Photograph courtesy NASA
Republished from the pages of National Geographic magazine
"It's not a boring white disk," says solar physicist Bernhard Fleck. Certainly not as seen by SOHO, the Solar and Heliospheric Observatory launched by the European Space Agency and NASA in 1995. SOHO captured this image in extreme-ultraviolet wavelengths, color-coded by temperature, with red showing the hottest. Why is the halo-like corona, visible from Earth only during a total eclipse, hundreds—even thousands—of times hotter than the surface? That's one of the questions that keep scientists looking straight at the sun.
Magnetism made visible: That describes virtually every feature on the sun, from sunspots to soaring structures called loops. Loops easily reach the height of ten Earths. Energy generated by the dynamics of smaller loops is likely the source of the corona's mysterious heat. The superheated gases that form the sun, mainly hydrogen and helium, exist in an electrified state called plasma. Below the surface, plasma can push and drag magnetic field lines. But when lines are strong enough to arc out, wildly conductive plasma follows.
Mercury aligned with the Earth and the sun on May 7, 2003, as it does 12 or 13 times a century. Far rarer: Venus could be seen crossing the sun last month for the first time since 1882. The transit of Venus in 1769 found observers around the globe—including Capt. James Cook in Tahiti—taking measurements in a joint effort to figure the distance from Earth to the sun. They were off by only 2 million miles (3.2 million kilometers). It's 93 million miles (149.7 million kilometers) away, on average, and its light makes the trip in about eight minutes.
Our star has been slow to give up its secrets. To study the sun is to enter a realm that is surpassingly weird.
They call it "good seeing." Squinting up into the luminous blue morning 8,000 feet (2,438 meters) above the Atlantic off the west coast of Africa, it's not hard to see why.
To astronomers good seeing means the air will permit a sharp and stable image of celestial objects. And indeed, it's almost surreal, the sapphire clarity over La Palma, one of the westernmost Canary Islands.
"Hold your arm out until your thumb just barely covers the sun," says Göran Scharmer, director of the Royal Swedish Academy of Sciences' Institute for Solar Physics. "When it's dark blue right up to the edge of your thumb, it's going to be a coronal sky."
Coronal sky itself doesn't guarantee good seeing, but it's one sign of a calm, dust-free atmosphere. That's why Scharmer and his team are here atop the rim of an ancient caldera, half a mile (0.8 kilometers) above the cloud deck, continuing a quest as old as man: studying the fire in the sky.
It has been burning for 4.6 billion years, even before there was an Earth to bask in its all-sustaining glow. Yet it is only in the past two decades that scientists truly have begun to understand the thermonuclear reactor we call the sun.
By big-time galactic standards, our star is quite undistinguished. Sure, it's so huge that a million Earths would fit comfortably inside. And it's so dense that the sunbeams you see today began their journey from the center of the sun before the last ice age, taking hundreds of thousands of years to elbow their way out to the glowing photosphere before making the 8-minute, 93-million-mile (150-million-kilometer) trip across space to your eyes.
Yet the sun falls into the general stellar category of yellow runts called type G, a species so monotonously common that there are billions of them in the Milky Way alone. And it appears to be remarkably stable so far, with an energy output that varies no more than one-tenth of one percent over the course of a decade, and not much more over centuries.
But nothing else in the universe—save only our planet itself—is more immediately important to us. The sun is the origin of virtually all the energy that sustains life, the source of our weather, arbiter of our climate, and, of course, our closest connection to the processes that populate galaxies and power the cosmos.
"The sun is the Rosetta stone of astrophysics," says Scharmer, whose observations with the Swedish 1-meter (3.3-foot) Solar Telescope on La Palma keep setting world records for high resolution. "But it is a stone that we haven't been able to decrypt entirely."
Even today, four centuries after Galileo and others stunned Europe by revealing that a spatter of spots moved across the solar surface, many of the most profound aspects of our local star remain shadowed in mystery. Now scientists are on the cusp of finding answers, thanks to a surge of international interest over the past 20 years—and to advances in computer modeling and new, high-tech instruments on the ground and in space that can monitor subtle aspects of solar behavior that were previously unrecognizable, and sometimes unimaginable.
"Before, it was solar dermatology," says Scharmer. "Now it's really astrophysics."
But much finer telescopic resolution is still needed. Many scientists believe that some fundamental solar structures are only a few miles wide. The best resolution with the Swedish telescope is 50 miles (80.5 kilometers), so the team has been furiously upgrading its instruments. Ditto for investigators at some of the scores of terrestrial facilities from Sunspot, New Mexico, to the mountain summits of Maui and the forbidding Siberian outback. Off the planet there are nearly a dozen major space-based observatories—almost all of them launched since the mid-1990s.
