In this exercise, you will study the method that astronomers use to classify stars, which is called the MK spectral classification system, after the founders, W. W. Morgan and P. C. Keenan. A spectrum of a star is composed of its continuum emission as well as a number of 'lines' which can be either emission or absorption lines. The continuum emission is a product of the blackbody radiation at specific frequencies, caused by electrons in atoms dropping down into lower energy levels. They can also be caused by molecular transitions to lower energy levels. This sort of line appears brighter compared to the region of the spectrum around it.
Absorption lines cause dark features in the continuum emission where the radiation is removed.
This process causes the lines to look darker when compared to the region of the spectrum around them. Stars come in a wide range of sizes and temperatures. The hottest stars in the sky have temperatures in excess of 40,000 K, whereas the coolest stars that we can detect optically have temperatures on the order of 2,000-3,000 K. As you might guess, the appearance of the spectrum of a star is very strongly dependent on its temperature. They built on the Harvard classification system, developed by Annie Jump Cannon and her associates. Each class can be divided into subtypes running from 0 to 9.