Hydrogen compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways that seldom involve elementary hydrogen. Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H, and is used when
Carbon life as we know it. The CNO cycle is an additional hydrogen fusion mechanism that powers stars, wherein carbon operates as a catalyst. Rotational transitions of various isotopic forms of carbon monoxide (for example, CO, CO, and CO) are detectable in the submillimeter wavelength range, and are used in the study of newly forming stars in molecular clouds. Under terrestrial conditions, conversion of one element to another is very rare. Therefore, the amount of carbon on Earth is effectively constant. Thus, processes that use carbon must obtain it from somewhere and dispose of it somewhere else. The paths of carbon