Why Is The Sun So Bright?

The sun, an average, middle-aged star, provides the Earth with light, warmth and energy even though it's 150 million kilometers (93 million miles) distant. What makes the sun so bright is its power source: a process called nuclear fusion, which yields abundant energy. The fusion reaction, along with the sun's enormous size, means it will continue to shine brightly billions of years into the future.

The sun consists primarily of hydrogen and helium gas. At the center of the sun, the force of gravity squeezes hydrogen atoms with huge amounts of pressure. Under ordinary conditions, the positive electric charges at the center of hydrogen atoms make them repel each other strongly, but the sun's gravity is so great the atoms fuse together, forming deuterium and helium and releasing vast quantities of energy. The energy released by fusion is about 10 million times greater than combustion — the reaction that causes coal and gasoline to burn.

As stars go, the sun isn't the biggest or the brightest; it's fairly small and dim compared to others. Astronomers call stars like the sun yellow dwarfs and give them a classification code of "G V." If the sun were a larger, brighter star, it would engulf the Earth with its size and roast the planet with its energy. Larger stars also run out of energy much faster than smaller ones like the sun, rapidly consuming their stores of hydrogen and dying out within a few hundred million years.

The sun is a complex object with many layers, each of which has a characteristic temperature. The center, called the core, is where most of the fusion takes place; scientists estimate its temperature at 15 million degrees Celsius (27 million degrees Fahrenheit). The surface, called the photosphere, is the brightest part of the sun, although it is much cooler — about 6,000 degrees Celsius (more than 10,000 degrees Fahrenheit).

The sun produces a wide range of light wavelengths called a spectrum. In addition to the familiar colors people see, the sun's spectrum contains X-rays, ultraviolet and infrared light and radio waves. The Earth's atmosphere fortunately blocks most of the harmful wavelengths; without this shielding effect, life would not be possible.

Inside the sun, 600 million tons of hydrogen are converted into helium every second, producing enough energy to power 4 trillion trillion 100-watt light bulbs. Because a large distance separates the Earth and sun, however, the planet receives just a tiny fraction of this, amounting to 400 trillion watts, or about 1,000 watts per square meter at the Earth's surface.