Some stars are extremely hot, while others are cool. You can tell by the color of light that the stars give off. If you look at the coals in a charcoal grill, you know that the red glowing coals are cooler than the white hot ones. The same is true for stars. A blue or white star is hotter than a yellow star, which is hotter than a red star. So, if you look at the strongest color or wavelength of light emitted by the star, then you can calculate its temperature (temperature in degrees Kelvin = 3 x 106/ wavelength in nanometers). A star's spectrum can also tell you the chemical elements that are in that star because different elements (for example, hydrogen, helium, carbon, calcium) absorb light at different wavelengths.