![]() ![]() If the rate of fusion goes up, the rate at which energy is being generated also increases, and the luminosity of the star gradually rises. For the proton-proton cycle described in The Sun: A Nuclear Powerhouse, the rate of fusion goes up roughly as the temperature to the fourth power. As the temperature gets hotter, each proton acquires more energy of motion on average this means it is more likely to interact with other protons, and as a result, the rate of fusion also increases. When a star’s luminosity and temperature begin to change, the point that represents the star on the H–R diagram moves away from the zero-age main sequence.Ĭalculations show that the temperature and density in the inner region slowly increase as helium accumulates in the center of a star. This change of composition changes the luminosity, temperature, size, and interior structure of the star. It does, however, change the chemical composition in its central regions where nuclear reactions occur: hydrogen is gradually depleted, and helium accumulates. Since only 0.7% of the hydrogen used in fusion reactions is converted into energy, fusion does not change the total mass of the star appreciably during this long period. The zero-age main sequence is a continuous line in the H–R diagram that shows where stars of different masses but similar chemical composition can be found when they begin to fuse hydrogen. We use the term zero-age to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core. ![]() The left-hand edge of the main-sequence band in the H–R diagram is called the zero-age main sequence (see Figure \(18.4.1\) in Section 18.4). Some astronomers like to call the main-sequence phase the star’s “prolonged adolescence” or “adulthood” (continuing our analogy to the stages in a human life). Thus, all stars remain on the main sequence for most of their lives. Since hydrogen is the most abundant element in stars, this process can maintain the star’s equilibrium for a long time. Once a star has reached the main-sequence stage of its life, it derives its energy almost entirely from the conversion of hydrogen to helium via the process of nuclear fusion in its core (see The Sun: A Nuclear Powerhouse). We have already used the H–R diagram to follow the evolution of protostars up to the time they reach the main sequence. One of the best ways to get a “snapshot” of a group of stars is by plotting their properties on an H–R diagram. Describe what happens to main-sequence stars of various masses as they exhaust their hydrogen supply.\)īy the end of this section, you will be able to: ![]()
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