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How the Sun will destroy the Earth

April 06, 2002

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LONDON: Simple concepts of stellar evolution indicate that the sun will swell up and consume the Earth.

This hypothesis for the eventual fate of our planet has been widely accepted, all textbooks giving broadly the same picture. But now a group of researchers at the University of Sussex in the UK has shown that it’s not quite that simple, and Earth may escape the sun’s clutches.

To understand why, we must consider the factors controlling our star’s long-term behaviour.

It’s an astonishing thought, but the sun’s mass falls by more than 4m tons per second, due simply to the radiation it emits. That value derives from an application of Einstein’s famous E=3dmc2, given that we know the intensity of sunlight.

Once the usable hydrogen in the core has been consumed, the sun will expand to become a red giant.

Even though the energy generation rate will have dropped off, lowering its surface temperature (making it red rather than yellow), our star will emit more radiation than previously due to its much greater surface area.

This will cool the star but, paradoxically, as its outer layers expand, its core contracts.

This re-ignited energy liberation in the core again produces an inverse reaction in the size of the star: its overall radius drops again.

A knock-on effect is that the conditions become right for hydrogen fusion to resume, although now in a shell surrounding the helium-burning core. And so the star swells up once more.

No return, except that the stellar mass does not stay constant. As these expansions occur, a star’s surface gravity is reduced temporarily, and in consequence it can more easily lose matter into space.

It is expected that the sun will eventually cast off a good fraction of its bulk, about one-third in all. Then, when all its helium has been used up, our star will finally run out of fuel and slowly contract to become a white dwarf, ending its days by gradually shrinking and cooling.

So, what does this mean for the Earth? Before one gets too worried, note this: it will be more than 7bn years before the sun initially becomes a red giant. But it is interesting to ponder the eventual fate of our planet, even if humankind has long since perished or moved off, gone to colonize other parts of the galaxy.

If the Earth stayed where it is — orbiting 150m kms from the sun — it would be eaten up. But it won’t, because our planet will not stay at the same orbital distance.

The reason is connected with the mass the sun will lose as it enters its red giant phases.

The pertinent results are these. In about 7.5bn years, the solar radius will grow to about 167m kms (compared to the present 0.7m). If the Earth remained at its present distance it would be enveloped, but a gradual migration to 185m kms will have occurred due to the solar mass loss.

That is the first red giant phase. It’s interesting to note that while the Earth will remain outside the sun, the latter will fill about half the sky.

The sun will then shrink before again fattening about 120m years later. In that epoch its radius will maximize at 172m kms, but our planet will have shifted outwards to 220m. The closest call is therefore in the first instance.

With regard to the habitability of the Earth, the results are moot, however. Looking forward 5bn years, the sun’s luminosity will have doubled compared to the present, making our planet too hot for life (on the surface, at least). And in the final red giant phase, the solar-power output will be several thousand times more than now experienced, desiccating the planet. —Dawn/The Guardian News Service.