Red Giants And White Dwarfs
Red giants and white dwarfs come about because stars, like people, change with age and eventually die. For people, the cause of aging is the deterioration of biological functions. For a star, the cause is the inevitable energy crisis as it begins to run out of nuclear fuel.
Since its birth 4.5 billion years ago, the Sun’s luminosity has very gently increased by about 30%.3 This is an inevitable evolution which comes about because, as the billions of years roll by, the Sun is burning up the hydrogen in its core. The helium “ashes” left behind are denser than hydrogen, so the hydrogen/helium mix in the Sun’s core is very slowly becoming denser, thus raising the pressure. This causes the nuclear reactions to run a little hotter. The Sun brightens.
This brightening process moves along very slowly at first, when there is still ample hydrogen remaining to be burnt at the center of the star. But eventually, the core becomes so severely depleted of fuel that its energy production starts to fall regardless of the increasing density. When this happens, the density of the core begins to increase even more, because without a heat source to help it resist gravity, the only possible way the core can respond is by contracting until its internal pressure is high enough to hold up the weight of the entire star. Bizarrely, this emptying of the central fuel tank makes the star brighter, not dimmer, because the intense pressure at the surface of the core causes the hydrogen there to burn even faster. This more than takes up the slack from the fuel-exhausted center. The star’s brightening not only continues, it accelerates.