The Enduring Mysteries of the Sun

Thesun lies at the heart of our solar system, but it still holds back many secretsfrom science. Unlockingthese mysteries could shed light on puzzling activity seen in other stars andeven safeguard lives.

Anexplosive star

The sun isliterally bursting with energy, violently exploding with solar flares, coronal massejections and other kinds of eruptions up to hundreds of times per year. Thenumber of explosions and sunspots the sun experiences tends to rise and fall ina roughly 11-year-long "solar cycle,"the roots of which remain uncertain.

Astrophysicistsgenerally agree the solar cycle is driven by the solar dynamo?the flowing,electrically charged gas within the sun that generates its magnetic field?andthat magnetic fluctuations trigger solar explosions. "But which of themany dynamo models is right is uncertain," said solar physicist Paul Charbonneau of the University of Montreal.

Sheddinglight on the solar dynamo could help predict whensolar explosions happen, "which can endanger astronauts and satellitesin space and damage power lines on Earth," Charbonneausaid. But whether or not scientists can ever predict the solar cycle remainsunknown?some claim it is physically impossible to predict.

Thesuper-hot corona

Just as afire feels warmer the closer one gets to it, so is the core of the sun hotterthan its surface. Mysteriously, however, the corona?the sun's atmosphere?isalso far hotter than its surface.

The sun'ssurface is roughly 5,500 degrees Celsius. The corona, on the other hand, is oneto three million degrees C or more.

Why thecorona is super-hot is hotly debated. Some researchers suggest the sun'smagnetic fields heat the corona, while others propose that waves fromthe sun do. "I wouldn't be surprised if these mechanisms are at worktogether. They're not mutually exclusive," said BernhardFleck, project scientist for the Solar and Heliospheric Observatory (SOHO)spacecraft.

TheMaunder Minimum

Oddly, thesolar cycle once seemed to go on vacation for roughly 70 years. Only 50sunspots were seen during this Maunder Minimum between 1645 to 1715, as opposedto the expected 40,000 to 50,000.

Researchdoes suggest that similar phases of suppressed activity have occurred in thepast 10,000 years, with the sun in such "quiet" modes about 15percent of the time, Charbonneau said. Why these occur remains unclear, although there are modelsof the sun that suggest the solar dynamo can rev solar cycles up or down.

Also, theMaunder Minimum coincided in part with theLittle Ice Age, leading to debates over whether or not the sun was thecause of that past climatic shift or the current one the world is undergoing. "The agreement of the majority of the scientists isthat while the sun has had an influence on Earth's climate in the past, therecent dramatic change in climate is not caused by the sun but due to man-madegreenhouse gases," Fleck said.

Erraticsiblings

Most starslike the sun actually behave more erratically than our sun. "More thanhalf of sun-like stars either have cycles that are slowly increasing ordecreasing in how active they are over time instead of remaining steady, orthey're completely irregular," said solar physicist Karel Schrijver at Lockheed Martin Advanced Technology Center in PaloAlto, Calif. "We don't really know why."

NASA's upcomingSolar Dynamic Observatory spacecraft could shed light on the inner workings ofthe sun and therefore its siblings, Schrijver said,"and therefore shed light on these mysteries."

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