Mysteries Of The Sun

This Thursday will dawn like any other in Hawaii, with surfers catching the crests at Kona and sun worshipers settling in for some serious bronzing. Invisible in the bright morning light, the moon will slip from west to east unseen, so that when a nick appears in the sun's right side it will seem as if the very sky has taken a nibble. After 30 minutes, as half the sun appears to have melted into a chunk of midnight, the blue of the sky will grow duller; on the ground, the landscape will take on the gray metallic look of cheap cabinets. As the stealthy blackness creeps on, only a silver are will glow in the sky. The tiny spots of light between shadows of leaves will metamorphose into perfect crescents, images of the darkening sun.

As the last rays of sunlight beam through valleys between the mountains of the moon, the vanishing sliver will blaze with spots of luminescence. The leading edge of the moon will approach the left side of the sun, and the last point of light will glow like a diamond ring. Just before totality, the period when the sun is completely covered by the moon, a wall of darkness will come tearing over the western horizon at 7,600 miles an hour. Where the sun once shone there will be a black hole surrounded by a soft halo the color of good pearls. Birds may stop singing and roost. Temperatures will drop. Flowers may fold their petals as if for the night. The sky will be as dark as a night under a full moon. Around the blotted-out sun, plumes and streamers from the corona, or upper atmosphere, will arch out. Reddish prominences will curl around the black hole as hydrogen boils off the sun. The moon's shadow will race over the Pacific Ocean to Baja California, and more people will witness the eerie daytime darkness of a total eclipse of the sun than have ever done so before.

It will all happen on just the day that a Mayan astronomer-priest predicted some 1,200 years ago.

What the Maya could not have foreseen is that when the lights come up, the audience will be left with something more tangible than the feeling that the cosmos has gone awry. Astronomers along the path of totality will aim their telescopes for a once-in-a-lifetime opportunity to penetrate mysteries of our star that are literally lost in its usual brightness. Observations made during an eclipse often take solar astronomy into unmapped terrain, answering old puzzles and uncovering new ones, says astronomer Eric Becklin of the University of California, Los Angeles: "Eclipses let us make the first important measurements of a phenomenon. " It was on just such an afternoon 123 years ago, for instance, that an eclipse watcher discovered the element helium.

Researchers know volumes about the sun. Most notably, it generates heat and light by smashing hydrogen atoms together in its core to induce nuclear fusion-trillions of H-bombs a second. But everything they learn strips the cover off a deeper mystery. "It's pretty embarrassing," says astronomer John Harvey of Kitt Peak National Observatory in Arizona. "We are incredibly ignorant about what's driving solar activity." They know that the sun rings like a gong and shivers with "starquakes"-which may reveal as much about its interior as earthquakes do about Earth's-but are stumped by where the quakes and rings come from. They gather evidence that the sun is not the constant, reliable beacon that everyone assumed, but can't imagine what turns the stellar furnace up and down. They gape at the discovery that different layers of this 4.5 billion-year-old star rotate at different speeds. And theorists make themselves scarce when observers ask why.

Any solar eclipse lets scientists make measurements that would be impossible at ordinary times. But the 7-11 eclipse is extra lucky. Unlike previous passages of total darkness, which out of cosmic contrariness passed over such idyllic locales as northern Siberia (last July) and the Sahara (1973), the 1991 path of totality will pass over Mauna Kea, site of one of the world's premier telescope installations. (How much of a coincidence is this? A total solar eclipse passes over a particular spot on Earth once every 360 years, on average.) Even better, the eclipse will occur there early in the morning, before the sun warms the atmosphere and creates heat ripples that distort images. And this one will also last longer than most, four minutes and 10 seconds over Mauna Kea. "It's as though somebody mailed in a list of conditions they wanted and got this as a present," says astronomer Barry LaBonte of the University of Hawaii. Other spots along the path of darkness will see day turn to night even longer. That's great news for the thousands of solar groupies who plot their vacations not from Fodor's Guides but from astronomical tables (page 61). And cities as far north as Dallas and south as Lima will see at least half the sun blotted out.

