Ashes to Ashes Dust to Dust

In the chapters of this book's final section, we will consider questions of time and eternity as they relate to the universe. But as to the solar system, we know that it was born about 4.6 billion years ago, and that it will die when its source of energy (the sun) dies of old age.

Just as the specifics of the formation of the solar system depended on the formation of the sun, so its death will be intimately related to the future of our parent star. The evolution of the sun will presumably follow the same path of other stars of its size and mass (see Chapters 18, 19, and 20), which means that the sun will eventually consume the store of hydrogen fuel at its core. As this core fuel wanes, the sun will start to burn fuel in its outer layers, grow brighter, and its outer shell will expand. It will become a red giant (see Chapter 17), with its outer layers extending perhaps as far as the orbit of Venus. When the sun puffs up into a red giant, Mercury will slow in its orbit, and probably fall into the sun. Venus and the earth will certainly be transformed, their atmospheres (and, in the case of Earth, also water) being driven away by the intense heat of the swelling sun. Venus and Earth will return to their infant state, dry and lifeless.

Some recent models of solar evolution predict that the sun will slowly grow to this state sooner, giving us only another billion or so years before the earth becomes uninhabitable.

But don't fret. All of this is another one to five billion years away. The sun is in its midlife now, and, we hope, will avoid any crisis. The sun will then eject its outer layers (to become a planetary nebula), leaving behind a burned-out star called a white dwarf. A white dwarf does not have sufficient mass to continue fusing elements (as we will see some stars do in Chapter 19). It will slowly cool, radiating its internal heat into space, and eventually become a black dwarf—a strange object composed mostly of oxygen and carbon, the size of a planet with the mass of a star. Let's hope humanity has pushed on by then!

The planetary nebula NGC 1514 was discovered in 1790 by William Herschel. Planetary nebulae represent a brief phase late in the life of a low-mass star, when the outer layers of the star are ejected.

(Image from Tom Wickman)

The Least You Need to Know

V Based on studies of meteorites, astronomers believe the solar system is 4.6 billion years old.

V The philosopher Kant and the mathematician Laplace proposed, in the eighteenth century, the nebular hypothesis, a theory that the solar system was formed from a spinning cloud of interstellar dust.

V The early solar system was filled with dust grains that were pulled together by gravity to form planetesimals. The composition of the dust grains and thus the planetesimals depended on the distance from the sun.

V A gravitationally contracting nebula in which dust grains condense and collide to form planets can explain most of the patterns observed in our solar system, and even account for some of its irregularities.

Chapter 12

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