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. The Milky Way is about 0.5% carbon by mass.
It results from CNO cycle and the triple alpha process.
The carbon to oxygen ratio of stars is of interest in identifying systems similar to ours, i.e., planets Earth-like in composition.
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 seen in cooling clouds, being the result of fine structure atomic excitation from collisions, and escaping clouds easily, effectively turning thermal kinetic energy into escaping electromagnetic radiation. Thus they 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.