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Carbon (C) is the element with atomic number 6, symbol C. Its usual isotope has mass number 12, but mass number 13 is stable and 14 has a 5730-year half-life. It is the fourth most common element in the universe: Milky-Way baryonic matter is on the order of 0.5% carbon by mass. Carbon nuclei are synthesized by the CNO cycle and the triple alpha process.
The carbon forbidden lines [CII] (e.g., 1900 GHz, 158 μm) of singly-ionized carbon (CII, i.e., C+) and [CI] (e.g., 492 GHz) of neutral carbon (CI) are observed in cooling clouds, being the result of fine structure atomic excitation from collisions: the resulting photons have a high probability of escaping the clouds, effectively turning thermal kinetic energy into outgoing electromagnetic radiation. Thus they are used, along with carbon monoxide spectral lines, to locate molecular clouds. The former maps to a microwave atmospheric window at high redshifts, thus has been proposed for mapping the high-redshift universe.
The carbon to oxygen ratio of stars and protoplanetary disks is of interest in identifying systems similar to ours, i.e., regarding the potential for planets with Earth-like composition. A substantially higher such ratio could lead to rocky planets significantly different than those of the solar system.