?
Thetwoquestions are, of course, strongly coupled to each other. Dependingon your
point of view, you can either consider the first as the main question and the second
as a consequence or application of the first, or vice versa.
If you are interested in fundamental questions in particle physics you may take
the formerpoint ofview: youask the questionwhathappensto matter if yousqueeze
it more and more. This leads to fundamentalquestions because at some level of suf
f
icient squeezing you expect to reach the point where the fundamental degrees of
freedom and their interactions become important. That is, at some point you will
reach a form of matter where not molecules or atoms, but the constituents of an
atom, namely neutrons, protons, and electrons, are the relevant degrees of freedom.
This form of matter, and its variants, constitute one important topic of these lec
tures and is termed nuclear matter. If you squeeze further, you might reach a level
where the constituents of neutrons and protons, namely quarks and gluons, become
relevant degrees of freedom. This form of matter, termed quark matter or strange
quark matter, is the second important subject we shall discuss. By studying dense
matter, we shall thus learn a lot about the fundamental theories and interactions of
elementary particles. When trying to understand this kind of dense matter, we would
like to perform experiments and check whether our fundamental theories work or
whether there are new phenomena, or maybe even new theories, that we have not
included into our description. Unfortunately, there are currently no experiments on
earth which can produce matter at such ultra-high densities we are talking about.
However, this does not mean that this kind of matter does not exist in nature. On
the contrary, we are pretty sure that we have observed objects that contain matter
at ultra-high density, namely compact stars. We may thus consider compact stars
as our “laboratory”. Thinking about the first question has therefore led us to the
second.