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The transition in primary energy production from one form to the other spans a range difference of less than a single solar mass. In the Sun, a one solar-mass star, only 1.5% of the energy is generated by the CNO cycle. [32] By contrast, stars with 1.8 M ☉ or above generate almost their entire energy output through the CNO cycle. [33]
This core convection occurs in stars where the CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass. [25] The Sun produces on the order of 1% of its energy from the CNO cycle.
A self-maintaining CNO chain starts at approximately 15 × 10 6 K, but its energy output rises much more rapidly with increasing temperatures [1] so that it becomes the dominant source of energy at approximately 17 × 10 6 K. [4] The Sun has a core temperature of around 15.7 × 10 6 K, and only 1.7% of 4 He nuclei produced in the Sun are born ...
Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution A mass-radius plot ...
For stars at 1.5 M ☉ where the core temperature reaches 18 MK, half the energy production comes from the CNO cycle and half from the pp chain. [5] The CNO process is more temperature-sensitive than the pp chain, with most of the energy production occurring near the very center of the star.
Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr −1. Many LIRGs also emit radiation from an AGN. [135] [136] Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns.
The kinetic energy of an expanding supernova remnant can trigger star formation by compressing nearby, dense molecular clouds in space. [213] The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.
Many of the well-known bright stars are red giants because they are luminous and moderately common. The K0 RGB star Arcturus is 36 light-years away, and Gacrux is the nearest M-class giant at 88 light-years' distance. A red giant will usually produce a planetary nebula and become a white dwarf at the end of its life.