In addition there are new initiatives to understand and forecast space weather, the effects created by the billions of tons of plasma that can erupt from the sun and cause magnetoelectrical squalls throughout the solar system.
"In space weather we're about where terrestrial weather forecasters were 40 years ago," says Timothy Killeen, director of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and a principal investigator at the new Center for Integrated Space Weather Modeling at Boston University. "One of the most important things that we need is end-to-end modeling—a comprehensive view of what happens all the way from the interior of the sun to Earth's upper atmosphere." With today's observational and computing power, Killeen says, "we've got the resources to make significant progress within just a few years."
For those who operate sensitive satellite broadcast and communications systems, the global positioning system, military spacecraft, and sundry systems critical to modern life, it can't be soon enough.
Although nearly everything that happens in and on the sun affects our planet, two kinds of explosive solar events impact Earthlings most severely. One is a solar flare, in which a small area above the solar surface suddenly roars to tens of millions of degrees, throwing off a surge of radiation that can cause communications blackouts, disable satellites, or, theoretically, kill a spacewalking astronaut.
The other event is a coronal mass ejection (CME), in which billions of tons of charged particles escape from the sun's halo, the corona, at millions of miles an hour. When these behemoth clouds slam into Earth's protective magnetosphere, they squash the magnetic field lines and dump trillions of watts of power into Earth's upper atmosphere. This can overload power lines, causing massive blackouts, and destroy delicate instruments on anything in Earth's orbit.
Often flares and CMEs occur together, as was the case last October when the fourth most powerful flare ever observed exploded. Back-to-back CMEs then smacked the planet. Thanks to modern detection equipment, we had enough warning to take preventive action. The atmosphere was so electrically charged that the northern lights were seen as far south as the Mediterranean, but little damage was done. By contrast, in 1989, when a fierce CME struck the Earth, it blew out HydroQuebec's power grid, leaving almost seven million people without electricity, and a multimillion-dollar damage bill.
Not surprisingly, locating the causes of such events is a top priority among researchers. But our star has been slow to give up its secrets, and no wonder: To study the sun is to enter a realm that is surpassingly weird.
Most of the Earth is solid. By contrast, all of the sun is gas: about 70 percent hydrogen, 28 percent helium, and 2 percent heavier elements. The outer visible layer is called the photosphere. But in fact, the sun has no "surface," and its atmosphere extends all the way to Earth and beyond, thinning out as it goes.
Moreover, the sun is a madhouse of electromagnetic activity. On Earth very few materials are good conductors of electricity. But in the sun almost everything is electrically conductive because there aren't many intact neutral atoms. The overwhelming thermal and radiation energies excite electrons to the point at which they pop off their atoms, creating a seething stew of positively charged nuclei and free negative electrons—a gaseous mix called plasma that can carry current as easily as copper wire.
Like any electrically charged object, plasma produces magnetic fields when it moves. As those fields shift, they induce more currents to flow, which in turn produce more fields. This tangle of plasma and magnetic and electrical effects determines the forms of nearly everything in or above the sun, such as the bright coronal loops and the dark areas we call sunspots.
"Everything we see as solar activity," says Stephen Keil, director of the National Science Foundation's National Solar Observatory, a consortium of facilities in New Mexico and Arizona, with telescopes around the world, "is a magnetic field being acted on by plasma and vice versa." Both are forever in motion.
The source of this energy is nuclear fusion. Like all stars, the sun formed when local gas and dust drifted together, drawn by gravity, swirling into a sphere. As the mass became larger and larger, hydrogen at the center was crushed by the gigantic pressure, finally sparking a fusion reaction in which hydrogen nuclei come together in a multistep reaction to create helium. The resulting nuclei are just slightly less massive than the component hydrogen nuclei that formed them. The difference is converted to energy according to Einstein's famous E=mc2.
Much of that energy is carried away as light in the form of gamma rays—the most energetic wavelength of electromagnetic radiation. But the solar core is so dense that a single photon, the fundamental unit of light, can't go even a fraction of a millimeter before banging into some subatomic particle, where it is scattered or absorbed and re-emitted. As a result, it can take hundreds of thousands of years for a photon to ricochet its way nearly half a million miles (804,672 kilometers) to the sun's surface. By that time, it has shed so much energy that most of it emerges as the fairly puny radiation we call visible light.
