Stellar Classification

As we know, there are countless(in essence) stars in our universe, so naturally, there will be noticeable differences between them. In order to investigate stars, we can use their spectra to find out more about them. A spectrum(singular of spectra) is simply the wavelengths of light that a star emits. Using those wavelengths, we can make many inferences on the differing nature of stars which effectively allows us to classify them. In earlier times, astronomers used to analyze the spectra of stars and believed the differences in spectra can be attributed to the chemical makeups of the stars. However, this isn't the case because most stars have very similar chemical compositions, including our Sun. The differences in lines can mainly be attributed to differences in surface temperatures between stars. Higher surface temperatures lead to the ionization of hydrogen, the most abundant element in stars.

To analyze spectral lines from an element, the energy levels of the lines correspond to the difference in energy levels for electrons orbiting the atomic nucleus of the element. If an electron goes from a low energy orbit to a high-energy orbit, it emits a photon with a wavelength corresponding to the difference in energy between levels. These photons have wavelengths characteristic of certain elements, allowing us to analyze the chemical elements in a star. Now,

Now, back to the ionization of hydrogen. A neutral hydrogen atom is 1 proton and 1 electron. If you ionize a hydrogen atom, the hydrogen ion is just a proton. Thus, there are no electrons to spectroscopically analyze, meaning that the hydrogen spectral lines from hotter stars darken.

Since stars differ because of this mechanism, it is apt to classify them via the differences in their surface temperatures. The following table lists the stellar classification types for stars: