After perhaps a billion years of going from nebula to hydrogen-burning, a star of solar mass now lingers on the main sequence for perhaps 10 billion years. But let's drop the dry term "solar-mass star." Let's instead talk about the life and fate of the solar-mass star that we all care deeply about—the Sun.
Our own Sun has been burning for about 41/2 billion years and has generally been thought to be about halfway through its tenure of stable life on the main sequence. Some recent theories suggest that we might have much less time before changes in the Sun begin to cause critical problems for life on Earth—maybe just a paltry 1 billion years!
Although we may not know exactly how many billions of years from now the Sun will start to change critically, we do know that eventually this must happen. There is, after all, a finite amount of hydrogen in the Sun. What happens when all the hydrogen in the Sun's core has been converted to helium?
The answer is that gravity will begin to take over again (as it did with the proto-Sun), and the core of the Sun will start to contract. The rest of the Sun will still contain hydrogen, so fusion of that hydrogen will continue on in a shell around the core. In fact, the contraction of the core will produce greater heat, and this heat will prompt the hydrogen in the shell to fuse at a faster rate. What will happen is that the outer layers of the Sun will begin to expand. The total amount of light and energy coming from the Sun will greatly increase while the outer layers will expand so hugely that the surface will decrease in temperature. The Sun will swell and brighten but its surface will cool and therefore redden. It will move upward but also right-ward on the H-R diagram. It will become a red giant.
Just how large the Sun will eventually become is not known. It will never grow to the behemoth size of current red supergiants like Betelgeuse and
Antares, for those stars are many times more massive than the Sun. But the Sun will surely grow to engulf the orbits of Mercury and Venus and perhaps swell out as far as the Earth. By that time, life on Earth will, in any case, be boiled away (unless the human race—or some other agency—has moved the planet).
But let's switch to the present tense to follow these momentous future events.
After reaching its maximum red-giant size, the core of the Sun continues to collapse. Eventually it reaches a density and temperature at which a new fusion reaction can occur: fusing helium to create carbon.
The helium creates greater energy but also is used up more quickly. The Sun—or any star in its approximate mass-range—keeps generating a higher density and pressure as its core contracts and, in turn, fuses heavier and heavier elements. Throughout this period, a star is alternately swelling and shrinking to try to maintain a balance. The instability is what makes such stars—our Sun in the future and any star in this state now—fluctuate in brightness. In other words, this is what causes a star to become what we call a variable star.
The heaviest element that steady fusion can produce is iron. But the Sun is not massive enough to reach that stage. Its final pulsations cough off some of its mass, relatively gently, to become a planetary nebula. Fusion ceases, and without its outward pressure, the star collapses into a body roughly the size of Earth. The contraction stops when the star has been crushed into a substance in which the electrons of its atoms have been stripped and mingled but still cushion the atomic nuclei from one another. We call it degenerate matter. The energy from contraction has made the star white-hot. Its luminosity has decreased greatly, but the temperature has soared: so on the H-R diagram, the star—our Sun in the future—has zoomed far leftward and far downward. It has become a white dwarf.
The planetary nebula of the white-dwarf Sun (or other white-dwarf star) remains energized and glowing for many thousands of years before the nebula dissipates. Then the white-dwarf Sun continues to radiate its remaining great heat off for many billions of years—until it eventually fades out and becomes a dark remnant, a black dwarf.
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