Sunlight takes about eight minutes to travel the 93 million miles (150 million kilometers) from the sun to the Earth. The next closest star is about 250,000 times farther away. That is why the sun is by far the brightest object in the sky.

The sun is about average in size for a star. Scientists classify the sun as the type of star called a yellow dwarf. The sun's diameter, or distance across, is 865,000 miles (1,392,000 kilometers). This is about 109 times the distance across the Earth. The sun's mass, or amount of matter, is about 333,000 times greater than that of the Earth.

The core of the sun is about 100 times denser than water. It is also extremely hot. The temperature is probably about 28,080,000° F (15,600,000° C). Under such conditions, atoms cannot exist in stable form. Hydrogen particles collide and combine into helium particles in thermonuclear reactions. These reactions create enormous amounts of energy.

The outer region of the sun that is normally visible from the Earth is called the photosphere, which means “sphere of light.” At about 10,000° F (6000° C), the photosphere is much cooler than the core but still very hot by earthly standards. Solids and liquids cannot exist there. More than 90 percent of the molecules are hydrogen gas. Most of the rest is helium.

From time to time the surface of the sun displays cooler, darker patches called sunspots. Groups of sunspots might cover thousands of miles. Sunspots appear and disappear in 11-year cycles.

Above the surface is a slightly hotter region called the chromosphere, meaning “sphere of color.” This region has a reddish color. At times great blasts of energy called solar flares erupt from the chromosphere. Solar flares can be dangerous to space travelers. The radiation from them can penetrate spaceship walls and damage body cells.

Surrounding the chromosphere is a faint but very hot region called the corona. With a temperature between 1,800,000° F and 3,600,000° F (1,000,000°  and 2,000,000° C), the corona is much hotter than the photosphere. Without special equipment, the chromosphere and the corona are visible from Earth only during total eclipses of the sun. However, people should not view the sun directly even during eclipses.

The corona sends off tiny particles of matter into the solar system. These particles are known as the solar wind. The wind moves outward at about 250 miles (400 kilometers) per second. High solar winds disrupt radio signals on the Earth and cause the colorful bands of light in the polar skies known as the northern and southern lights.

The sun has been shining for at least 4 billion years. As the ages pass, its light is getting brighter, not dimmer. Billions of years in the future, the sun will use up the fuel at its core. As its surface burns, the sun will swell up into the kind of star called a red giant. In this stage, the sun's enormous shell may engulf the Earth.