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: Milky-Way baryonic matter is on the order of 0.5% carbon by mass. Carbon results from the CNO cycle and the triple alpha process.
The carbon forbidden lines [CII] (e.g., 1900GHz, 158um) of singly-ionized carbon (CII, i.e., C+) and [CI] (e.g., 492GHz) 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 is of interest in identifying systems similar to ours, i.e., planets with Earth-like composition.