The field of science that studies spectrum is called spectroscopy. One of the primary results from the field of spectroscopy was the discovery that the spectrum of an object changes with temperature. All objects emit energy in the shape of Planck's curve, where the amount and the peak energy vary only as the temperature of the body. The problem with Planck's curve is that it does not agree with Rutherford's model of the atom. Atoms absorb and emit light through a process called scattering.
Photons fly near the atoms and are deflected.
However, even the nature of atoms, the photons should receive more energy than the atoms, so there should be more and more blue photons, but clearly the Planck curve drops off at short wavelengths. This is called the UV catastrophe, which was resolved by quantum physics. The fact that the temperature and energy generation of an object can be determined by its Planck curve made the study of spectrum the key component to stellar astronomy. The amount of energy emitted from stars is determined by measuring their brightness or the amount of light they emit. This is called photometry.