The word eclipse comes from the Greek for "abandonment," and captures the sense of foreboding the sun's vanishing act inspired even in civilizations whose astronomers had figured out why and when the darkness would fall. By 2400 B.C., Stonehenge had two rings of holes that could be used to count off the length of the 29 1/2 -day lunar month, as well as massive stones placed in such a way that ancient astronomers could use them to calculate when the moon's orbit would intersect the plane of Earth's. With those data, the astronomer-priests could predict eclipses. No one knows whether they did. The architects of Stonehenge left no written records, so the Babylonians get credit for the first approximate eclipse forecasts, by about 2000 B.C. The Mayas did even better. Their astronomers had worked out the cycles of the heavens so exactly that they could predict not merely the year or month of a total solar eclipse, but the precise date.

To understand how solar eclipses occur, look up. From Earth, the sun and moon appear to be almost the same size. While the sun's diameter is about 400 times the moon's (885,000 miles vs. 2,160), it is also a serendipitous 400 times as far away (93 million miles vs. 238,000). When the full moon insinuates itself directly in the line of sight from Earth to sun, as it does every 18 months or so from at least one place on Earth, a total solar eclipse occurs. Those who resent that Copernicus displaced Earth from the center of the solar system can take heart: ours is the only planet that has a moon of exactly the right size to block out the sun but leave the glowing corona. Take that, Saturn.

Astronomers have been preparing for this week like players before a once-in-a-lifetime game. "It's like a rocket shot, only worse," says LaBonte. "If something goes before a rocket shot, you hold. This rocket is going whether we're ready or not." The goal: to learn what the different layers of the sun are made of, how hot they are and why. As the moon moves across the sun's face, astronomers will be able to analyze the rays coming from ever-deeper regions that are ordinarily nearly impossible to see because of the blinding light.

The star of the show is the sun's corona (diagram). Beginning a few thousand miles above the surface, it extends millions of miles into space and is usually outshone by the sun's disc. But during an eclipse it glows like the halo around a Giacometti saint, and that will let astronomers focus on one of the sun's more perplexing mysteries. Imagine a well-insulated voyager, thermometer in hand, on an odyssey out from the sun's core. Core: 27 million degrees Fahrenheit. Photosphere (the transparent layer from which light begins its journey to Earth): 10,000 degrees. Bottom of the chromosphere, or surface: 8,000 degrees. So far, so good. Temperatures should fall as one travels away from the heat source. But at the sun's corona, the mercury soars to 2 million degrees. It's like finding that the grill on your Fourth of July barbecue was hotter than the coals. "It's bass-ackward," says Harold Zirin of the California Institute of Technology. "And we don't understand the physical mechanism at all."

One possibility is that something is carrying the heat up, up, up through the sun and then releasing it in the corona, sort of like lumps of asbestos-coated coal being thrown into the air and then shedding their insulation. The coals in this case would be microflares, small packets of intense heat carrying magnetic energy which fire quickly before dying away and being replaced by others. Whatever the cause, the corona is so hot the sun can't even hold onto it. Its energetic particles blast off, headed throughout the solar system as an electrically charged "solar wind" that creates brilliant multicolor northern lights on Earth. A team headed by LaBonte will train the 2.2-meter optical telescope on Mauna Kea at a glob of sunspots. These freckles on the sun's face are places where magnetic energy pierces the star's surface. By examining them, the astronomers hope to see evidence of microflares that may be responsible for the hellish corona.

They'll have to work fast, as researchers led by Drake Deming of NASA's Goddard Space Flight Center keep reminding themselves. These astronomers also suspect that magnetism is the key to understanding the corona and, probably, the rest of the sun. Deming has targeted one mysterious wavelength of infrared light that is sensitive to magnetism. Using the latest in spectrography, the team will shoot the equivalent of five frames per second during the height of totality. When the infrared wave vanishes, they'll know that the moon has just covered its source. Since the infrared wave carries information about its source's magnetic field, believed to be the cause of the corona's heat, this experiment should help map the sun's magnetism. The challenge: protect the telescope until the sun is sufficiently darkened, then whip off the protective filter 40 seconds before the shooting is to start. Too soon and Deming will burn out the instruments; too late and he'll miss the show. Deming has been on Mauna Kea since this past weekend, practicing his moves like an America's Cup sailor rehearsing tacks.

