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. It is the fifth most common element in the Earth's crust. Calcium is commonly detected in stars, F-type star and G-type star showing ionized calcium, e.g., the Sun, which has strong absorption lines of singly ionized calcium (Ca II), including the H line (396.85 nm) and K line (393.37 nm). K-type star and M-type star often show neutral calcium (Ca I).