It took decades to comprehend the physics of this process, which was ridiculed as outlandish in the 1920s when it was first suggested by the great British astronomer Sir Arthur Eddington and others, who were convinced that the source of the sun's power was some subatomic phenomenon requiring enormous heat. "We do not argue with the critic who urges that the stars are not hot enough for this process," Eddington wrote in 1926. "We tell him to go and find a hotter place."
By the 1950s, however, the fusion model had been convincingly verified, except for one infuriating mystery: the output of wraithlike subatomic particles called neutrinos that are produced in the fusion process. Despite decades of painstaking searches, researchers were able to detect only a third of the neutrinos that theory predicts should strike the Earth every day. Finally, three years ago, a remarkable international effort involving facilities in Japan and Canada solved the problem by demonstrating that the "missing" neutrinos had mutated into different types that had not been detectable until the latest instruments became available. Solar physicists are still rejoicing.
Elation indeed is the feeling in the science community for what today's explorations are adding to our knowledge of the sun. Peter Gilman, a veteran sun researcher with NCAR's High Altitude Observatory, sums it up: "This is the golden age of solar science."
As the neutrino resolution illustrates, it's an international affair. The workhorse of the solar space fleet, for instance, is the Solar and Heliospheric Observatory (SOHO), a satellite run jointly by the European Space Agency and NASA. Launched in 1995, its arsenal of instruments has contributed to the research of scientists around the world.
Breakthroughs have been made on all solar fronts. But nearly every hard-won answer has revealed new puzzles: The ceaseless dance between plasma and magnetic fields makes it maddeningly difficult to tease apart cause and effect. Each major level of solar phenomena is influenced by the others, each has a direct effect on Earth, and each is still not completely understood. The momentum toward solving what solar physicists think of as the "big questions" isn't likely to slow, given our ever greater need to predict space weather. And because, as astronomer John Harvey of the National Solar Observatory puts it: "The sun is the only astronomical object that critically matters to humankind.
Among the big questions (in no particular order) are:
What interior mechanisms produce the sun's mighty magnetic dynamo?
The magnetic field drives virtually everything on the sun. Our star has an overall main magnetic field, with opposite north and south magnetic poles like the Earth's. Geophysicists believe that the Earth's field is formed by the dynamolike motion of molten iron in the outer part of our planet's ultrahot core. Similarly, the sun's overarching field seems to be produced by internal motion of plasma.
Until recently, however, it was impossible to see anything beneath the blazing photosphere. Then in the early 1980s scientists developed a technique called helioseismology—a sort of ultrasound scan of the solar innards that allows researchers to analyze the propagation of sound waves through the sun using the techniques geologists use to understand the interior structure of the Earth.
"Nobody dreamed 30 years ago that there would be the possibility of looking beneath the surface of a star," says John Leibacher, program director for the Global Oscillation Network Group (GONG), a worldwide array of automated observation stations funded by the National Science Foundation and positioned about 60 degrees apart around the Earth to view the sun 24 hours a day.
The idea of analyzing sound waves originated in the 1960s, when a Caltech physicist named Robert Leighton used Doppler imaging techniques to show that the solar surface throbbed with rhythmic oscillations like the skin of a drum, with a frequency of about one beat every five minutes. Solar astronomers later found more and different waves that resonate throughout the sun, and in the 1990s began to apply the science of acoustics to data from GONG and from space-based instruments like SOHO. As a result, "we're seeing structures inside the sun that nobody expected," says solar physicist Craig DeForest of Boulder's Southwest Research Institute (SwRI).
Perhaps the biggest surprise is how the innermost layers revolve—especially when compared with the sun's peculiar outer rotation. It takes roughly 26 days for the visible photosphere and the convection zone just below it to make a complete revolution at the equator, at about 4,400 miles per hour (7,100 kilometers per hour), but about 36 days near the poles at a sluggish 545 miles per hour (877 kilometers per hour.)
Many scientists had long suspected that the inner layers of the sun—the core and the vast radiation zone—were spinning faster than the upper layers. That turned out to be partly right. The inner layers are rotating as if they were a solid body, at one revolution per 27 days—slower than the upper layers at the equator but faster than at the polar regions. That means that the radiation zone and convection zone are spinning at very different rates as they slide past one another. Many experts now think this "shear" area, known as the tachocline, forms the dynamo that generates the sun's main magnetic field.
"We hope that we're not being too optimistic," says Jack B. Zirker, former director of the National Solar Observatory, "but we now have a fair idea of how and where the dynamo comes about."
