11-2 Most stars shine throughout their lives by converting hydrogen into helium through nuclear fusion

When a protostar has evolved into a stable main-sequence star, regardless of mass, all stars are fundamentally similar. The fundamental energy source inside stars is the release of energy from the conversion of hydrogen into helium. When hydrogen atoms collide with each other at extremely high temperatures and pressures, they can combine to form larger helium atoms and release tremendous amounts of energy in the process. This process occurs in the very center of Sunlike stars.

The total time that a star will spend fusing hydrogen into helium in its core, and thus the total time that it will spend as a main-sequence star, is called its main-sequence lifetime. For our Sun, the main-sequence lifetime is about 12 billion (1.2 × 10¹⁰) years. Hydrogen fusion has been going on in the Sun’s core for the past 4.56 billion (4.56 × 10⁹) years, so our Sun is less than halfway through its main-sequence lifetime.

Stars Form in Clusters

Dark nebulae contain tens or hundreds of solar masses of gas and dust, enough to form many stars. As a consequence, these nebulae tend to form groups or clusters of young stars. One such cluster is M16, shown in Figure 11-10.

Figure 11-10: RIVUXG A Star Cluster with an H II Region The star cluster M16 is thought to be no more than 800,000 years old, and star formation is still taking place within adjacent dark, dusty clouds. The inset shows three dense, cold pillars of gas and dust silhouetted against the glowing background of the red emission nebula (called the Eagle Nebula for its shape). The pillar at the upper left extends about 0.3 pc (1 ly) from base to tip, and each of its “fingers” is somewhat broader thanour entire solar system.

In addition to being objects of great natural beauty, star clusters give us a unique way to compare the evolution of stars with differing masses, because clusters typically include stars with a range of different masses, all of which began to form out of the parent nebula at roughly the same time. Thus, they have roughly the same chronological age and similar initial chemical composition.

All the stars in a cluster may begin to form nearly simultaneously, but they do not all become main-sequence stars at the same time. As you can see from their evolutionary tracks (see Figure 11-7), high-mass stars evolve more rapidly than low-mass stars. Consistently, the more massive the protostar, the sooner it develops the central pressures and temperatures needed for steady hydrogen fusion to begin. As we shall see, stars in the same cluster also do not age at the same rate nor do they leave the main sequence at the same time.

Figure 11-11a shows a young star cluster called the Pleiades. The photograph shows gas that must have initially surrounded this cluster but has since dissipated into interstellar space, leaving only traces of dusty material that forms nebulae around the cluster’s stars. This implies that that the Pleiades must be older than NGC 2264, the cluster shown in Figure 11-12a, which is still surrounded by a nebula. The H-R diagram for the Pleiades in Figure 11-11b bears out this idea. In contrast to the H-R diagram for NGC 2264 (Figure 11-12b), nearly all the stars in the Pleiades are on the main sequence. The cluster’s age is about 100 million years, which is how long it takes for the least massive stars to finally begin hydrogen fusion in their cores.

Figure 11-11: The Pleiades and Its H-R Diagram (a) The Pleiades star cluster is 117 pc (380 ly) from Earth in the constellation Taurus and can be seen with the naked eye. (b) Each dot plotted on this H-R diagram represents a star in the Pleiades whose luminosity and surface temperature have been measured.

(Note: The scales on this H-R diagram are different from those in Figure 11-12b.) The Pleiades is about 100 million (10 × 10⁸) years old. (NASA, ESA and AURA/Caltech)

Figure 11-12: A Young Star Cluster and Its H-R Diagram (a) Newborn stars, hidden behind thick dust, are revealed in this image of the NGC 2264 from NASA’s Spitzer Space Telescope. These infant stars appear to have formed in regularly spaced intervals along linear structures in a configuration like the pattern of a snowflake. The youngest protostars form straight lines and have yet to scatter from their location of formation. (b) Each dot plotted on this H-R diagram represents a star in NGC 2264, whose luminosity and surface temperature have been determined. This star cluster probably started forming only 2 million years ago.

Go to Video 11-3

What happens to a star like the Sun after the core hydrogen has been used up, so that it is no longer a main-sequence star? As we will see, it expands dramatically to become a red giant. To understand why this happens, it is useful to first look at how a star evolves during its main-sequence lifetime. The nature of that evolution depends on whether its mass is less than or greater than about 0.4 M_⊙.

