The arrangement of a star's position in the temperature classification system (O, B, A, F, G, K, M), ranging from hot to cool.
In homological algebra, especially in algebraic topology or group cohomology, a spectral sequence is a means of computing homology groups by taking successive approximations. Spectral sequences are a generalization of exact sequences, and since their introduction by Jean Leray in 1946, they have become an important research tool, particularly in homotopy theory. However, they have a reputation for being abstruse and difficult to comprehend.