The end of the world: Five to seven billion years from now, the Sun will loom in our sky as a blazing 'red giant'.

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PREDICTING HOW HUMAN SOCIETY will evolve is fraught with difficulty. Our accelerating science and technology hint at unprecedented wealth and leisure, while the danger of natural disasters, epidemics, wars and countless other unpredictable factors could lead to a much darker future.

When it comes to purely physical systems – the Sun and its planets, the galaxy, the universe – we can see more clearly what lies ahead. The Solar System, for example, turns out to be a fairly simple physical system, and astronomers have understood stellar physics well enough for several decades now to predict, with fairly high confidence, the fate of our home star and its family of planets.

Our planet's fate is inexorably tied to that of our Sun, which has been shining for five billion years, with at least five billion to go. The Sun shines by burning hydrogen in its core, fusing it into helium. As the Sun's nuclear fuel supply starts to run out, it will begin some peculiar contortions.

Gravity will at first cause it to shrink in size – but this will make the Sun's core hotter, which will actually cause its outer layers to expand significantly. At this stage, five to seven billion years from now, the Sun will loom in our sky as a blazing 'red giant'.

A few hundred million years later – a short period in terms of the Sun's lifetime – it will undergo yet another phase of heating and expansion, shedding much of the material in its outer layers, and finally collapsing into a so-called 'white dwarf'. By this time, its mass will still be about three-quarters of its current value, but compressed into a sphere the size of the Earth.

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The Sun's initial swelling during the onset of the red giant phase will destroy our blue planet. The additional sunlight reaching our atmosphere will cause global warming beyond Al Gore's worst nightmares. The oceans will evaporate into space, leaving only deserts; life as we know it will not be able to sustain itself. As astronomer Fred Adams of the University of Michigan, Ann Arbor, put it: "Within a few billion years, our world – nowgreen and flowering with life – will closely resemble present-day Venus, with a hellish atmosphere fuelled by a runaway greenhouse effect."

According to recent calculations by Klaus-Peter Schröder, at the University of Guanajuato, Mexico, the Sun's diameter will eventually swell from its current 1.4 million km to as much as 358 million km. The inner planets, Mercury and Venus, will be swallowed outright by the raging Sun.

Given that the diameter of the Earth's orbit is only about 300 million km, our own prospects don't seem much better. But it's not quite that simple: because of the Sun's weakening gravitational attraction, the Earth's orbit will have expanded to about 370 million km. So we won't be engulfed by the swelling Sun – not yet.

What is left of our planet, however, will be scorched beyond recognition, baked by a crimson Sun that takes up half the sky. At its brightest, the Sun will shine with an intensity more than 4,000 times greater than today. As Adams put it in a recent paper: "Current estimates indicate that our biosphere will be essentially sterilised in about 3.5 billion years, so this future time marks the end of life on Earth."

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