The sun is an ordinary star, and is just one of about 100 billion in our galaxy, the Milky Way. However, the sun has extremely important influences on our planet: It drives weather, ocean currents, seasons, climate, and makes plant life possible through photosynthesis. Without the sun's 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 about 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 not only has a much larger radius than Earth—it is also much more massive. The sun's mass is more than 333,000 times that of Earth. The solar system is the region of space that is home to the eight planets and eight moons that orbit the sun. The sun is so large it contains about 99.8 percent of all of the mass in the entire solar system!

The Sun Is a Combination of Gases

The sun is made up of a blazing combination of gases. About three quarters of the sun is hydrogen, which is constantly fusing together and creating helium by a process called nuclear fusion. Helium makes up almost the entire remaining quarter. A very small percentage—1.69 percent—of the sun's mass is made up of other gases and metals: iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium.

The sun is not a solid mass. It does not have easily identifiable boundaries like rocky planets such as Earth. Instead, the sun is composed of layers 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 around its own axis, 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. Its orbit is between 24,000 and 26,000 light-years away from the galactic center. A light-year is a unit of astronomical distance equal to the distance that light travels in one year, which is about 9.5 trillion kilometers (5.9 trillion miles). The sun takes about 225 million to 250 million years to orbit once around the galactic center.

The Sun Sends Energy to Earth

The sun's energy travels to Earth at the speed of light 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). The most harmful of the UV rays are almost completely absorbed by Earth's atmosphere. Less harmful UV rays travel through the atmosphere, and can cause sunburn.

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

The Evolution of the Sun

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

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

At this stage in the sun's life, its core is about 74 percent hydrogen. Over the next five billion years, the sun will burn through most of its hydrogen, and helium will become its major source of fuel.

When almost all of the hydrogen in the sun's core has been consumed, the core will contract 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 engulfed 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, and the sun will 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 phase, the sun will shrink greatly in size, leaving only the hard, carbon core.

The Sun's Six Layers

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

Core

The sun's core is a huge furnace. Temperatures in the core exceed 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 far greater than that released in the explosion of hundreds of thousands of hydrogen bombs.

Radiative Zone

The radiative zone of the sun starts at about 25 percent of the radius, and extends to about 70 percent of the radius. In this broad zone, heat from the core cools dramatically.

Transition Zone: Tachocline

Between the radiative zone and the next layer, the convective zone, there is a transition zone called the tachocline.

Convective Zone

At around 70 percent of the sun's radius, the convective zone begins. In this zone, the sun's temperature 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, through thermal columns. When the gases reach the outer limits of the convective zone, they cool down, and plunge back to the base of the convective zone, to be heated again.

Photosphere

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

Photosphere: Sunspots

A sunspot is just what it sounds like—a dark spot on the sun. A sunspot forms when intense magnetic activity in the convective zone breaks a thermal column. At the top of the broken column, temperature is temporarily decreased because hot gases are not reaching it. This top part is visible in the photosphere and appears darker than the surrounding area.

Photosphere: Solar Flares

The process of creating sunspots opens a connection between the corona—the very outer layer of the sun—and the sun's interior. Solar matter surges out of this opening in formations called solar flares. These explosions are massive: In the period of a few minutes, solar flares release the equivalent of about 160 billion megatons of TNT.

Clouds of ions, atoms, and electrons erupt from solar flares, and reach Earth in about two days. Solar flares can cause disturbances to Earth's atmosphere and magnetic field. They also can interfere with satellite and telecommunications systems.

Photosphere: Coronal Mass Ejections

Coronal mass ejections (CMEs) are another type of solar activity caused by the constant movement and disturbances within the sun's magnetic field. CMEs typically form near the active regions of sunspots. Their cause is still unclear, however.

Photosphere: Solar Prominence

Solar prominences are bright loops of solar matter. They can burst far into the coronal layer of the sun, expanding hundreds of miles per second. These curved and twisted features can reach hundreds of thousands of miles in height and width, and last anywhere from a few days to a few months.

Solar prominences are cooler than the corona, and they appear as darker strands against the sun. For this reason, they are also known as filaments.

