Oh! Be A Fine Girl — Kiss Me!

This phrase is a mnemonic device used by students and astronomers to learn the spectral classification of stars developed by Annie Jump Cannon.

So what does this classification sequence mean?

It is mainly based on temperature. The O stars are the bluest and hottest stars, and the M stars are the coolest red stars. The stars are the beginning of the sequence are traditionally called “early-type” stars and stars at the end are called “late type” stars. With in each type there are subdivisions by taking the letter and adding a decimal digit, such as B0, B1, ... B9 stars. As we know from “Understanding Her Work” different temperatures cause the distinction in star spectra due to electrons occupying different orbitals in atoms of the atmospheres of the stars. Patterns of spectral lines are also related to the composition of the stars and can be quite complicated. The following two tables summarize the main differences among the spectra and stars representative of the different spectral classes:

table of characteristics of spectra in each class

from: Carroll & Ostlie, An Introduction to Modern Astrophysics; Reading Massachusetts: Addison-Wesley Publishing Company, 1996.

table of characteristics of stars in each spectral class

from: Chaisson & McMillian, Astronomy Today, 2nd Edition; Upper Saddle River, New Jersey: Prentice Hall, 1996.