DENVER (AP) – Glowing steadily for more than 4 billion years and rising unfailingly every morning, the sun is something even astronomers can take for granted. Among the 100 billion stars in the Milky Way, ours is rather lackluster.

But the sun certainly is demanding everyone’s attention now, three weeks into perhaps the most dramatic and unexpected chain of eruptions ever observed venting from its seething, bubbling surface.

There have been as many as 11 salvos since Oct. 19. And the fireworks could reach a new crescendo by Thanksgiving, the nation’s busiest holiday for air travel, just one of the things that can be disrupted.

“There’s been nothing quite like this,” said Bill Murtagh, a space weather forecaster for the National Oceanic and Space Administration in Boulder, Colo. “Another big blow is not what anyone needs.”

NASA scientists compare it to a blizzard in July – in California.

It sounds incredible, but “something like that just happened on the sun,” says David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center in Alabama.

The biggest solar storm to affect Earth in the recent cycle was Oct. 28. It caused little damage, largely because it was forecast, and electric utilities and satellite companies took precautions.

Even so, it caused a blackout in Sweden, damaged two Japanese satellites and upset radio communications and navigation systems for jets and ships. Airlines in the northern latitudes flew lower to protect passengers from extra doses of radiation.

It is a startling reminder of who’s really in charge of the solar system. Scientists worry that a new round of eruptions could do more of the same or worse.

Each solar burst hurls into space huge clouds of superheated, charged particle clouds that are 13 times the size of Earth. One explosion on Nov. 4 ranks as the most powerful solar flare to be recorded by orbiting instruments – although it was pointed away from Earth.

“This period will go into the history books as one of the most dramatic,” said Paal Brekke, deputy project scientist for SOHO, a joint U.S.-European observatory between Earth and the sun.

What will the sun do next? Astronomers can only watch and wait. Early civilizations from the Sumerians to the Aztecs worshipped the sun for its life-nourishing properties. Its furious dynamics weren’t discovered until Galileo and others in the 17th century began to directly observe the sun through the first telescopes, sacrificing their eyesight for their discoveries.

In 1613, Galileo published three letters on sunspots, the cooler, dark, irregular spots that resemble cancerous moles on the sun’s fiery face. By recording the sunspots’ disappearance around the far side, Galileo was the first to demonstrate that the sun rotates.

But how do sunspots form and how do they trigger solar explosions? How do they affect Earth? Researchers still aren’t entirely sure.

The sun is not solid, but a dense and torrid ball of gas. It rotates in sections at different latitudes as if the layers of a cake were turning at different speeds, with the equator’s layer moving faster than the poles.

This phenomenon tangles and twists the sun’s magnetic field. The migration of hot plasma from the sun’s interior dynamo up to the surface is somehow inhibited in these distortions, producing sunspots.

Sunspots erupt and fade in 11-year cycles. But that’s just an average; some cycles last 15 years.

New studies suggest sunspots also work in longer patterns of 100 and 1,000 years. The sun’s luminosity can change slightly during those cycles, possibly affecting Earth’s climate and, some argue, contributing to global warming. If true, those details will take years to work out.

The current 11-year solar cycle, No. 23, peaked quietly in 2000. By late 2003 it was supposed to be on its downside. Researchers were labeling it a dud.

Until now.

Sunspots’ magnetic distortions intensify until something explodes. Some sunspots reload and fire again. And again. That is what’s happening now with the current sunspot clusters, 484 and 486.

From 93 million miles away, they look like tiny smudges on the sun’s chin. Yet each rival Jupiter in size.

The SOHO observatory is also photographing clouds of gas being thrown over and around the sun’s far side by unseen explosions. And astronomers are warning satellite and power grid operators to be ready for more fireworks.

Forecasters in Boulder are analyzing past cycles to determine whether powerful sunspots similarly have appeared late.

“In 1984, we had a bout of activity four years after the solar max in that cycle,” Murtagh said. “What’s different with cycle 23 is today’s events are more intense than what occurred at the cycle’s maximum.”

Sunspots are best known for spawning solar flares, which are akin to space tornadoes. They last for hours, extend for tens of thousands of miles and reach millions of degrees.

In recent years, astronomers have identified a second and even more powerful tempest – the coronal mass ejection. Like a cosmic Molotov cocktail, that is the phenomenon that has been bombarding Earth lately.

A CME bursts from the sun’s corona, the wispy, outermost and hottest layer. Often, CMEs trigger solar flares below, too.

CMEs belch huge clouds of superheated particles at speeds exceeding 1 million mph. Earth can orbit straight into the speeding particles and they can envelop the planet for hours.

If these incoming particles have a southward magnetic orientation, they slice against the grain of Earth’s north-pointing magnetic field, and travel deep into the atmosphere. This causes electrical and radio disturbances, as well as colorful aurora displays in the night sky.

Space forecasters measure the intensity of CMEs on three scales. Each scale is 1-5.

The G-scale measures the geomagnetic storm generated when the particle cloud slams into Earth’s magnetic field. A G5 storm can knock out electrical power grids.

The S-scale measures radiation pulses. In an S5 storm, airline passengers flying though the incoming fallout would receive the equivalent dose of 100 chest X-rays.

The R-scale measures radio blackouts. At R5, the entire sunlit side of Earth would experience a high-frequency radio blackout.

The CME on Oct. 28 measured G5-S4-R4.

“It was almost the perfect space storm,” Murtagh said. Yet, because utilities and satellite companies were ready for it, its damage was limited.

A fourth scale measures the intensity of X-ray emissions from solar flares.

During the current sunspot period, solar flares erupting on Oct. 19 rated X3 and X5. On Oct. 28-29, the major CME triggered a one-two solar flare punch. The first measured X17.2 – it was the third-largest flare ever recorded. A day later, the second flare measured X11.

But Sunspot 486 was just warming up.

Beginning on Nov. 3, it triggered three flares over several hours. The final flare on Nov. 4 saturated the X-ray detectors on NASA’s GOES satellite, which produces a new image of the sun every minute. The satellite was blinded for 11 minutes.

Luckily for Earth, the sunspot cluster was rotating off the sun’s face and out of view. Most of its fury was directed harmlessly into space.

Officially, it is ranked as X28. But some researchers believe it might’ve registered an astonishing X40.

Previously, the most intense flares were a pair of X20s.

The X-scale has been in use only since the 1970s, so historical comparisons are difficult. But the Nov. 4 flare rivals an event in 1859 that knocked out telegraph service across in the United States.

“What is clear is that the latest flare is the strongest ever recorded,” Brekke said.

Scientists are certain sunspots 484 and 486 will surf back around the sun’s face and take aim at Earth again. Though hidden from view, their explosiveness can be gauged by researchers much as seismic waves from earthquakes are measured.

The SOHO observatory is also photographing clouds of gas being thrown over and around the sun’s far side by unseen explosions.

Astronomers are warning satellite and power grid operators to be ready for more fireworks.

“Society is becoming more dependent on systems that could fail during these events,” Brekke said. “People should not be afraid, but we should learn to live with our closest star and how it is varying.”

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On the Net:

NOAA Space Environment Center:

http://www.sec.noaa.gov

Stanford Solar Center: http://solar-center.stanford.edu

AP-ES-11-14-03 0950EST