Color of the stars
A star's color is critical in identifying the star because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K. Stars are fuelled by the nuclear fusion reactions at their core. There is a dynamic equilibrium maintained throughout the star’s life between the expanding heat of the reactive core and gravitational forces holding the star together. Fusion produces extremely high energy. Fusion releases some of the energy that binds the particles of the nucleus together, unleashing remarkable power. Stars begin as a mass of dust and gas dense enough to start collapsing inwards under the pressure of its own gravity. If this protostar is massive enough, it will eventually initiate a nuclear reaction in its hot, dense core. This initiates the main sequence of a star’s life cycle when hydrogen forms helium at the star’s core through the process of nuclear fusion. Heat from the star’s core radiates outwards through the layers of the star to the photosphere, the visible surface, which emits electromagnetic energy and charged particles as the solar wind.
A star does not stay the same color throughout its lifecycle since the surface temperature alters depending on the type of fusion reaction fuelling the star at the time. Depending on the initial mass of the star, it will evolve along the lines of one of three main star types: low-mass stars, intermediate-mass stars (like our sun) and high-mass stars.