The sun is an ordinary star. It is just one of about 100 billion in our galaxy, the Milky Way. However, the sun is very important for our planet. Without its heat and light, life would not exist on Earth.

The sun is about 150 million kilometers (93 million) miles from Earth. Light from the sun takes eight minutes and 19 seconds to reach Earth.

The radius of the sun, or the distance from the very center to the outer limits, is about 700,000 kilometers (432,000 miles). That distance is about 109 times the size of Earth's radius.

The sun is also much more massive than Earth. Its mass is more than 333,000 times that of our planet. Indeed, it contains about 99.8 percent of all of the mass in the entire solar system, the region of space that is home to the eight planets and eight moons that orbit the sun.

What Is the Sun Made of?

The sun is made up of a blazing hot combination of gases. About three quarters of the sun is hydrogen. The remaining quarter is mostly helium.

The hydrogen in the sun is constantly fusing, or joining, together. This process is called nuclear fusion. It turns hydrogen into helium.

The sun is not a solid mass. It does not have stable boundaries like rocky planets such as Earth. Instead, the sun has various layers, each made up almost entirely of hydrogen and helium.

The sun is white, but appears orangish yellow because of the blue light it gives off. Astronomers call the sun a "yellow dwarf" star.

The sun rotates, just like Earth. It rotates counterclockwise, and takes between 25 and 35 days to complete a single rotation.

The sun also orbits clockwise around the center of the Milky Way. It takes about 250 million years to orbit one time around the galactic center. That is much longer than the one year it takes Earth to orbit one time around the sun!

The Sun Sends Energy to Earth

The sun's energy travels to Earth at the speed of light. It travels in the form of electromagnetic waves. The vast majority of these waves are invisible to us. They include gamma rays, x-rays and ultraviolet radiation (UV rays). Some UV rays travel through Earth's atmosphere, and can cause sunburn.

The sun also gives off infrared radiation. Most heat from the sun arrives as infrared energy.

The Sun Has Changed over Billions of Years

The sun has existed for about 4.5 billion years. It will not shine forever, though.

Nuclear fusion in the sun creates the heat and light that make life on our planet possible. It is also slowly changing the sun's composition. Through nuclear fusion, the sun is constantly using up the hydrogen in its core. Every second, the sun turns around 684 million tons of hydrogen into helium.

At this stage in the sun's life, its core is about three-quarters hydrogen. Over the next five billion years, however, the sun will burn through most of its hydrogen. Helium then will become its major source of fuel.

When almost all of the hydrogen in the sun's core has been used, the core will shrink and heat up. That, in turn, will increase the amount of nuclear fusion that takes place. The outer layers of the sun will expand from this extra energy.

The sun will expand to about 200 times its current radius, swallowing Mercury and Venus. Our own planet could be swallowed up by the sun as well.

As the sun expands, it will spread its energy over a larger surface area, which will have an overall cooling effect. This cooling will shift the sun's visible light to a reddish color. The sun will then become what astronomers call a "red giant."

Eventually, the sun's core will reach a temperature so low that it will no longer create or give off energy. At that point, the sun will become what is called a "white dwarf." During this last period, the sun will shrink greatly in size. Only the hard carbon core will be left behind.

The Sun has Six Layers

The sun is made up of six layers. These are the core, the radiative zone, the convective zone, the photosphere, the chromosphere, and the corona.

The Center of the Sun Is Its Core

The sun's core is a huge furnace. Temperatures in the core rise above 15.7 million degrees Celsius (28 million degrees Fahrenheit). The core is more than 1,000 times the size of Earth. It extends to about 25 percent of the sun's radius.

The core is the only place where nuclear fusion reactions can happen. The sun's other layers are heated from the nuclear energy created there. The energy released during one second of solar fusion is enormous. It is far greater than the energy that would be released by the explosion of hundreds of thousands of hydrogen bombs.

