19-2 When core hydrogen fusion ceases, a main-sequence star like the Sun becomes a red giant

Like so many properties of stars, what happens at the end of a star’s main-sequence lifetime depends on its mass. If the star is a red dwarf of less than about 0.4 M_⊙, after hundreds of billions of years the star has converted all of its hydrogen to helium. It is possible for helium to undergo thermonuclear fusion, but this requires temperatures and pressures far higher than those found within a red dwarf. Thus, this red dwarf will end its life as a ball of helium, which has no further nuclear reactions but still glows due to its internal heat. As it radiates energy into space, it slowly cools and shrinks. This slow, quiet demise is the ultimate fate of the red dwarfs, which are 85% of the stars in the Milky Way. (As we have seen, there has not yet been time in the history of the universe for any red dwarf to reach this final stage in its evolution.)

What is the fate of stars more massive than about 0.4 M_⊙, including the Sun? As we will see, the late stages of their evolution are far more dramatic. Studying these stages will give us insight into the fate of our solar system and of life on Earth.

Stars of 0.4 M or Greater: From Main-Sequence Star to Red Giant

When a star of at least 0.4 solar masses reaches the end of its main-sequence lifetime, all of the hydrogen in its core has been used up and hydrogen fusion ceases there. In this new stage, hydrogen fusion continues only in the hydrogen-rich material just outside the core, a situation called shell hydrogen fusion. At first, this process occurs only in the hottest region just outside the core, where the hydrogen fuel has not yet been exhausted. Outside this region, no fusion reactions take place.

Strangely enough, the end of the core hydrogen fusion process leads to an increase in the core’s temperature. Here’s why: When thermonuclear reactions first cease in the core, nothing remains to generate heat there. Hence, the core starts to cool and the pressure in the core starts to decrease. This pressure decrease allows the star’s core to again compress under the weight of the outer layers. As the core contracts, its temperature again increases, and heat begins to flow outward from the core even though no nuclear reactions are taking place there. (Technically, gravitational energy is converted into thermal energy, as in Kelvin-Helmholtz contraction; see Section 8-4 and Section 16-1).

This new flow of heat warms the gases around the core, increasing the rate of shell hydrogen fusion and causing the shell of fusion to “eat” further outward into the surrounding matter. Helium produced by reactions in the shell falls down onto the core, which continues to contract and heat up as it gains mass. Over the course of hundreds of millions of years, the core of a 1-M_⊙ star compresses to about one-third of its original radius, while its central temperature increases from about 15 million (1.5 × 10⁷) K to about 100 million (10⁸) K.

Figure 19-4: Red Giants (a) The present-day Sun produces energy in a hydrogen-fusing core about 100,000 km in diameter. Some 7.6 billion years from now, when the Sun becomes a red giant, its energy source will be a shell only about 30,000 km in diameter within which hydrogen fusion will take place at a furious rate. The Sun’s luminosity will be about 2000 times greater than today, and the increased luminosity will make the Sun’s outer layers expand to approximately 100 times their present size. (b) The cluster M103 contains a red giant star. M103 is 14 ly across and lies around 8000 ly from Earth in the constellation Cassiopeia.

(NOAO/Science Source)

Figure 19-5: R I V U X G
A Mass-Loss Star As stars age and become giant stars, they expand tremendously and shed matter into space. This star, HD 148937, is losing matter at a high rate. Other strong outbursts in the past ejected the clouds that surround HD 148937. These clouds absorb ultraviolet radiation from the star, which excites the atoms in the clouds and causes them to glow. The characteristic red color of the clouds reveals the presence of hydrogen (see Section 5-6) that was ejected from the star’s outer layers.

(Australian Astronomical Observatory/David Malin Images)

During this post–main-sequence phase, the star’s outer layers expand just as dramatically as the core contracts. As the hydrogen-fusing shell works its way outward, egged on by heat from the contracting core, the star’s luminosity increases substantially. This increases the star’s internal pressure and makes the star’s outer layers expand to many times its original radius. This tremendous expansion causes those outer layers to cool down, and the star’s surface temperature drops (see Box 19-1). Once the temperature of the star’s bloated surface falls to about 3500 K, the gases glow with a reddish hue, in accordance with Wien’s law (see Figure 17-7a). The star is then appropriately called a red giant (Figure 19-4). Thus, we see that red giant stars are former main-sequence stars that have evolved into a new stage of existence. We can summarize these observations as a general rule:

Stars join the main sequence when they begin hydrogen fusion in their cores. They leave the main sequence and become giant stars when the core hydrogen is depleted.

Red giant stars undergo substantial mass loss because of their large diameters and correspondingly weak surface gravity. This makes it relatively easy for gases to escape from the red giant into space. Mass loss can be detected in a star’s spectrum, because gas escaping from a red giant toward a telescope on Earth produces narrow absorption lines that are slightly blueshifted by the Doppler effect (review Figure 5-26). Typical observed blueshifts correspond to a speed of about 10 km/s. A typical red giant loses roughly 10^–7 M_⊙ of matter per year. For comparison, the Sun’s present-day mass loss rate is only 10^–14 M_⊙ per year. Hence, an evolving star loses a substantial amount of mass as it becomes a red giant. Figure 19-5 shows a star losing mass in this way.

The Distant Future of Our Solar System

We can use these ideas to peer into the future of our planet and our solar system. The Sun’s luminosity will continue to increase as it goes through its main-sequence lifetime, and the temperature of Earth will increase with it. One and a half billion years from now Earth’s average surface temperature will be 50°C (122°F). By 3½ billion years from now the surface temperature of Earth will exceed the boiling temperature of water. All the oceans will boil away, and Earth will become utterly incapable of supporting life. These increasingly hostile conditions will pose the ultimate challenge to whatever intelligent beings might inhabit Earth in the distant future.

About 7 billion years from now, our Sun will finish converting hydrogen into helium at its core. As the Sun’s core contracts, its atmosphere will expand to envelop Mercury and perhaps reach to the orbit of Venus. Roughly 700 million years after leaving the main sequence, our red giant Sun will have swollen to a diameter of about 1 AU—roughly 100 times larger than its present size—and its surface temperature will have dropped to about 3500 K. Shell hydrogen fusion will proceed at such a furious rate that our star will shine with the brightness of 2000 present-day Suns. Finding themselves inside the bloated Sun, some of the inner planets, possibly including Earth, will be vaporized. Even the thick atmospheres of the outer planets will evaporate away to reveal tiny, rocky cores. Thus, in its later years, the aging Sun may destroy the planets that have accompanied it since its birth.