Core fusion If the star is massive enough, gravity can compress the core enough to create high enough temperatures, 100 million K, to start fusing helium, or temperatures of billions of Kelvin to fuse heavier elements if it is repeating this stage. In low mass stars (like the Sun), the onset of helium fusion can be very rapid, producing a burst of energy called a helium flash . Eventually the reaction rate settles down. Fusion in the core during this stage releases more energy/second than the core fusion of the main sequence stage, so the star is bigger, but stable! Hydrostatic equilibrium is restored until the core fuel runs out. Stars entering and leaving this stage can create conditions in their interiors that trap their radiated energy in their outer layers. The outward thermal pressure increases enough to expand the outer layers of the star. The trapped energy is able to escape when the outer layers are expanded and the thermal pressure drops. Gravity takes over and the star shrinks, but it shrinks beyond the
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