-- Submitted by Noelle Yeazel, Grade 6, Whitehorse Middle School

A. Like all stars, the sun changes over time, and some day -- when it has consumed all of its hydrogen fuel and becomes what astronomers call a "red giant star " -- its outer layers could reach as far as the Earth and swallow our planet.

But not to worry. Even the most pessimistic estimates suggest it will be a billion years or so before the sun makes the Earth uninhabitable. The life history of a star such as our sun is determined by the rate at which it converts matter to energy.

Astronomers calculate that when it was formed 5 billion years ago, the sun had enough fuel to keep producing energy for about 10 billion years, said James Lattis, director of UW-Madison 's Space Place. As the sun consumes its hydrogen fuel, its core will gradually shrink and become hotter. That, in turn, causes its outer layers to expand and increase the sun 's luminosity. In a billion years, that increased solar radiation will make things seriously uncomfortable on Earth.

And when all of its hydrogen fuel is consumed in about 5 billion years, extreme compression of the sun 's core will inflate its outer layers and transform it into the red giant star that will very likely determine the ultimate fate of our planet, says Lattis.

-- Produced in cooperation with University Communications

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