The world will eventually end in fire, not ice, say two ISU astronomers.

In about 5 billion years, the sun will start growing larger and eventually engulf the Earth, said George Bowen, professor emeritus of physics and astronomy.

Bowen constructed computer models of the future sun, and Lee Anne Willson, professor of physics and astronomy, compared those models with ancient stars. Bowen said he and Willson are fairly confident about the fate of the solar system.

“What we’re talking about is a time very late in the life and development of the sun,” he said. “Stars do change because they use up their sources of energy.”

The sun is currently about halfway through its longest stage of life. This first period lasts about 10 billion years.

Bowen said the first changes will begin after the sun has exhausted all of its energy during the first stage, when it finally uses all of the available hydrogen in the inner core.

“That’s when the fun really begins,” he said. At that time, the sun will be slightly more luminous than it is now, Bowen believes.

In the next stage, the “red giant stage,” the structure of the sun will begin to change, he said.

“As energy sources change, the structure of the star has to change,” Bowen said.

About 6 to 7 billion years from now, the sun will grow, he said.

“We know from observation what happens,” he said. “The sun will become much, much bigger.”

Bowen said the sun will be an enormous star at this point — perhaps 35,000 times more luminous than it is now.

“These changes are slow by our time scale,” he said. “As stars go, it’s hardly more than the blink of the eye.”

The sun will get bigger and more luminous, he said. This will continue more and more rapidly until finally it strips the outer part about 7 billion years from now, losing about 40 percent of its current mass. He said most of what is left will just blow away.

Willson said this spells certain doom for the planet.

“The Earth is almost certainly done for at that point,” she said.

In this last stage, it will get very hot, until all that’s left of the Earth is a slowly vaporizing rock.

“That happens unless the sun manages to lose a lot of its mass before that, which I don’t think will happen,” Willson said.

As for the other planets, Mercury and Venus will be gone before Earth. Willson and Bowen were uncertain about Mars, although Bowen said it might escape the other planets’ fate.

“It’s going to be a close escape, depending on the details,” he said.

Jupiter and Saturn are far enough away to be safe, although the radiation from the sun will make them hotter, forcing some changes, they said.

Bowen said high-speed computers have allowed scientists to really study and understand the stars.

“This whole picture of how a star ages and changes is really, to me, one of the wonderful things about astronomy,” he said.

He said as stars go through these stages, the leftover material eventually makes new stars with new compositions.

“Because the sun is only one of enormously many stars like this, we are changing the composition of the galaxy,” Bowen said. “That’s why I think it’s so exciting.”

Willson will be presenting the results at a session she organized titled “The Far Future Sun and the Ultimate Fate of the Earth” at an annual meeting of the American Association for the Advancement of Science in Washington D.C. The event will be held Feb. 20.