The primary physics task of STAR is to study the formation and characteristics of the quark gluon plasma (QGP), a state of matter believed to exist at sufficiently high energy densities. Detecting and understanding the QGP allows us to understand better the universe in the moments after the Big Bang, where the symmetries (and lack of symmetries) of our surroundings were put into motion.
Unlike other physics experiments where a theoretical idea can be tested directly by a single measurement, STAR must make use of a variety of simultaneous studies in order to draw strong conclusions about the QGP. This is due both to the complexity of the system formed in the high-energy nuclear collision and the unexplored landscape of the physics we study. STAR therefore consists of several types of detectors, each specializing in detecting certain types of particles or characterizing their motion. These detectors work together in an advanced data acquisition and subsequent physics analysis that allows final statements to be made about the collision.
The physics of STAR 
Scientists believe that, in the fractions of a second after the Big Bang, the expanding matter was so hot and dense that protons and neutrons couldn't exist yet. Instead, the early universe was composed of tiny quarks and gluons, which in today's cool universe are confined to exist only within other particles like protons. Collisions of heavy nuclei at sufficiently high energies allow us to explore whether quarks and gluons do in fact become deconfined when subjected to high densities, and if so, what the properties of this matter (a.k.a. quark-gluon plasma) are.
See also 
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