All of the stars in the observable Universe, both large and small, live out their entire nuclear-fusing main-sequence “lives” by keeping a very delicate and necessary balance between two ancient foes–gravity and radiation pressure. The main-sequence refers to hydrogen-burning stars on the Hertzsprung-Russell Diagram of Stellar Evolution. Main-sequence stars still have enough nuclear-fusing hydrogen fuel to keep themselves bouncy against the crush of their own gravity.
The outward push of a star’s radiation pressure forces everything away from the star, while simultaneously gravity tries to mercilessly pull everything inward. The radiation pressure of a star is the result of the process of nuclear-fusion which commences with the burning of hydrogen, the lightest and most abundant atomic element in the Cosmos, into helium–which is the second lightest. This process of stellar nucleosynthesis continually fuses increasingly heavier and heavier atomic elements out of lighter ones. Indeed, all of the atomic elements heavier than helium–termed metals in the jargon of astronomers–formed within the searing-hot nuclear-fusing cores of the billions of stars inhabiting our vast Universe. Alternatively, the heaviest metals of all–such as gold and uranium–form in the spectacular supernova explosions heralding the death of a star.
Many supernovae are triggered when a lone, especially massive star runs out of its necessary supply of nuclear-fusing fuel, and rips itself apart in a spectacular core-collapse explosion. The progenitor of a core-collapse (Type II) supernova is usually a massive star that contains an extremely heavy core that weighs-in at about 1.4 times solar mass. Smaller stars normally do not die this way. In fact, smaller stars live much longer than more massive stars. This is because less massive stars are not as hot, and hence burn their fuel more slowly, than their heavier stellar kin. Small stars of our Sun’s mass generally last for about 10 billion years. More massive stars, however, live fast and die young–frequently living for millions (as opposed to billions) of years.
Like all stars, our Sun is doomed to run out of its necessary hydrogen fuel. It is a middle-aged star of about 4.56 billion years of age, and it can continue to fuse hydrogen in its core for another 5 billion years, or so.
When small stars, like our sun, finally have managed to fuse most of their necessary hydrogen fuel into heavier things, they first swell into glaring, bloated red giant stars. This evolved sun-like star at this late stage of development contains a worn-out heart composed of helium, surrounded by a shell in which there is still a small amount of hydrogen left to be burned into helium. This shell begins to travel outward, and the dying heart of the small star grows ever larger, as the star ages. At last, the helium heart begins to shrink. As it does so, its temperature soars at its center to the point that the helium is fused into the even heavier metal carbon. The star ends up with a very small, but extremely hot heart, that produces more energy that it did when it was still a hydrogen-burning star on the main-sequence. The star is now doomed, and its outer layers of gas are now swollen and red. The temperature at the glaring surface of this bloated red giant is cooler than it was when it was still a young star.