Re: How do we know stars are made of plasma ?

Plasmas are gases made up of charged particles. Astronomers can determine the outside/surface temperture of a star by analyzing its starlight (spectrum) and we find that the outside temperatures of stars always lie between 2500 and 100,000 K (4500-180,000 F). These tempertures have to increase inward from the surface and reach tens to millions of degrees at the center. At these temperatures the gases (hydrogen, helium and a little bit of all other elements) are ionized, consisting of negatively charged electrons and positively charged nuclei. Sucn a gas is called a plasma. Astronomers determine the interior structure of a star (that is the temperature, density and pressure at differents points inside the star) by mathematical models. These models explain the observations astronomers can make of stars, such as the observed oscillations of the sun.

REFERENCES: any introductory astronomy text book discusses stellar models and how they are made, for example:
Foundations of Astronomy, 4^th ed, by Michael Snow, p. 243
There is also a nice website which calculates stellar models for you (the diagrams have very little explanations with them).