Modern instruments can measure very precisely the color of a star. This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths. The hottest stars are blue and violet, then white, then yellow, and the coolest are red. Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it. 