Introduction

Abstract

Hot plasmas appeared in our universe only a few hundreds of millions of years after its birth, according to Big Bang theoreticians. Since then, active stars have been emitting light and generating chemical elements through fusion processes, that produce increasingly heavier nuclei as the stars pass through their life cycles. Hot plasmas consist of ionized atoms and free electrons at high temperatures. In the center of the Sun, for example, the temperature exceeds ten million Kelvin. These high temperatures allow nuclei to get close enough together to fuse. The far-UV photons produced cannot be observed, because that energy is successively absorbed and re-emitted, at various increasing wavelengths, by the superposed gas shells of the Sun. Ultimately, what we see is some visible light escaping from the solar surface.