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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