THE SUN: Our Neighborhood Star - 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 Sun.

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?

Trip 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 rises.

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 Sun's surface.

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 fuel.

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.