The sun lies at the heart of our solar system, but it still holds back many secrets from science. Unlocking these mysteries could shed light on puzzling activity seen in other stars and even safeguard lives.

An explosive star

The sun is literally bursting with energy, violently exploding with solar flares, coronal mass ejections and other kinds of eruptions up to hundreds of times per year. The number of explosions and sunspots the sun experiences tends to rise and fall in a roughly 11-year-long "solar cycle," the roots of which remain uncertain.

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

Shedding light on the solar dynamo could help predict when solar explosions happen, "which can endanger astronauts and satellites in space and damage power lines on Earth," Charbonneau said. But whether or not scientists can ever predict the solar cycle remains unknown?some claim it is physically impossible to predict.

The super-hot corona

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

The sun's surface is roughly 5,500 degrees Celsius. The corona, on the other hand, is one to three million degrees C or more.

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

The Maunder Minimum

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

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

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

Erratic siblings

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

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

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