text: Carbon is a necessary component of all known life. C, a stable isotope of carbon, is abundantly produced in stars due to three factors:
  # The decay lifetime of a Be nucleus is four orders of magnitude larger than the time for two He nuclei (alpha particles) to scatter.
  # An excited state of the C nucleus exists a little (0.3193 MeV) above the energy level of Be + He. This is necessary because the ground state of C is 7.3367 MeV below the energy of Be + He; a Be nucleus and a He nucleus cannot reasonably fuse directly into a ground-state C nucleus. However, Be and He use the kinetic energy of their collision to fuse into the excited C (kinetic energy supplies the additional 0.3193 MeV necessary to reach the excited state), which can then transition to its stable ground state. According to one calculation, the energy level of this excited state must be between about 7.3 MeV and 7.9 MeV to produce sufficient carbon for life to exist, and must be further "fine-tuned" to between 7.596 MeV and 7.716 MeV in order to produce the abundant level of C observed in nature. The Hoyle state has been measured to be about 7.65 MeV above the ground state of C.
  # In the reaction C + He →  O, there is an excited state of oxygen which, if it were slightly higher, would provide a resonance and speed up the reaction. In that case, insufficient carbon would exist in nature; almost all of it would have converted to oxygen.
  Some scholars argue the 7.656 MeV Hoyle resonance, in particular, is unlikely to be the product of mere chance. Fred Hoyle argued in 1982 that the Hoyle resonance was evidence of a "superintellect"; Leonard Susskind in The Cosmic Landscape rejects Hoyle's intelligent design argument. Instead, some scientists believe that different universes, portions of a vast "multiverse", have different fundamental constants: according to this controversial fine-tuning hypothesis, life can only evolve in the minority of universes where the fundamental constants happen to be fine-tuned to support the existence of life. Other scientists reject the hypothesis of the multiverse on account of the lack of independent evidence.