ALMA allows scientists to determine the radius and the rotation of stars and identify the chemical composition of dust being emitted from red supergiant stars (a star which is 10 to 1,000 times the diameter of our Sun, such as Betelgeuse).
One way in which a star is classified is by color (or spectrum). The stars were first classified in the 1890s by Annie Cannon. She used letters to classify the different spectra of light that the stars emit. At first the letters used ranged from A to Q, since then, astronomers have restricted and rearranged the classification index to the letters O, B, AF, G, K, M. Representing O the hottest stars and M the coldest ones.