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KnowledgeNews
SciencePhiles
Firing Up the Sun

The sun, in ultraviolet light. The colors reveal temperature,
from 1.8 million (blue) to 3.6 million (red) degrees Fahrenheit. 

Friends, sun scientists have announced the start of a new solar cycle.
(It's "Solar Cycle 24," if you're keeping track.) What does that mean?

You might not know it, but solar activity waxes and wanes in 11-year
cycles. Recently, we've been at the end of a cycle, with a comparative
lack of solar
activity. The start of a new cycle means that the frequency of solar
storms and other activity should start to pick up--though the new cycle
won't reach
"Solar Max" until 2011 or 2012.

There's nothing to fear--though NASA scientists warn that "solar storms
can disable satellites that we depend on for weather forecasts and GPS
navigation."
There is, however, plenty to learn about the sun. You know that the
bright ball in the sky is crucial to life on Earth. But do you know what
it's actually
made of? Today, let's slather on some SPF 15,000,000 and journey to the
center of the sun.

3 . . . 2 . . . 1 . . . Blastoff!

First, get comfortable. It's going to be a long trip--about 93 million
miles (150 million km), depending on where Earth is in its elliptical
orbit. Since
we've got time to burn, let's brush up on some basic sun facts.

Size facts: Astronomers say our sun is nothing special as stars go--just
another middle-aged yellow dwarf. Even so, it's by far the biggest thing
in our
neck of the cosmos, accounting for more than 99 percent of our solar
system's total mass. More than a million Earths could fit inside the
sun, which has
a diameter of 865,000 miles (1.4 million km).

Age facts: The sun has been burning for 4.6 billion years and has a life
expectancy of nine or ten billion. In another four or five billion, our
yellow
dwarf will expand into a hot red giant. Then it will contract into a
white dwarf, smaller than it is now. Finally, when all its thermal
energy is spent,
it will become a cold black dwarf.

Inside facts: The sun is made almost entirely of hydrogen and helium.
About 70 percent of its mass is hydrogen. Another 28 percent is helium.
The rest consists
of elements like carbon, nitrogen, and oxygen. The sun's core process is
a nuclear fusion reaction in which hydrogen atoms fuse to produce helium
atoms
(and release energy when they do).

Touching the Surface

If you're starting to feel hot, it's because we've reached the corona,
the outermost part of the sun's atmosphere (not including the solar
wind, which blows
out well past Pluto). Temperatures here regularly reach 3.6 million
degrees Fahrenheit (2 million degrees Celsius). The corona extends
millions of miles
into space, so we still have a way to go to get to the next layer.
Luckily, we're flying fast.

Beneath the corona is the chromosphere (from the Greek word chromos, or
"color"). It has a reddish tint, and it's far cooler than the
corona--thousands
of degrees hot instead of millions. In fact, scientists still don't
understand why the corona is so hot. From Earth, we can't see the
chromosphere, or
the corona, except during a solar eclipse.

Having traveled through several thousand miles of chromosphere, we reach
the photosphere, which is only a few hundred miles thick. Here, the
temperature
is a comparatively cool 10,000 degrees F (5,500 degrees C), and energy
is given off as visible light. The photosphere is the part of the sun we
ordinarily
see from Earth. Some call it the sun's "surface," because beneath it,
the sun's gases are thick enough to be opaque. Above it, they are
transparent.

Inside Stuff

Beneath the photosphere, we reach the sun's interior, which has three
layers. First is the convective zone, where the temperature heats up
again to around
3.6 million degrees F (2 million degrees C). Here, energy circulates in
large cells. This part of the sun is a bit like a pot of boiling water,
only with
hot plasma bubbling up toward the surface.

Further in, we reach the radiative zone. It's as hot as 12.6 million
degrees F here (7 million degrees C), and energy radiates out from the
sun's core at
the speed of light. Still, this deep, the sun is so dense that each
photon of energy may bounce from particle to particle for a million
years before reaching
the convective zone.

After fighting through that traffic jam, we finally arrive at the sun's
core, which burns at 27 million degrees F (15 million degrees C). Talk
about a high-pressure
environment! The pressure here is 250 billion times that of Earth, so
great that hydrogen atoms fuse to form helium atoms. Every second, 700
million tons
of hydrogen become 695 million tons of helium, with the extra 5 million
tons released as energy.

Only one half of one-billionth of the sun's energy will travel to the
Earth's surface, but that's still enough to sustain life on our planet.
Sunlight makes
the 93-million-mile trip in around eight and a half minutes--almost as
fast as you just made it.

--Steve Sampson and Kris Herbert

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