The internal shearing motion stretches and twists the north-south magnetic field lines, wrapping them around the sun. Doing so adds energy to them, just as stretching a rubber band stores energy in it. Sometimes this action creates powerful ropes of field lines that are buoyant enough to rise. They poke out into the photosphere as loops, prominences, and those enigmatic signposts of solar activity—sunspots.
Why do sunspots fluctuate in 11-year cycles, and what effect does this have on terrestrial climate?
When these titanic bundles of magnetic field lines bulge up and protrude, hernia-like, through the photosphere, they can range in diameter from 1,500 miles (2,414 kilometers) to several times the size of the Earth. Sunspots are visible because the bundled field lines impede the flow of convection. The center of the spot, the umbra, appears dark because it's a thousand or more degrees cooler than the surrounding 10,0000 degrees Fahrenheit (5,538 degrees Celsius) photosphere.
Reliable references to sunspots date from first century B.C. China, and they were seen by telescope in the early 17th century, but no one made a systematic count until a German astronomer, Samuel Heinrich Schwabe, began a tally in 1826. By 1843 he was confident enough to report that their number goes from minimum to maximum and back to minimum in about a decade's time.
By 1915 American astronomer George Ellery Hale and colleagues at California's Mount Wilson Observatory had shown that the spots usually appear in pairs, aligned roughly parallel to the sun's equator, and that each half of a pair has the opposite magnetic polarity. Further, they determined that all spot pairs in the sun's northern hemisphere have the same orientation and that all the spot pairs in the southern hemisphere have the opposite orientation. Clearly, the arrangement of sunspots is directly influenced by the internal wrapping of the sun's main north south magnetic field.
Every 11 years, on average, the sun reverses its overall magnetic polarity: Its north magnetic pole becomes a south pole, and vice versa. So a complete magnetic solar cycle—returning the sun to its initial orientation—actually lasts an average of 22 years. No one completely understands the entire process, just as no one understands why the Earth's field also reverses itself at seemingly random intervals, most recently about 780,000 years ago.
That's unfortunate, because there's evidence that sunspot cycles have direct consequences for human life. Witness the sobering case of the Maunder Minimum, the eerie stretch from 1645 to 1715 in which records show that practically no sunspots appeared on the solar face.
It was named after British astronomer E. Walter Maunder, who in the 1890s tried in vain to stir up interest in this aberration. In the 1970s American solar physicist Jack Eddy revisited Maunder's work, noting that the Minimum offered "a good test case for solar influence on climate." Eddy, like most solar scientists at the time, wasn't convinced that variations in sunspot numbers—the most visible indicator of solar activity—had any link to terrestrial climate. he examined data on the growth rings of trees from the 70-year-long minimum.
They contained significantly more carbon 14 than trees before and after the period. That meant that higher amounts of cosmic radiation had been reaching Earth during that time. (A magnetically active sun reduces the cosmic radiation we receive.) So, Eddy concluded, there might be a connection after all.
Eddy's investigation also drew attention to another sunspot dearth from 1460 to 1550. Putting that episode next to the Maunder dates, scientists realized that these extended minimums coincided with the core of a famously frigid period in Europe and elsewhere known as the Little Ice Age (1400-1850), during which the Thames River in London and the Lagoon of Venice regularly froze.
It might seem as if fewer sunspots should mean a brighter sun. But the sun's luminosity is actually greater when there are more sunspots, because their magnetism creates extra-bright areas called faculae.
Sunspot activity has indeed been high over the past century as Earth's temperatures have climbed. But according to a recent NASA report, greater luminosity seems to account for only half of the global temperature increase before 1940, and less than that in later years as greenhouse gases have continued to rise. Swings in solar activity are only part of the puzzle.
Moreover, our knowledge of those swings is limited. Our best helioseismological studies and high-tech spacecraft observations only cover about 15 years. And as Joel B. Mozer, senior physicist at the Air Force Research Laboratory at Sacramento Peak, New Mexico, points out, "Since the beginning of the space age in the 1950s, we've had only four solar cycles. All our understanding is based on that. But there's plenty of evidence that these don't represent the extremes."
From computer simulations, scientists have a fair idea of how sunspots might arise and dissipate. But there are still too few highly detailed observations to compare with theory.
"The hope is that helioseismology will eventually give us better magnetic field observations at crucial depths," says Spiro Antiochos of the Naval Research Laboratory in Washington, D.C., who models the physics of solar outbursts. "Now we have to infer from the surface what's going on below. Even the simple question of the structure of the magnetic field under a sunspot—we just don't know."
