Passage IV

NATUAL SCIENCE: This passage is adapted from Just Six Number: The Deer Forces That Shape the Universe by Martin Rees (©2000 by Martin Rees ).

The sun is fueled by conversion of hydrogen (the simplest atom, whose nuclear consists of one proton) into helium (the second-simplest nucleus, consisting of two protons and two neutrons). Attempts to harness fusion as a power source (controlled fusion) has so far been stymied by the difficulty of achieving the requisite temperatures of many millions of degrees. It is even more of a problem to confine this ultra-hot gas physically in a laboratory—it would obviously melt any solid container—and it has instead to be trapped by magnetic forces. But the Sun is so massive that gravity holds down the overlying the cooler layers, and thereby ‘keep the lid on’ the high-pressure core. The Sun has adjusted its structure so that nuclear power is generated in the core, and diffuses outward, at just the rate needed to balance the heat lost from the surface—heat that is the basis for life on Earth.

The fuel has kept the sun shining for nearly five billion years. But when it starts to run out, in another five billion years or so, the Sun’s core will contract, and the out layer expand. For a hundred million years—a brief interval relative to its overall life time—the Sun will brighten up and expand into the kind of star known as a “red giant”, engulfing the inner planets and vaporizing any life that remains on Earth. Some of its outer layer will be blown off, but the core will then settle down as a dwarf, shining with a dull blue glow, no brighter than a full moon today, on the parched remains of the Solar System.

Astrophysicists have compute what the inside of our Sun should be like, and have achieved a gratifying fit with its observed radius, brightness, temperature and so forth. They can tell us confidently what conditions prevail in its deep interior; they can also calculate how it will evolve over the next few billion years. Obviously these calculation can’t be checked directly. We can, however, observe other stars like the Sun that are at different stages in their evolution. Having a single “snapshot” of each star’s life is not a fatal handicap if we have a large sample, born at a different time, available for study. In the same way, a newly handed Martian wouldn’t take long to infer the life-circle of humans (or of trees), by observing large numbers at different stages. Even among the nearby stars, we can discern some that are still youngsters, no more than a million years old, and others in a near-terminal stage, which may already have swallowed up any retinue of planets that they once possessed. Such inferences are based on the assumption that atoms and their nuclei are the same everywhere.

Astrophysicists can compute, just as easily as the Sun’s evolution, the life-circle of a star that is (say) half, twice, or ten times the mass of the Sun. Smaller stars burn their fuel more slowly. In contrast, stars ten times as heavy as the Sun—the four blue Trapezium stars in the constellation of Orion, for instance—shine thousands of times more brightly, and consume their fuel more quickly. Their lifetime are much shorter than the Sun’s, and they expire in a more violent way, by exploding as supernovae. They become, for a few weeks, as bright as several billion suns. Their outer layer, blown off at 20,000 kilometers per second, form a blast wave that ploughs into the surrounding interstellar gas.

Supernovae represent cataclysmic events in the life of the stars, involving some “extreme” physical processes; so supernovae naturally fascinate astronomers. But only one person in ten thousand is an astronomer. What possible relevance could these stellar explosions thousands of light-year away have to all the others, whose business lies purely on or near the Earth’s surface? The surprising answer is that they are fundamental to everyone’s environment. Without them, we would never have existed. Supernova have crated the ’mix’ of atoms that the Earth is made of and that are the building blocks for the intricate chemistry of life. Ever since Darwin, we’ve been aware of the evolution and selection that preceded our emergence, and of our links with the rest of the biosphere. Astronomers now trace our Earth’s origin back to stars that die before the Solar System formed. These ancient stars made the atoms of which we and our planet are composed.