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The Sun will not continue to shine for ever (see also Chapter 1). Like most stars, it is undergoing gradual evolution on the so‐called main sequence. Calculations based on the amount of hydrogen in the Sun and the rate at which it is being consumed, allied with theories of stellar evolution based on observations of other Sun‐like stars, indicate that it is currently almost halfway through its active life. Eventually, the Sun will leave the main sequence and swell into a red giant as it begins the final, dramatic phase of its existence.

Figure 2.45 The Cat's Eye nebula in the constellation Draco may resemble the remains of our Sun some 7 billion years from now. One of the first planetary nebulas discovered, the Cat's Eye is also one of the most complex examples of this kind of nebula, with 11 rings, or shells, of gas surrounding the dying star at the center. The star ejected a series of concentric shells in a series of pulses at 1,500‐year intervals. (NASA, ESA, HEIC, and The Hubble Heritage Team STScI/AURA)

Some 7 billion years from now, the Sun will have grown to 2.3 times its present diameter and shine 2.7 times brighter than it does today. Within a few hundred million years, it will balloon outward, expanding to more than 100 times its current size, so that it engulfs Mercury, and possibly Venus, whilst swelling almost to the present orbit of Earth.¹² Meanwhile, the surface temperature will drop to around 4,000°C, making it appear red in color, and the bloated star will lose up to 25% of its mass in the form of a powerful solar wind.

The first red giant phase will end abruptly when there is no longer enough hydrogen available to continue the fusion process that leads to the creation of helium. As the core temperature soars to 100 million degrees Celsius, the intense heat and pressure initiate nuclear reactions that use helium as fuel. In this process, three helium atoms are fused to create one carbon atom.

The Sun will remain a fully fledged red giant for only a few hundred million years. During this time it will steadily burn its supply of helium, until this, too, begins to run out, leaving the star with a core made of nuclear fusion by‐products – carbon and oxygen.

As the energy output begins to drop, the star balloons outward again, once more extending to at least the present orbit of Venus. During this “asymptotic branch” stage on the sequence of stellar evolution, it will experience enormous pulsations known as helium flashes. However, these 10,000‐year‐long bursts of energy are almost the last throw of the dice.

Figure 2.46 The evolution of the Sun will culminate in a red giant phase, beginning about 7 billion years in the future. The Sun will expand enormously in the first phase as hydrogen burning accelerates, possibly reaching the orbit of Venus. Once helium fusion takes over, it will suddenly shrink. A few hundred million years later, when most of the helium is converted to carbon and oxygen, it will expand once more, marked by periodic outbursts known as helium flashes. When the star runs out of helium it collapses to form a white dwarf.

(I‐J Sackman, Caltech)

The second red giant phase, which lasts for perhaps 100 million years, is marked by another huge mass loss as the remaining atmosphere is ejected into space, creating a beautiful, but short‐lived, planetary nebula. Eventually, all nuclear fusion ceases. Without the outward push of radiation pressure, the stellar remnant collapses under its own gravity to form a white dwarf.

All that is left is an extremely hot (100,000°C) core no larger than Earth. This is composed of highly compressed matter which is so dense and hot that the atoms are stripped of almost all their electrons. One teaspoon of this degenerate matter would weigh around 5 tons.

Over hundreds of millions of years, the white dwarf will gradually cool and fade until it emits only a tiny amount of heat or light. The Sun will end its life as a black dwarf, a cold, dark object hidden amongst the surrounding stars and surrounded by what remains of its retinue of planets and satellites.