Photosphere: Solar Cycle

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, which loop around the sun and connect at the two poles. Every 11 years, the magnetic fields reverse, causing a disruption that leads to solar activity and sunspots.

The solar cycle can have effects on Earth's climate. For example, the sun's ultraviolet light splits oxygen in the stratosphere—earth's upper atmosphere—and strengthens Earth's protective ozone layer. During the solar minimum, there are low amounts of UV rays, which means that Earth's ozone layer is temporarily thinned. This allows more UV rays to enter and heat Earth's atmosphere.

Solar Atmosphere

The solar atmosphere is the hottest region of the sun. It is made up of the chromosphere and corona, and a transition zone called the solar transition region that connects the other two.

The solar atmosphere is hidden by the bright light given off by the photosphere. It can rarely be seen without special instruments. Only during solar eclipses, when the moon moves between Earth and the sun and hides the photosphere, can these layers be seen with the unaided eye.

Chromosphere

The pinkish-red chromosphere is about 2,000 kilometers (1,250 miles) thick and riddled with jets of hot gas.

At the bottom of the chromosphere, where it meets the photosphere, the sun is at its coolest, at 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, and reaches 25,000 degrees Celsius (45,000 degrees Fahrenheit) at the outer edge of the region.

The chromosphere gives off jets of burning gases called spicules, similar to solar flares. These fiery strands 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, but 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 controlled and 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 controlled by magnetic forces. These magnetic forces can cause solar events such as coronal loops and the solar wind.

Corona

The corona is the wispy outermost layer of the solar atmosphere, and can extend millions of kilometers into space. Gases in the corona burn at about one million degrees Celsius (1.8 million degrees Fahrenheit), and 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 gravitational pull 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 are coronal holes. These areas are colder and darker than other regions of the sun, and allow some of the fastest-moving parts of the solar wind to pass through.

Solar Wind

The solar wind is a stream of extremely hot, charged particles that are thrown out from the upper atmosphere of the sun. This means that every 150 million years, the sun loses a mass equal to that of Earth. However, even at this rate of loss, the sun has only lost about 0.01 percent of its total mass from solar wind.

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 collide with atmosphere, these charged particles set the atmosphere aglow with color, creating auroras, colorful light displays 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 known in cultures all over the world as a god or goddess, and a symbol of life.

To the ancient Aztecs, the sun was a powerful god known as Tonatiuh, who required human sacrifice to travel across the sky. In Baltic myths, the sun was a goddess named Saule, who brought fertility and health. The Chinese believed our sun was once one of 10 sun gods, but is now the only one remaining.

In C.E. 150, Greek scholar Claudius Ptolemy created a geocentric model of the solar system in which the moon, planets, and sun revolved around Earth. It was not until the 16th century that Polish astronomer Nicolaus Copernicus used mathematical and scientific reasoning to prove that planets rotated around the sun. This heliocentric model is the one we use today.

In the 17th century, the telescope allowed people to examine the sun in detail. The sun is much too bright to be studied with unprotected eyes. With a telescope, it was possible for the first time to project a clear image of the sun onto a screen for examination.

Studying the Sun from the Sky and Space

Over the following centuries, technology continued to improve, allowing scientists to uncover new features of the sun. Infrared telescopes were invented in the 1960s, giving scientists the ability to observe energy outside of the visible spectrum of light. Twentieth-century astronomers used balloons and rockets to send specialized telescopes high above Earth. Once in space these telescopes 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 about the star.

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 forecast space weather.

Energy from the Sun

Photosynthesis

Sunlight provides necessary light and energy to plants. The sun's radiation is absorbed by plants and converted into energy through a process called photosynthesis.

Fossil Fuels

Photosynthesis is also responsible for all of the fossil fuels on Earth. Scientists estimate that about three billion years ago, the first plants evolved. After the plants died, they decomposed and shifted deeper into the earth. This process continued with new plants for millions of years.

Under intense pressure and high temperatures, these plant remains became what we know as fossil fuels—petroleum, natural gas, and coal. We use these fuels as an important source of energy, though their use also causes serious environmental problems.

Solar Energy Technology

Solar energy technology harnesses the sun's radiation and converts 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, and depends on sunny and cloudless local weather to be effective. Methods of harnessing the sun's energy are still being developed and improved.