Heat Cools in the Radiative Zone

The radiative zone of the sun starts at about 25 percent of the radius. It extends to about 70 percent of the radius. Heat from the core cools greatly in the broad radiative zone.

The Transition Zone Is the Tachocline

Between the radiative zone and the next layer, the convective zone, there is a transition zone. This is called the tachocline. It separates the other two zones.

Inside the Convective Zone

The convective zone begins at around 70 percent of the sun's radius. In this zone, the sun is not hot enough to transfer energy by thermal radiation. Instead, it transfers heat though thermal convection.

This process is similar to water boiling in a pot. Gases deep in the sun's convective zone are heated and "boil" outward, away from the sun's core. When the gases reach the outer limits of the convective zone, they cool down. They then plunge back to the base of the convective zone, to be heated again.

The Photosphere Is Bright Yellow

The photosphere is the bright yellow, visible "surface" of the sun. It is about 400 kilometers (250 miles) thick. Temperatures in the photosphere reach about 5,700 degrees Celsius (10,300 degrees Fahrenheit).

Why Do Sunspots Appear?

A sunspot is a dark spot on the sun. It appears dark because it is slightly cooler than the surrounding area.

Solar Flares Erupt from the Corona

Solar flares are eruptions of material out of openings in the corona. The corona is the very outer layer of the sun. Solar flare explosions are huge. One lasting a few minutes has the force of about 160 billion megatons of dynamite.

Solar flares unleash clouds of ions, atoms, and electrons. These clouds reach Earth in about two days, and can effect Earth's atmosphere and magnetic field. They also can interfere with satellite and telecommunications systems.

Coronal Mass Ejections Form

Coronal mass ejections (CMEs) are another type of solar activity. They are caused by the constant changes within the sun's magnetic field. CMEs usually form near sunspots. Their connection with sunspots is still unclear, however.

Solar Prominences Look Like Dark Lines

Solar prominences are curved and twisted loops of solar matter. They sometimes appear on the corona from within the sun. Solar prominences are cooler than the corona. For this reason, they look like dark lines.

Solar prominences can expand hundreds of kilometers per second. They can reach hundreds of thousands of kilometers in height and width. They last anywhere from a few days to a few months.

Solar Cycles Last about 11 Years

The sun does not constantly give off solar flares. It goes through a cycle of about 11 years. During this solar cycle, the frequency of solar flares changes. During solar maximums, there can be several flares per day. During solar minimums, there may be fewer than one a week.

The solar cycle is defined by the sun's magnetic fields. Every 11 years, the magnetic fields reverse. This causes a disruption that leads to increased solar activity and sunspots.

The Sun's Hottest Region

The solar atmosphere is the hottest region of the sun. It is made up of the chromosphere, the corona and the solar transition region. The solar transition region comes between and connects the other two zones.

The solar atmosphere is hidden by the bright light given off by the photosphere. It can only be seen without special equipment during a solar eclipse. During solar eclipses, the moon moves between Earth and the sun and hides the photosphere.

Chromosphere

The pinkish-red chromosphere is about 2,000 kilometers (1,250 miles) thick. Its bottom meets the photosphere.

At the bottom of the chromosphere, the sun is at its coolest. It is "only" about 4,100 degrees Celsius (7,500 degrees Fahrenheit). This low temperature gives the chromosphere its pink color. The temperature in the chromosphere increases with altitude. It reaches 25,000 degrees Celsius (45,000 degrees Farenheit) at the outer edge of the region.

The chromosphere gives off jets of burning gases called spicules. These fiery jets of gas reach out from the chromosphere like long, flaming fingers. They are usually about 500 kilometers (310 miles) in diameter. Spicules only last for about 15 minutes. Even so, they can reach thousands of kilometers in height before collapsing and dissolving.

Solar Transition Region

The solar transition region (STR) separates the chromosphere from the corona.

Below the STR, the layers of the sun are stable. They stay separate because of gravity, gas pressure, and the different processes of exchanging energy. Above the STR, the motion and shape of the layers are much more ever-changing. Their shifts are largely caused by magnetic forces.

