In the past few years the big cosmic picture has come into focus. We know (more or less) how the universe was created, where it is heading and how it will end. We don't (yet) know if there is intelligent life elsewhere.by Martin Rees / October 20, 1999 / Leave a comment
Centuries ago, maps of the Earth had blurred boundaries, where the cartographers wrote “there be dragons.” But after the pioneer navigators had traversed the globe there was no expectation of a new continent, nor that we would ever drastically revise our estimates of the Earth’s size and shape. At the end of the 20th century we have, remarkably, reached this stage in mapping our universe-its spatial extent, its structure, its main constituents, and its huge span in time. For the first time the big cosmic picture is coming into focus. This story-a collective achievement of thousands of astronomers, physicists and engineers-can now be presented (in outline) with conviction.
Our own star
How did our own star, the sun, begin; and how will it end? The proto-sun condensed from a cloud of dusty gas in our Milky Way. Gravity pulled it together until its centre got squeezed hot enough to trigger nuclear fusion-the process that makes hydrogen bombs explode. Even though it is already 4.5 billion years old, less than half the sun’s central hydrogen has been used up. The sun will keep shining for a further 5 billion years. It will then swell up to become a red giant, large and bright enough to engulf the inner planets, and to vaporise all life on Earth. After this “red giant” phase, some outer layers are blown off, leaving a white dwarf-a dense star no larger than the Earth, which will shine with a dull glow on whatever remains of the solar system.
Not everything in the cosmos happens slowly. Stars more than ten times as heavy as our sun expire violently, by exploding as supernovae. The nearest supernova of modern times was in 1987. On 23rd-24th February, a new bright “star” appeared that had not been visible before. Astronomers have studied this supernova closely, especially how it fades and decays. When a heavy star has consumed all its available hydrogen, its core contracts and heats up, releasing energy via a succession of reactions involving progressively heavier nuclei, of helium, carbon, oxygen, silicon and so on. The star then faces an energy crisis when it cannot draw on any further nuclear sources. The consequences are a supernova explosion, throwing back into space a mixture of atoms, in roughly the proportions observed on Earth. Why are carbon and oxygen atoms so common here on Earth, but gold and uranium so rare? The…