A star of 1 solar mass remains there for roughly 10 billion years, while a star of about 0.4 solar mass has a main-sequence lifetime of some 200 billion years, which is longer than the current age of the universe. This table shows that the most massive stars spend only a few million years on the main sequence. The main-sequence lifetimes of stars of different masses are listed in Table 22.1. Like new rock stars with their first platinum album, they spend their resources at an astounding rate. You can also understand now why the most massive main-sequence stars are also the most luminous. Although massive stars have more fuel, they burn it so prodigiously that their lifetimes are much shorter than those of their low-mass counterparts. The higher the temperature in the central regions, the faster the star races through its storehouse of central hydrogen. Higher pressure, in turn, is produced by higher temperature. And what determines how hot a star’s central regions get? It is the mass of the star-the weight of the overlying layers determines how high the pressure in the core must be: higher mass requires higher pressure to balance it. The reason massive stars are such spendthrifts is that, as we saw above, the rate of fusion depends very strongly on the star’s core temperature. This is why many lottery winners who go on spending sprees quickly wind up poor again.) In the case of stars, more massive ones use up their fuel much more quickly than stars of low mass. (In the same way, how long people can keep spending money depends not only on how much money they have but also on how quickly they spend it. The lifetime of a star in a particular stage of evolution depends on how much nuclear fuel it has and on how quickly it uses up that fuel. You might think that a more massive star, having more fuel, would last longer, but it’s not that simple. How many years a star remains in the main-sequence band depends on its mass. The temperature would increase by a factor of 256 0.25 (that is, the 4 th root of 256), or 4 times. Initially, however, these changes are small, and stars remain within the main-sequence band on the H–R diagram for most of their lifetimes. 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 21.12). 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.By the end of this section, you will be able to:

25m series main sequence