Today, our Sun is still sufficiently youthful and bouncy to go on burning hydrogen in its heart by way of nuclear fusion–which continually creates heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). But our Sun’s looks will change when it finally begins to run out of its necessary supply of hydrogen fuel. At this point, our Sun will evolve into an elderly star. In the dying heart of our Sun, there will exist a core of helium that is encased within a shell composed of hydrogen that is still in the process of being fused into helium. Eventually, the shell will begin to travel outward, and the core will grow larger as our Sun grows older.
The helium core itself will ultimately shrivel under the intense pull of its own weight–until, finally, it grows seething-hot enough to trigger a new stage of nuclear-fusion. At this point, the atoms existing in our dying Star’s helium core will begin to be fused into the heavier atomic element, carbon. In another five billion years, our Star will possess a small and searing-hot core that will be emitting more energy than it does now. The outer gaseous layers of our doomed Sun will have ballooned to hideous proportions, and it will no longer be the same Star we are familiar with today. Alas, it will have experienced a sea-change into a ghastly red giant that will go on to engulf and incinerate Mercury, then Venus, and then (possibly) Earth.
The temperature at the surface of our Star, in its future red giant phase, will be quite a bit cooler than it is now–which will account for its comparatively cool red hue. Nonetheless, our evolved Star will still be hot enough to convert the currently frigid, icy inhabitants of the distant Kuiper Belt–such as the dwarf planet Pluto–into delightful tropical havens of refuge for what may (or may not) be left of humanity. The dying hot heart of our Sun will continue to shrivel, and since it can no longer produce radiation by way of nuclear fusion, all further evolution will be governed only by the force of gravity.
Our Star will then finally hurl off its outer layers–but its core will stay in one piece. All of our Sun’s material will finally collapse into this small relic body–the newly-formed white dwarf. The baby white dwarf will be encircled by a lovely expanding shell of multicolored gases termed a planetary nebula. White dwarfs radiate away the energy of their collapse, and our future white dwarf Sun will likely be composed of carbon and oxygen atomic nuclei swimming around in a swirling sea of degenerate electrons. The equation of state for degenerate matter is “soft”. This means that any mass added to the body will cause it to grow smaller in size. Continuing to add mass to a white dwarf causes it to shrink further, even as its central density grows larger. Our Sun’s radius will finally shrink to only a few thousand kilometers. Our Sun, and other stars like it, will grow progressively cooler over time when they evolve into white dwarfs.