The spectrum of stars, although close to the spectrum of a black body, contains specific absorption lines that depend on the chemical composition, temperature, and pressure in the star's atmosphere. These absorption lines distinguish the stellar spectrum from the ideal black body spectrum and enable detailed classification of stars based on their spectral characteristics. Thus, stars can be grouped into different spectral types, from the hottest to the coolest, with each type having specific spectral lines that are key for identifying the elements present in the stellar atmosphere. The application of Planck's law to stellar radiation is described, providing an idealized model of black body radiation. While the actual stellar spectrum deviates from this model due to the presence of spectral lines, Planck's law remains a useful tool for estimating the surface temperature of stars. By applying this model, it is possible to determine the approximate temperature of a star based on the shape of its spectrum and to observe how temperature affects the color of the star. This paper provides an understanding of the connection between a star’s spectrum, its temperature, and its position on the Hertzsprung-Russell (H-R) diagram, emphasizing the importance of these tools in modern astrophysics.