The Children's Encyclopedia
2. Journey to the Center of the Sun
On a clear night, you can see hundreds
of stars in the sky. During the day, though, you can see only one star –
The stars at night are all just tiny points of light. But the Sun is a big yellow
disk, extremely bright. Since the Sun is a star, why does it look so much bigger
and brighter than all the other stars? The answer is that, compared to other stars,
the Sun is very, very close. It is our "neighborhood star."
The Sun in Space
An express train, going 100 miles (160 kilometers) per hour, would take 106 years
to reach the Sun. The same train would reach the Moon in about 3 months and go
around the Earth in a little more than 10 days. But do you know how long it would
take our train to get to the next nearest star after the Sun? 28 million years!
Compared with this distance, the Sun is truly just a space hop away.
Our planet Earth is an inner member of the Sun's family – the solar system.
Earth orbits, or moves around, the Sun, in a path that is roughly a circle, along
with eight other planets. It takes Earth one year to go all the way around the
Sun. Our planet also spins on its axis like a top and takes one day to do a full
turn in this way.
At night, our side of the Earth is turned away from the Sun. Then we see only
the blackness of space, the stars, and sometimes the Moon. During the day, though,
our side of the Earth is facing the Sun. The Sun's light makes the sky too bright
for us to see any of the other stars.
Our Friendly, Neighborhood Star
Even at 93 million miles (150 million kilometers) – the distance from the
Sun to the Earth – the Sun looks big and bright. But can you imagine what
the Sun would be like close up? Its giant face is 865,000 miles (1,392,000 kilometers)
in diameter – wider than a hundred Earths. And, if we could set it on a
scale, we would find that it weighed over five hundred times more than the rest
of the solar system put together!
There are other things, too, apart from size and weight, that make the Sun different
from its family of planets. The Sun is very hot. At its surface, the temperature
is around 11,000°F (6,000°C) – hot enough to boil any metal. Deeper
down, it is much hotter still.
Because it's so hot, the Sun isn't a solid object like the Earth. It is an enormous
ball of gas that gives off a great deal of light and heat of its own. Planets,
moons, and smaller objects, on the other hand, shine only by reflecting sunlight.
They have no light of their own.
This difference leads us to an important question. How does the Sun make all that
light and heat? Is it on fire? Is it perhaps exploding?
Inside the Sun
To find the answer, we will journey to the Sun. We'll travel in an imaginary spaceship
that can stand any temperature, no matter how high. And we'll take with us some
useful gadgets for making measurements.
When we arrive, our first task is to find what the hot gas of the Sun is made
of. Using a special scoop, we gather up a sample and test it. Here are the results.
At the Sun's surface, three-quarters of the gas is hydrogen, one-quarter is helium,
and only a tiny amount is anything else.
We can guess, then, that either hydrogen or helium must be the raw fuel from which
the Sun makes its light and heat. But exactly how does the Sun "burn"
its fuel? And where is the fiery furnace in which it makes the light and heat
that gives us life?
Scientists know that the Sun isn't burning like a fire. If it were, even though
it is very big, it would have burnt to a dead cinder long ago. The Sun has a much
better way of making energy out of its gassy fuel.
To find out what it is, we must leave the Sun's surface for a while and go below.
Deeper and deeper we must go, towards the very core of our neighborhood star.
At the surface, the Sun's gas is much thinner than the air we breathe on Earth.
But as we go deeper into the Sun, the gas around us gets thicker. Its density
The Sun, because of its gravity, is always trying to pull itself together. Parts
of the Sun that are deep inside are squashed by the weight of the parts that are
farther out. As we go deeper, the weight of the outside parts increases. The inside
gas is squashed more and more, making it thicker. Gas right in the middle of the
Sun is so dense that a box full of it would weigh 12 times more than a box full
of lead the same size!
Inside the Sun
Something else, too, is unusual about the gas in the middle of the Sun. It is
very hot. Its temperature is at least 27,000,000°F (15,000,000°C).
Hot hydrogen in the Sun's core is broken up into tiny pieces called protons. At
the high temperature of the core, the protons dash around. They bump into each
other a great deal, and sometimes they stick together.
When four protons have managed to stick together, they form a piece of helium.
But they aren't exactly the same. Two protons have become neutrons. Together,
all four pieces weigh a bit less than when they were all apart. The bit that they
lose is turned into energy – light and heat that, in time, escape from the
Here, then, in the core, is the Sun's furnace. Protons sometimes strike each other,
stick together, and build into pieces of helium. In this process, a tiny bit of
matter is turned into a lot of energy that is given off by the Sun as life-giving
light and heat.
Scientists use the word fusion to describe how the Sun makes energy. Protons are
fused to form helium. Slowly, the middle of the Sun is losing hydrogen and gaining
helium. But it will take a very long time for the Sun to run out of its hydrogen
While fusion goes on in its core, the Sun has a way to stop itself from being
squeezed by gravity. The Sun stays the same size by balancing its inward force
of gravity with the outward pressure of light and heat.