The sun rings like a bell, and that has inspired the new science of helioseismology. Bubbling globs of superhot gas, or heat cells, generate pressure fluctuations that produce millions of sound waves, each with a different frequency. They zigzag near the surface or plunge 80 percent of the way to the core before bouncing up and breaking along the sun's surface. There, they make the surface rise and fall with a rhythm characteristic of the wave, like the pulsing of a heart. And that's music to astronomers' ears. By analyzing the undulating surface, scientists can map the solar interior just as geoscientists dissect Earth's insides by listening to seismic waves during earthquakes.

These solar sound waves have already shown that different layers rotate at different speeds. Although earthlings go for a spin around the planet's axis once every 24 hours no matter whether they stand on Mount Everest or deep in a coal mine, the sun is a lot more complicated. The top 30 percent rotates once every 25 earth days, but the deeper 70 percent takes 27 days. And the poles take 35 days, while the equator whips around in 25. "We've got this ball of gas spinning faster in the middle than at the poles," says Ken Libbrecht of Caltech. "I rib my theorist friends to tell me why, and they throw up their hands."

Scientists suspect that this uneven rotation helps drive the solar dynamo, the mysterious engine that somehow produces sunspots and ignites flares. In an earthly dynamo, or generator, powerful currents of electricity create magnetic-field lines. The sun's dynamo originates in or just below what's called the convection layer, the outer 30 percent of the star, where hot gases rise as cool ones sink. Because the sun spins at many different rates, its hydrogen and helium gases are a mishmash of motion that causes shearing where layers rotating at one speed meet those rotating at a different speed. This shearing, suggests astronomer Robert Noyes of Harvard University, winds up magnetic lines like thread around a spool. "But the lines can't get infinitely strong, " says Noyes. So as the fields become more and more taut over the course of 11 years, something eventually gives. The magnetic-field lines become so strong that they repel gases below and rise to the surface, according to one theory. When they reach the surface the erupting magnetism may appear as sunspots, dense points of magnetism some 2,000 times stronger than elsewhere that may block some of the sun's heat and light from escaping. Or else the field may self-destruct, going out with style in the form of a solar flare.

Why do the magnetic-field lines twist up for 11 years in one direction before snapping? The question became more urgent in the early '80s when the satellite Solar Max found that the sun's brightness changes by about l percent during the sunspot cycle. More spots, more heat. So somehow magnetism, the cause of sunspots, is linked to the sun's energy output, the basis for life on Earth. "We believe that magnetism distributed over the surface of the sun causes changes in brightness," says Noyes. "But we don't understand the details." Nor do they know whether the sunspot cycle causes the brightness cycle, or whether both get their rhythm from an undiscovered metronome.

The next total solar eclipse over North America will darken a swath from British Columbia to the Carolinas on Aug. 21,2017. By then, astronomers will undoubtedly have new solar mysteries to solve. They'd better hurry. The moon is spiraling away from Earth and will be too distant to completely cover the sun-in a million years or so. Catch the eclipse while you can.

Eclipses terrified the ancients. The builders of Stonehenge sank holes and erected monoliths to measure lunar months and orbits, so they could predict when the moon might eclipse the sun.

In Amos 8:9, the prophet quotes God: "I will cause the sun to go down at noon and I will darken the earth in a clear day." Scholars have determined that this refers to the eclipse of June 15, 763 B.C.

After five years of war in Turkey, the Medes and Lydians were so frightened when the sun disappeared on May 28 that they negotiated a peace treaty and sealed it with a double wedding.

1868

During an eclipse, Briton Norman Lockyer found an unexpected color emerging from the corona. He deduced it was from an unknown element and called it helium, from the Greek for sun.

Scientists embarked on an elaborate eclipse expedition to Brazil, where they measured starlight bending around the darkened sun, proving Einstein's theory of general relativity.

Our nearest star is 4.5 billion years old and probably has that much longer to burn. Astronomers know its diameter (885,000 miles), distance from us (93 million miles) and energy source (nuclear fusion of hydrogen), but other basics remain a mystery.

The core is the hottest part of the sun; the temperature of surrounding layers cool the farther they are from the center. Why, then, is the corona hotter than layers closer to the core?

A million different frequencies ring within the sun. They have revealed some of the star's hidden structure (as seismic waves have Earth's). But where do the waves come from?

The sun is all gas, and its different layers exhibit a bizarre independence. The surface rotates faster than the depths, the equator faster than the poles. How can this happen?

They speckle the sun's face, growing more numerous and then rarer over 11 years. They have something to do with magnetism, but what causes them, and why do they wax and wane every 11 years?