How is it possible that the corona—the ultra-rarefied halo of ions that extends millions of miles into the chill of space—is typically hundreds of times hotter than the solar surface?
For the most awesome extremes of solar output, scientists look to the most inscrutable of the sun's features: the corona. Invisible except during a total eclipse, the corona and its lower altitude neighbor, the chromosphere—a 1,500-mile-thick (2,414-kilometer-thick) band of plasma just above the visible photosphere—utterly defy the common-sense assumption that things ought to be cooler if they're farther from the surface of the sun.
The chromosphere is only one-millionth as dense as the photosphere. The corona is one-hundredth as dense as that. And yet, between the photosphere and the corona, "the proportional contrast in temperatures is about the same as if you were standing with your feet in liquid helium and your head encased in a blast furnace," says SwRI physicist Craig DeForest. The photosphere is about 5700 degrees Celsius (10,292 degrees Fahrenheit), the chromosphere averages 10,000 degrees Celsius (18,032 degrees Fahrenheit), and temperatures in the corona regularly top two million.
Where is that stupendous heat coming from? The leading suspect is a process called magnetic reconnection, a splicing of magnetic field lines that causes energy to be released.
"A key SOHO discovery was that small-scale magnetic fields are constantly generated all over the sun just under the surface," says SOHO's U.S. project scientist Joseph Gurman of NASA's Goddard Space Flight Center. This "magnetic carpet" is made up of small loops arcing up from the photosphere. The bases of the loops are pushed around by plasma. When two lines are shoved together, their stored electrical energy grows to unmaintainable levels. The lines break and reconnect with each other to form a lower energy configuration. The excess energy—sometimes billions of kilowatt-hours—is released in an instant.
"After decades of not being able to come up with enough energy for a coronal-heating model," says Gurman, "we now have a thousand times more energy than needed."
What explains flares and the coronal mass ejections that are responsible for electrical tempests on Earth? How can these storms be predicted?
The corona can produce what Robert Lin, professor of physics at the University of California at Berkeley, calls "the most powerful particle accelerators in the solar system—flares and CMEs. The biggest flares are equivalent to billions of megatons of TNT, all on a timescale of 10 to 1,000 seconds."
Flares expel much of their energy as x-rays and are presumably generated when electrical currents are suddenly released as one or more magnetic field loops in the corona become strained to the breaking point and snap into a new shape. Traveling at the speed of light, the radiation reaches Earth in eight minutes and can disrupt radio communications and navigation systems. A small percentage of flares also hurl fast-moving high-energy protons that can cripple satellites.
But most of the four-alarm worry in the space weather community is devoted to CMEs and their particle barrage. Although CMEs often follow flares, these massive eruptions of plasma also frequently occur on their own. "CMEs fluctuate by many orders of magnitude," says Joel Mozer, the Air Force Research Lab physicist, "and their flavor and character vary."
They ordinarily take one to three days to reach Earth, where they smash into the planet's magnetosphere, deforming it and—if circumstances are right—producing a multimillion-ampere ring current in the belts of charged particles that continually circle the Earth. Even more threatening to communications satellites than flares, CMEs can also take out terrestrial power grids, leaving us in the dark.
It's still not possible to predict when or if CMEs will erupt, because the trigger mechanism isn't known. But with SOHO and other satellites now constantly monitoring solar activity, "we can see these storms leaving the sun in a way we never could before," says Joseph Kunches, chief of space weather operations at the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder. "We can predict with 80 percent accuracy whether or not they will hit the Earth."
Space meteorologists are also getting at least some warning on the velocity and magnetic orientation of the ejection. The magnetic polarity of a CME can change during its journey. If the polarity is the opposite of Earth's, it does the most damage on impact because the collision of opposite-moving field lines produces enormous charges. Scientists get those readings only an hour or less before a CME strikes, when it passes a satellite called the Advanced Composition Explorer, or ACE. Like SOHO, ACE orbits around a fixed point in space a million miles (1.6 million kilometers) from Earth, and is built to weather the storm.
The worst storms often come in the waning years after the solar maximum. The most recent solar max ended in 2001; November 2003 marked the strongest x-ray flare ever observed.
Scientists have been measuring flares for only a few decades, and CMEs weren't even identified until the early 1970s. Have we really seen the outermost limits of what the sun can do? We can't be sure. But by the time the next solar max rolls around—seven or so years from now—a new generation of solar observatories will be watching our stormy star, building on an era that for solar physicists has amounted to 20 years of good seeing.
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