The internal heating within stars is so great that (after an initial phase of gravitational contraction) they ignite and sustain thermonuclear reaction of hydrogen (with itself) to form helium, and can make heavier elements (see Stellar nucleosynthesis). The Sun for example has a core temperature of 13,600,000 K. The more massive and older the stars are, the more internal heating they have. During the end of its lifecycle, the internal heating of a star increases dramatically, caused by the change of composition of the core as successive fuels for fusion are consumed, and the resulting contraction (accompanied by faster consumption of the remaining fuel). Depending upon the mass of the star, the core may become hot enough to fuse helium (forming carbon and oxygen and traces of heavier elements), and for sufficiently massive stars even large quantities of heavier elements. Fusion to produce elements heavier than iron and nickel no longer produces energy, and since stellar cores massive enough to attain the temperatures required to produce these elements are too massive to form stable white dwarf stars, a core collapse supernova results, producing a neutron star or a black hole, depending upon the mass. Heat generated by the collapse is trapped within a neutron star and only escapes slowly, due to the small surface area; heat cannot be conducted out of a black hole at all (however, see Hawking radiation).