Corona Extends Millions of Miles into Space

The corona is the wispy outermost layer of the solar atmosphere. It can extend millions of miles into space. Gases in the corona burn at about one million degrees Celsius (1.8 million degrees Fahrenheit). They move about 145 kilometers (90 miles) per second.

Some of these gases' particles reach a speed of 400 kilometers (249 miles) per second. At that speed, they escape the sun's gravity and become the solar wind. The solar wind blasts from the sun to the edge of the solar system.

Other particles form coronal loops. Coronal loops are bursts of particles that curve back around to a nearby sunspot.

Near the sun's poles there are coronal holes. These areas are colder and darker than other regions of the sun. They allow some of the fastest-moving parts of the solar wind to pass through.

Solar Winds Are Extremely Hot

The solar wind is a stream of extremely hot, charged particles that are thrown out from the upper atmosphere of the sun. The solar wind blows in all directions. It continues moving at the same speed for about 10 billion kilometers (six billion miles).

Some of the particles in the solar wind slip through Earth's magnetic field and into its upper atmosphere near the poles. As they enter the atmosphere, they make the atmosphere glow with color. These colorful light displays are called auroras. They are also known as the Northern Lights and Southern Lights.

Solar winds can also cause solar storms. These storms can interfere with satellites and knock out power grids on Earth.

Studying the Sun

The sun was not always considered something to be studied. For thousands of years, it was thought of as a god or goddess.

To the ancient Aztecs, the sun was a powerful god known as Tonatiuh. The Aztecs offered human sacrifices to this sun god. The Chinese believed our sun was once one of 10 sun gods. The others had disappeared, leaving only one mighty sun god.

For around 1,400 years, Europeans believed the moon, planets, and sun revolved around Earth. In the 1500s, Polish astronomer Nicolaus Copernicus proved this was wrong. He used mathematical and scientific reasoning to show that planets rotated around the sun. This model of the universe is called the heliocentric model. It is the one we use today.

In the 1600s, the telescope was invented. It allowed people to examine the sun in detail for the first time. The sun is much too bright to be studied with unprotected eyes.

Viewing the Sun from Space

Twentieth-century astronomers used balloons and rockets to send special telescopes high above Earth. Once in space, these telescopes were trained on the sun. They were able to examine the sun without any interference from Earth's atmosphere.

Solrad 1 was the first spacecraft designed to study the sun. It was launched by the United States in 1960. That decade, the U.S. space agency NASA sent five Pioneer satellites to orbit the sun and collect information.

In 1980, NASA launched a mission during the solar maximum. Its purpose was to gather information about the gamma rays, UV rays and x-rays that are given off during solar flares.

The Solar and Heliospheric Observatory (SOHO) was developed in Europe and put into orbit in 1996 to collect information. SOHO can collect data and forecast space weather.

Energy from the Sun

Photosynthesis

Sunlight provides necessary light and energy to plants. The sun's radiation is absorbed, or taken in, by plants and turned into energy through a process called photosynthesis. Plants use this energy to grow.

Fossil Fuels

Photosynthesis is also responsible for all of the fossil fuels on Earth. Fossil fuels include natural gas, coal, and petroleum (or oil). Scientists estimate that about three billion years ago, the first plants evolved. After the plants died, they decomposed and shifted deeper into Earth. This process continued with new plants for millions of years.

Over time, great pressure and high temperatures turned these plant remains into fossil fuels. We use these fuels as a very important source of energy. However, their use also causes serious environmental problems.

Solar Energy Technology

Solar energy technology takes the sun's radiation and turns it into heat, light, or electricity. It does this without producing the kind of pollution fossil fuels produce.

In one hour, Earth's atmosphere receives enough sunlight to power the electricity needs of all people for a year. However, solar technology is expensive. It also needs sunny and cloudless local weather to be effective. Ways of using the sun's energy are still being developed and improved.