Structure of the Sun: A Complete Guide from Core to Atmosphere

1. The Sun's Interior Structure

If we could slice the Sun in half, what would we see?

The interior is divided into three main layers: the Core, the Radiative Zone, and the Convective Zone. Each layer has a unique method of creating or transporting energy. Think of it like an egg with a yolk, white, and shell—each part plays a distinct role.

1-1. The Core: The Power Plant

The Core occupies about 20-25% of the Sun's radius. While it's only a quarter of the size, this is where all the energy is born.

The conditions here are beyond imagination. The temperature is about 27 million degrees Fahrenheit (15 million degrees Celsius), and the pressure is roughly 250 billion times that of Earth's atmosphere. In this extreme environment, matter exists as "plasma"—a soup of electrons and atomic nuclei buzzing around.

Here, hydrogen nuclei collide at high speeds and fuse to form helium. This is Nuclear Fusion.

When four hydrogen nuclei fuse into one helium nucleus, a tiny bit of mass is lost. Where does it go? Following Einstein's famous equation E=mc², that mass is converted directly into energy.

The Sun converts about 600 million tons of hydrogen into helium every second. Of that, about 4 million tons constitute pure energy released into space. Despite this massive consumption, the Sun is so huge it won't run out of fuel for another 5 billion years.

1-2. The Radiative Zone: The Long Journey

Energy born in the core doesn't just fly out instantly. It must first pass through the "Radiative Zone."

This zone covers about 70% of the Sun's radius. Here, energy travels as photons (light particles). However, the plasma is so dense that photons cannot travel in a straight line.

A photon travels a tiny distance, hits a particle, gets absorbed, and is re-emitted in a random direction. This game of "pinball" happens trillions of times. Because of this "random walk," it takes about 170,000 years for a photon to escape from the core to the surface (Source: Stanford Solar Center).

The sunlight hitting your face today was actually created in the center of the Sun before human civilization even began.

1-3. The Convective Zone: The Boiling Pot

The outer 30% of the solar interior is the "Convective Zone." Here, the transport of energy changes dramatically.

As the temperature drops from about 3.5 million °F to 10,000 °F near the surface, the plasma becomes too opaque for radiation to work efficiently. Instead, convection takes over. Hot plasma rises, releases heat at the surface, cools down, and sinks back down.

Imagine a pot of boiling soup or oatmeal. The heat from the bottom causes the liquid to bubble up and circulate. The Sun does the same thing.

This movement creates patterns on the surface called "granules." Each granule is about the size of Texas (roughly 600 miles or 1,000 km across) and lasts only 8 to 20 minutes. The surface of the Sun is literally boiling.

2. The Solar Atmosphere

Surrounding the main body of the Sun is its atmosphere, consisting of three layers: the Photosphere, the Chromosphere, and the Corona.

2-1. The Photosphere

When we "look" at the Sun (never look directly!), we are seeing the Photosphere. This is the visible surface.

It is relatively thin, only about 300 miles (500 km) deep. The temperature here is about 10,000°F (5,500°C), giving the Sun its yellow-white color.

The most famous features here are Sunspots. These are dark patches on the surface that appear black only because they are cooler (~6,000°F) than the surrounding area. They are caused by intense magnetic fields inhibiting the flow of hot plasma.

2-2. The Chromosphere

Just above the photosphere lies the Chromosphere ("Sphere of Color"). Usually invisible due to the glare of the photosphere, it can be seen as a reddish ring during a total solar eclipse.

The red color comes from hydrogen emitting light at a specific wavelength (H-alpha). Interestingly, unlike a campfire where it gets cooler as you move away, the temperature in the chromosphere actually rises as you go further out, reaching up to 36,000°F.

2-3. The Corona

The outermost layer is the Corona (Latin for "Crown"). This pearly white halo extends millions of miles into space and is best seen during a total solar eclipse.

The Corona presents a major scientific mystery: its temperature soars to 1.8 to 5.4 million degrees Fahrenheit. Why is the atmosphere millions of degrees hotter than the surface? It's like walking away from a fireplace and feeling the air get hotter instead of colder.

NASA's Parker Solar Probe is currently flying through the corona to solve this mystery, studying how magnetic waves might be heating this outer layer.

3. Solar Activity

The Sun is not a quiet, static ball of light. It is a dynamic, active star.

3-1. Sunspots and the 11-Year Cycle

The number of sunspots increases and decreases in a regular cycle of about 11 years. This is known as the Solar Cycle.

During "Solar Maximum," sunspots are numerous, and solar flares are frequent. During "Solar Minimum," the sun can be spotless for days. This rhythm is driven by the Sun's magnetic dynamo, though the exact mechanics are still being researched.

3-2. Solar Flares and CMEs

Sometimes, magnetic energy near sunspots is released explosively. This is a Solar Flare. A flare can release the energy equivalent of millions of hydrogen bombs in minutes.

Often accompanying flares are Coronal Mass Ejections (CMEs), where billions of tons of plasma are hurled into space. If directed at Earth, these can cause geomagnetic storms.

3-3. Solar Wind and Auroras

The Sun constantly emits a stream of charged particles called the Solar Wind. Moving at speeds of roughly 1 million miles per hour, this wind buffets Earth's magnetic field.

When these particles are trapped by Earth's magnetosphere and guided toward the poles, they interact with our atmosphere to create the **Aurora Borealis (Northern Lights)** and Aurora Australis (Southern Lights). The beautiful green and red lights are actually the glow of oxygen and nitrogen atoms being excited by solar particles.

▶ Related Article: What is a Solar Flare? Mechanisms and Effects on Earth

4. The Lifecycle of the Sun

Like all stars, the Sun has a birth and will eventually have a death.

4-1. Birth to Present

About 4.6 billion years ago, a giant molecular cloud collapsed under gravity to form our Sun. It is currently a "Main Sequence" star, happily burning hydrogen into helium. It is about halfway through its life.

4-2. The Red Giant Phase

In about 5 billion years, the hydrogen fuel in the core will run out. The core will shrink, but the outer layers will expand massively. The Sun will become a Red Giant.

It will grow so large that it will swallow Mercury and Venus, and possibly Earth. Even if Earth isn't swallowed, the oceans will boil away, and the planet will become uninhabitable.

4-3. The Beautiful End: White Dwarf

After the Red Giant phase, the Sun will shed its outer layers into space, creating a beautiful glowing shell of gas called a Planetary Nebula.

What remains of the core will be a White Dwarf. It will be about the size of Earth but incredibly dense—a teaspoon of white dwarf matter would weigh as much as a small car on Earth.

The Sun will not explode as a supernova; it isn't massive enough. Instead, it will slowly fade away over billions of years.

5. Common Misconceptions

5-1. "The Sun is on Fire"

Fire requires oxygen. There is no oxygen burning in space. The Sun glows because of nuclear fusion (atoms smashing together), which is far more powerful than chemical burning.

5-2. "The Sun is Yellow"

If you viewed the Sun from space, it would look white. It appears yellow or orange on Earth because our atmosphere scatters blue light (Rayleigh scattering), leaving the warmer colors to reach our eyes.

5-3. "The Sun has a Solid Surface"

You cannot stand on the Sun. It is plasma all the way through. The "surface" we see (the photosphere) is just the layer where the gas becomes transparent enough for light to escape.