by Mary Hrovat

When I was 17, I took an introductory course in physical geology at a community college. I was enchanted by the descriptions of the physical processes that created land forms, and also by the vocabulary: eskers and drumlins, barchan dunes, columnar basalt. I like to know how things form and what they’re called. My strongest memory of this class, though, centers on the final lecture. The professor put Earth and its landforms and minerals in a larger context. He told us about the life cycles of stars, which have produced most of the elements on Earth.
The central fact of the lecture was that the mass of a star is a key characteristic determining how long it exists and what happens as it ages. Stars are formed when gravity causes a portion of a gas cloud to collapse until its internal pressure, and thus its temperature, are high enough for nuclear fusion to begin. The energy released when, for example, two hydrogen nuclei fuse to form helium supports the mass of the star against the pull of gravity. A star’s life unfolds as a story of the equilibrium (or loss of equilibrium) between these two forces pulling inward and pushing outward. As one fuel source is depleted (for example, as hydrogen is converted to helium) other types of fusion occur in the core of the star using the new fuel source (and creating increasingly heavier elements). At the same time, hydrogen continues to fuse in a shell surrounding the core. Mature stars may have shells dominated by various elements undergoing different fusion reactions, although the available reactions depend on the mass of the star.
The professor probably described stars according to their type. I don’t remember if he mentioned the Hertzsprung–Russell diagram, although it seems likely that he would have. The HR diagram plots the luminosity of stars (the amount of light they emit) versus their temperature. When stars are plotted this way, most of them fall on a curve called the Main Sequence, which runs from hot blue stars to cool red stars along the sequence O-B-A-F-G-K-M. (In some HR diagrams, the stellar type or color is plotted on the horizontal axis as a proxy for temperature.) Stars remain on the Main Sequence as long as the gravitational and thermal forces are in equilibrium. The larger and hotter a star is, the shorter its time on the Main Sequence, because hotter stars consume their fuel more rapidly.
As they age and leave the Main Sequence, stars undergo different processes depending on their size. The universe is still too young for the very smallest stars to have exhausted their fuel, so they’re still on the Main Sequence. Stars with masses ranging from slightly less than that of the sun to 10 times the mass of the sun go through a red giant phase, ultimately undergoing core collapse and forming dense white dwarfs. Larger stars have more complicated end-of-life scenarios, typically exploding in supernovae and leaving behind superdense neutron stars or black holes. Some elements are created only in supernova explosions. Read more »