Can you imagine life on Earth without the Sun? It might be fun for about an hour. You could play an awesome game of flashlight tag with your friends and then tell spooky stories around a fire.

Before long, though, it wouldn't be so fun. You'd soon need to keep a fire going around the clock, because Earth would become a much colder place without the Sun's heat. In addition, food would become scarce over time, since plants wouldn't grow without the sunlight they need for photosynthesis.

The Sun's energy also powers Earth's weather and, through evaporation, the water cycle. What Earth would be like without these essential processes working properly is anyone's guess. It's safe to say that life on Earth wouldn't exist very long if the Sun went out.

Scientists have long known that the Sun makes life on Earth possible. We use the Sun's energy in many different ways. But just how does the Sun produce all that energy? To find out, we need to get to the heart of the matter and travel all the way to the Sun's core.

Unlike Earth, which is made up mostly of iron, rock, water, and many other elements, the Sun is made up primarily of hydrogen gas. The Sun is similar to Earth in one way, however: its core is by far its hottest part.

In addition to intense heat, there is an incredible amount of pressure at the Sun's core. In fact, the vast amounts of hydrogen atoms in the Sun's core are compressed and heated so much that they fuse together.

This reaction, known as nuclear fusion, converts hydrogen atoms into helium. The by-product of nuclear fusion in the Sun's core is a massive volume of energy that gets released and radiates outward toward the surface of the Sun and then into the solar system beyond it.

The nuclear fusion reaction that occurs in the Sun's core is similar to the reaction that produces the mighty explosion in a hydrogen bomb. Of course, the fusion reactions that occur in the Sun's core are almost-infinitely more powerful than a hydrogen bomb. So why doesn't the Sun explode?

The outward pressure created by the fusion reactions in the Sun's core are balanced by the inward pressure created by the gravitational force of all the gases that surround the core. Stars can and do collapse or explode if the fusion reactions at their cores become too weak or too strong.

Fortunately for those of us living on Earth, the Sun's outward and inward pressures are nicely balanced, resulting in a steady stream of energy coming our way. Of all the energy that the Sun produces, scientists estimate that Earth receives only about two billionths of it.

Of that amount, about a third is reflected back into space by clouds and snow. A little over 40% warms Earth, while about 25% is used by the water cycle. Winds and ocean currents absorb about 1%, while all the plants on Earth use only about 0.023% for photosynthesis!

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