A. "Technically, there is no surface of the sun," says UW-Madison's Sanjay Limaye.

The senior scientist and educator with the Space Science and Engineering Center explains that unlike the hard, physical boundary here on Earth, the sun's surface is a hot mass of gas that is more or less continuous with its atmosphere.

"The sun is a star and so we have to look first at what a star is: Usually it's mostly hydrogen gas that is being converted into helium," Limaye says. If you could travel from the sun's outermost regions toward its core, this gas -- though very thin at first -- would gradually become so thick that you could no longer move.

"But the temperature and pressure is so high, there's no possibility of anybody standing on what one thinks of as a surface," he adds.

Still, the sun does have a surface we can see, called the photosphere. Up close, it resembles water bubbling in a pot on the stove, except that while water boils at 100 degrees Celsius, the photosphere is more than 500 times hotter.

From this roiling surface, says Limaye, streams of particles shoot upward occasionally and then loop back down, forming giant arcs that can be thousands of miles long and contain millions of tons of material.

-- Produced in cooperation with University Communications.

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