Mystery of light and energy

THE BALTIMORE SUN

This is the time of the equinox, when the Northern night grows longer than the day, and the sun yields to the stars.

At 1:27 p.m. Friday, the midday sun passed precisely over the equator on its way south for the Southern summer. The South Pole has seen its first sunrise in six months. But in the far North, the same sun has set, and a long, dark winter of icy stars and silent auroras has begun.

Those stars, those pinpoints of cold, white light, would seem to have no kinship at all with the blinding glare and blistering heat of the sun.

But of course the sun and stars are siblings. And the sun is one of our richest sources of knowledge about the stars.

The sun is, after all, the easiest star to observe. It is as little as 91.4 million miles away during Earth's nearest approach each January. Light from its surface takes barely eight minutes to reach our skin.

The next-closest star in our neighborhood is Proxima Centauri, some 270,000 times farther away. Light from Proxima Centauri takes more than four years to get here.

Astrophysicists say the sun has been shining for more than 4.5 billion years, a late arrival in a universe of stars and galaxies then already 9 or 10 billion years old.

Although it is often described as an "average star," astronomers have found that our sun is in fact much larger, brighter and hotter than most other stars.

Old Sol began as a dense patch in a cloud of cold interstellar gas (mostly hydrogen) and dust. As the gas' own gravity gathered in more and more hydrogen, the temperature and pressure at the center began to climb.

Finally, when the heat and pressure reached a critical level, hydrogen nuclei at the sun's core began to smash together until they fused. That fusion created helium nuclei and prodigious energy, mostly in the form of gamma rays.

The sun's thermonuclear engine had kicked over, and it has been roaring ever since. Every second, the nuclear fusion at the sun's core converts about 700 million tons of hydrogen into helium. Another 5 million tons is transformed into pure energy. Lots of it.

In all, the power released each second is the equivalent of 92 billion one-megaton bombs. A tiny fraction of that energy falls across the Earth, warming our atmosphere and oceans, driving our weather and the photosynthesis in our crops. In short, it sustains our lives. Investigators are trying to learn how its cyclic variations may alter our climate.

Despite the sun's appetite for hydrogen, there is plenty to feed its fusion reactor for another 5 billion years or so. It is still 92 percent hydrogen, and 7.8 percent helium, with traces of oxygen, carbon, nitrogen and heavier elements.

The sun is huge -- 864,000 miles in diameter, or 109 Earths laid end-to-end. More than 1.3 million Earths could be stuffed inside.

And all that mass is in motion. The sun rotates west to east, just like the Earth, turning once every 25.4 days at its equator.

At its core, where the fusion takes place, the sun's temperature is 27 million degrees Fahrenheit. The pressure is 250 billion times that at the Earth's surface, but the heat keeps the hydrogen in the core in a gaseous state, even though it is compressed to eight times the density of gold.

Energy from the core radiates outward through 70 percent of its radius, into a churning region called the convection zone occupying the outer 30 percent. It emerges at the surface, or photosphere, mostly as light.

There, the sun's plasma cools to a mere 9,900 degrees Fahrenheit. But it is the birthplace of violent solar flares and other eruptions of matter and energy that can billow out across the solar system like passing squalls. They can alter the space environment around the Earth and rattle modern power, communications and satellite systems.

It's just above the surface that scientists encountered one of the sun's greatest mysteries.

For more than 60 years physicists had puzzled over data, gathered during total solar eclipses, indicating that the sun's corona - that pale, shimmering outer atmosphere visible during eclipses - is 1 or 2 million degrees hotter than the sun's surface.

"Probably the most intuitive law in all physics is that heat can't go from a colder object to a hotter one," said Joe Gurman, U.S. project scientist for the Solar and Heliospheric Observatory (SOHO) satellite.

So how could heat from the sun's relatively cool surface move millions of kilometers higher and produce temperatures there hundreds of times hotter?

All kinds of mechanisms were proposed for transporting the heat: sound waves, solar flares, large magnetic loops and something called magnetic hydrodynamic waves. "But all the models came up 10 to 100 times short" of the needed energy, Gurman said.

The breakthrough came in 1997, when time-lapse images made by instruments aboard SOHO began to reveal thousands of smaller magnetic loops carpeting the sun's surface - lines of magnetic fields like McDonald's arches connecting surface points of opposite polarity.

These loops are small by solar standards. But they are thousands or tens of thousands of miles long and extend high into the corona.

As the "feet" of those loops - about half the size of the Earth - move about in the churning photosphere, the loops move with them. But every few days, close encounters with nearby loops will cause them to snap and reconnect with other surface points.

That reconnection releases electrical and magnetic energy into the thin gas of the corona. "Do the math," said Gurman, "and it's more than enough to heat the corona."

SOHO has helped to resolve yet another solar mystery - the internal structure of the sun itself. The sun is, after all, opaque. Scientists could make educated guesses about its innards. But they could not see inside.

In the 1960s, however, observers studying the sun's light with Doppler instruments that measure tiny surface movements discovered that it undulates, like boiling fudge. By the 1970s, they understood that the undulations move outward like ripples on a pond, and down into the sun's interior, like the pressure waves released by an earthquake and recorded on seismographs.

These pressure waves are, in effect, sound waves. The place is roaring. "But it would be roaring at a lower frequency than you can hear," said Phil Scherrer, principal investigator for the Michelson Doppler Imager (MDI) instrument aboard SOHO.

With MDI and instruments on the ground, scientists have learned to identify and track the waves as they ripple across the sun's surface, or echo around inside the sun - sometimes for days - like sound in a gong.

They can follow waves as they penetrate the interior and emerge elsewhere on the surface. And they can detect when the waves are deflected by interior regions of varying temperature and density, like light in a lens.

Like geologists using seismic waves to map the Earth's interior, solar scientists use pressure waves to map the sun's interior. "It's like figuring out what's inside a piano by listening to it fall down the stairs," Scherrer said.

"Helioseismology" has yielded many discoveries, and forced many theorists back to their computer models, he said. But with new understanding of the sun's exotic physics, newer, better theories are emerging to explain how our sun, and the stars, work.

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