The Fate of the Sun

Some say the world will end in fire,

Some say in ice.

From what I've tasted of desire

I hold with those who favor fire.

But if it had to perish twice,

I think I know enough of hate

To say that for destruction ice

Is also great

And would suffice.*

Robert Frost wrote Fire and Ice in 1923, some four decades before astrophysicists were able to fathom how the sun -- and thus the earth -- would die. Nonetheless, he was basically correct: first fire, then ice. The fire will not be an explosion like the one now brightening the Large Magellanic Cloud; the sun is thought to have only about a tenth of the mass necessary to become a Type II supernova and has no stellar companion to contribute the mass necessary to turn it into a Type I blast. But that will be of little comfort to whatever creatures exist on earth when the sun is in its death throes; the $ final solar convulsions, while feeble compared to those of a supernova, will wipe out all life on the planet.

Fortunately for the earth's current inhabitants, the sun is enjoying a stable middle age, about halfway between its formation some 4.5 billion years ago and its demise about 5 billion years hence. Its radiation may fluctuate by a few hundredths of a percent here and there (data from the Solar Max satellite indicate that the sun's radiation declined from 1980 through 1985). But solar behavior has never been erratic enough to threaten all terrestrial life with extinction.

The real trouble will begin as the sun nears the 10 billion-year mark, when the thermonuclear fires that have been burning since its birth have fused all the hydrogen fuel in the solar core into helium. As the fuel runs out, the nuclear fire will die down, and the now largely helium core -- which has been kept distended by the heat -- will begin to contract under its own gravitational pull.

As the core contracts, however, its internal pressures will rise, forcing the temperature rapidly back up again until the intense heat ignites the unfused hydrogen gas surrounding the core. The interior of the sun will now be hotter than ever, a dense core of incandescent helium surrounded by a thin shell of hot, fusing hydrogen. Over the next few hundred million years, heat from the core will drive surface layers of the sun so far outward that they will cool to about two-thirds of the current 6,000 degrees C surface temperature, and redden. The sun will have become a red giant, so large that it will engulf the planet Mercury, perhaps extending to encompass the orbit of the earth. Even if the swollen sun stretches no farther out than Mercury, however, the heat reaching earth will be from 500 to 1,000 times as great as it is today. Oceans will boil, and life will be incinerated.

Finally, after a cycle of contraction and re-expansion, the sun's surface gravity will be so low the outer layers will boil off into space, leaving behind only the naked core, a lump of matter about as big as the earth, but with 60% of the sun's original mass, glowing blue-hot at perhaps 120,000 degrees C. That stage will mark the end of the sun's active life; its nuclear fires will never again turn on. Slowly it will cool until it is first a white dwarf, still glowing, then a cold black dwarf, a cinder. In the blackness of space, as in Fire and Ice, the lifeless earth will pass into an eternal deep freeze.

! FOOTNOTE: *Reprinted from The Poetry of Robert Frost by permission of Henry Holt and Co.