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.
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