510
Sun factS
Earth–sun 92.9 million miles
distance (149.6 million km)
Diameter 864,950 miles
at equator (1,392,000 km)
Time to 25.4 days
rotate once
Temperature 10,000°F (5,500°C)
at surface
Temperature 27,000,000°F
at center (15,000,000°C)ThE nighT sky is full of stars, so distant that they are mere points
of light. The sun is also a star, but we are closer to it than to any other
star. Along with the other planets of the solar system, Earth moves
around the sun, trapped in orbit by the force of gravity. The sun is a
ball of glowing gases, roughly three-quarters hydrogen and one-quarter
helium, along with traces of other elements. Within its hot, dense core,
hydrogen particles crash together. This produces nuclear reactions that
release enormous amounts of energy, keeping the core of the sun very
hot. The energy travels outward and leaves the sun’s surface mainly
as light, and infrared and ultraviolet radiation. The sun sustains
nearly all life on Earth with its light and heat. Energy sources
that humans use to provide power originate from
the sun. For example, coal is the
remains of ancient plants,
which trapped the
sun’s energy.sTory oF ThE sun
The sun formed just under 5,000 million
years ago from a cloud of hydrogen and
helium, and dust that contracted (shrank)
under its own gravity. The contraction heated
the cloud until nuclear reactions began,
converting hydrogen into helium. At this
point, the sun began to shine steadily. it is
believed that the sun will continue to shine
for another five billion years before it runs
out of hydrogen fuel and begins to die.Great streamers of
glowing hydrogen gas,
called prominences,
frequently soar up from
the sun. Prominences
are often about more
than 37,000 miles
(60,000 km) long.solAr EClipsEs
When the moon passes between Earth and the sun, the
sun is hidden. This is called a solar eclipse. A total solar
eclipse occurs at places on Earth where the sun appears
to be completely hidden (although prominences,
chromosphere, and corona can be seen). Elsewhere the
eclipse is partial, and parts of the sun can be seen.Umbra is the center of the
moon’s shadow, where the
sun is completely hidden.Penumbra is the outer part
of the moon’s shadow,
where part of the sun
can be seen.Energy travels outward
in the form of heat and
electromagnetic waves
such as infrared, light,
and radio waves.Relatively cool and dark
areas, called sunspots,
form on the surface of the
sun. Sunspots develop in
places where the sun’s
magnetic field becomes
particularly strong.Warning: never look at the sun,
either directly or through dark
glasses. The intense light could
seriously damage your eyesight.The sun’s diameter
is 109 times that of Earth.
More than 1,300,000
globes the size of Earth
could fit into the sun.Light from the
sun takes about
eight minutes to
reach Earth.Sun
Find out more
Astronomy
Energy
starsSolar energy
Electronic devices called solar cells
convert sunlight into electricity.
panels of solar cells produce
electricity to power many things
from satellites to people’s homes. in
2015, the solar-powered aircraft
solar impulse 2 (below) began
its journey around the world
from Abu Dhabi.solAr FlArEs
huge explosions on the
sun’s surface, called solar
flares, fire streams of
electrically charged
particles into space.CoronA AnD
solAr WinD
A thin pearl-white
atmosphere of gases,
called the corona,
extends for millions
of miles around the sun.
A blast of electrically
charged particles, called the
solar wind, blows out from the
corona at a rate of millions of
tons each second. Earth is
protected from these particles by
its magnetic field, but they can
damage spacecraft and satellites.
Coronal Mass Ejections are
sudden blasts of great clouds
from the corona. These are
thought to cause auroras—
colored lights in the sky above
Earth’s poles—and
magnetic storms.Core extends to about
110,800 miles (175,000 km)
from the sun’s center.The hot, glowing surface of the sun is
called the photosphere (sphere of light).
It is about 250 miles (400 km) deep.A glowing red layer of hydrogen gas called
the chromosphere (sphere of color) lies
above the photosphere. The chromosphere
is a few thousand miles deep.US_510_Sun.indd 510 12/02/16 5:40 pm