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Calcium (Ca) is the element with atomic number 20, symbol Ca. Its most common isotope has mass number 40, but 42, 43, and 44 are more stable and 46 is stable as well, and 48 virtually so. Calcium is the fifth most common element in the Earth's crust. It is commonly detected in stars, F-type stars and G-type stars showing ionized calcium, e.g., the Sun, which (among the Fraunhofer lines) has strong absorption lines of singly ionized calcium (Ca II), including the (Fraunhofer) H line (396.85 nm) and K line (393.37 nm). K-type stars and M-type stars often show neutral calcium (Ca I). Calcium is an alpha element, produced in early-type stars and their supernovae and much of it produced and spread in calcium-rich supernovae.