The roles of specific cosmic objects in producing these elemental abundances are clear for some elements, and heavily debated for others. For example, iron is believed to originate mostly from thermonuclear supernova explosions (also called supernovae of type Ia), and carbon and oxygen is believed to originate mostly from massive stars and their explosions. Lithium, beryllium, and boron are believed to originate from spallation reactions of cosmic-ray nuclei such as carbon and heavier nuclei, breaking these apart. Elements heavier than nickel are produced via the slow and rapid neutron capture processes, each contributing roughly half the abundance of these elements. The s-process is believed to occur in the envelopes of dying stars, whereas some uncertainty exists regarding r -process sites. The r -process is believed to occur in supernova explosions and compact object mergers, though observational evidence is limited to a single event, GW170817, and relative yields of proposed r-process sites leading to observed heavy element abundances are uncertain.