Main-Sequence Stars of 0.4 M[sub]⊙[/sub] or Greater: Consuming Core Hydrogen

A protostar becomes a main-sequence star when steady hydrogen fusion begins in its core and it achieves a stable balance between the inward force of gravity and the outward pressure produced by hydrogen fusion. From the point in time a star has stabilized its core hydrogen fusion, a star slowly, but predictably, undergoes noticeable changes in luminosity, surface temperature, and radius during its main-sequence lifetime. These changes are a result of hydrogen fusion in the core, which alters the chemical composition of the core. As an example, when our Sun first formed, its composition was the same at all points throughout its volume: by mass, about 74% hydrogen, 25% helium, and 1% heavy elements. But as Figure 11-13 shows, the Sun’s core now contains a greater mass of helium than of hydrogen. (There is still enough hydrogen in the Sun’s core for another 5.4 billion years as a main-sequence star.)

Figure 11-13: Changes in the Sun’s Chemical Composition These graphs show the percentage by mass of (a) hydrogen and (b) helium at different points within the Sun’s interior. The dashed horizontal lines show that these percentages were the same throughout the Sun’s volume when it first formed. As the solid curves show, over the past 4.56 × 10⁹ years, thermonuclear reactions at the core have depleted hydrogen in the core and increased the amount of helium in the core.

Thanks to hydrogen fusion in the core, the total number of atomic nuclei in a star’s core decreases with time: In each reaction, 4 hydrogen nuclei are converted to a single helium nucleus. With fewer particles bouncing around to provide the core’s internal pressure, the core contracts slightly under the weight of the star’s outer layers. Compression makes the core denser and increases its temperature. As a result of these changes in density and temperature, the pressure in the compressed core is actually higher than before.

As the star’s core shrinks, its outer layers expand and shine more brightly. Here’s why: As the core’s density and temperature increase, hydrogen nuclei in the core collide with one another more frequently, and the rate of hydrogen fusion in the core increases. Hence, the star’s luminosity increases. The radius of the star as a whole also increases slightly, because increased core pressure pushes outward on the star’s outer layers. The star’s surface temperature changes as well, because it is related to the luminosity and radius. As an example, theoretical calculations indicate that over the past 4.56 × 10⁹ years, our Sun has become 40% more luminous, grown in radius by 6%, and increased in surface temperature by 300 K (Figure 11-14).

Figure 11-14: The Zero-Age Sun and Today’s Sun Over the past 4.56 × 10⁹ years, much of the hydrogen in the Sun’s core has been converted into helium, the core has contracted a bit, and the Sun’s luminosity has gone up by about 40%. These changes in the core have made the Sun’s outer layers expand in radius by 6% and increased the surface temperature from 5500 K to 5800 K.

As a main-sequence star ages and evolves, the increase in energy outflow from its core also heats the material immediately surrounding the core. As a result, hydrogen fusion can begin outside the core in this surrounding material. By tapping this fresh supply of hydrogen, a star manages to eke out a few million more years of main-sequence existence.

Main-Sequence Stars of Less Than 0.4 M[sub]⊙[/sub]: Consuming All Their Hydrogen

The story is somewhat different for the smallest and least massive main-sequence stars, with masses between 0.08 M_⊙ (the minimum mass necessary for sustained thermonuclear reactions to take place in a star’s core) and about 0.4 M_⊙. These stars, of spectral class M, are called red dwarfs because they are small in size and have a red color due to their low surface temperature. They are also very numerous; about 85% of all stars in the Milky Way Galaxy are red dwarfs.

In a red dwarf, helium does not accumulate in the core to the same extent as in the core of a larger star, like our Sun. The reason is that in a red dwarf there are convection cells of rising and falling gas that extend throughout the star’s volume and penetrate into the core. These convection cells drag helium outward from the core and replace it with hydrogen from the outer layers (Figure 11-15). The fresh hydrogen can undergo thermonuclear fusion that releases energy and makes additional helium. This helium is then dragged out of the core by convection and replaced by even more hydrogen from the red dwarf’s outer layers.

Figure 11-15: A Fully Convective Red Dwarf In a red dwarf—a main-sequence star with less than about 0.4 solar masses helium (He) created in the core by thermonuclear reactions is carried to the star’s outer layers by convection. Convection also brings fresh hydrogen (H) from the outer layers into the core. This process continues until the entire star is helium.

As a consequence, over a red dwarf’s main-sequence lifetime essentially all of the star’s hydrogen can be consumed and converted to helium. The core temperature and pressure in a red dwarf is less than in the Sun, so thermonuclear reactions happen more slowly than in our Sun. Calculations indicate that it takes hundreds of billions of years for a red dwarf to convert all of its hydrogen to helium. The present age of the universe is only 13.7 billion years, so there has not yet been time for any red dwarfs to become pure helium.