High mass stars go through a similar process to low mass stars in the beginning, except that it all happens much faster. They have a hydrogen fusion core, but much of the hydrogen fusion happens via the CNO cycle. After the hydrogen is exhausted, like low mass stars, a helium core with a hydrogen shell forms, then a carbon core, with helium and hydrogen shells. Then unlike low mass stars, they have enough mass that gravity contracts the core raising the temperature and carbon can fuse into neon, then neon into oxygen, then oxygen into silicon, then iron. Each stage of burning lasts a shorter time than the previous one. For example, in a 25 solar mass star, hydrogen burning would take about 7 × 106 years, helium burning 7 × 105 years, carbon burning 600 years, neon burning 1 year, oxygen burning 6 months and silicon burning one day. Once silicon has fused into iron, no more fusion occurs, as the fusion of iron requires more energy than it releases. The core therefore collapses and releases a huge amount of energy in an explosion called a supernova. In the centre of the debris from the explosion is an incredibly dense neutron star. If the star is massive enough, the neutron star will collapse further and form a black hole.