What Is The Sun Made Of, and How Hot Is It?

The Sun has been steadily generating light for the entire solar system for at least 4.5 billion years

Showing flares on the Surface of the Sun, captured in ultraviolet light
The Sun in ultraviolet light (Source)

We know light from the Sun takes 8 minutes to reach Earth. But how are this light and heat generated in the first place? 

These are some of the questions that scientists have been pondering for a long time. Let’s take a look at what they’ve found and the latest research surrounding the workings of the Sun.

Table Of Contents

What is the Temperature of the Sun?

This is not an easy question to answer because it depends very much on where you stick your thermometer!

At the heart of the Sun, the temperature reaches 17 million °F (9.4 million °C), hot enough for nuclear fusion to be sustained. At the Sun’s visible surface, the temperature can be as ‘cool’ as 1,100°F (600°C).

Finally, as we make it to the outer reaches of the corona, the temperature rises once again to as much as 44,000°F (24,400°C).

Keep reading to get more details on the temperature in each of the Sun’s layers.

What is the Sun Made Of?

When the Sun began forming 4.5 billion years ago, it consisted of only gas and dust. However, as it evolved, nuclear fusion resulted in hydrogen, helium, and other elements inside the Sun. Let’s peel its complex structure one layer at a time.

How Was the Sun Formed?

4.5 billion years ago, the Sun and its system of planets that we see today only existed as a cloud of dust. When a shockwave from a nearby supernova struck this dust cloud, it collapsed to form a solar nebula. 

This nebula began spinning, causing the material to flatten out into a disk. At the center of the disc, the Sun began forming

Gravity was at its highest here, and it eventually accumulated 99.8% of the available matter in the solar system to create the Sun. The remaining 0.2% formed the planets and asteroids.

Nearly all of the matter creating the Sun is just two elements: 70.6% is hydrogen, while 27.4% is helium

As the density of material increased at the heart of the spinning disc, so did its temperature and pressure. Eventually, the right combination of extreme temperature and pressure, which can only be brought about through having enough mass (which is why Jupiter isn’t a failed star), caused the Sun to ignite.

In terms of star formation, ignition means that nuclear fusion was triggered nuclear. This process turns hydrogen into helium and releases a tremendous amount of energy in the process.

We perceive this energy release as light and heat coming from the Sun.

Does the Sun Have Different Layers Like a Planet?

The Sun is truly enormous and is made of six different layers. Let’s unwrap each of them in detail from the inside out.

The Core

The Sun’s core, which is 200,000 miles across, is where all the action takes place. 

The core’s temperature of 27 million degrees Fahrenheit (9.4 million degrees Celsius) makes it possible for nuclear fusion to occur. Hydrogen is converted to helium, and the energy released by this is what we see as visible light. 

The Radiative Zone

The radiative zone is nestled between the core and the convective zone.

This zone carries the energy created from nuclear fusion as electromagnetic radiation from the core to the convective zone. Although these photos travel at the speed of light, they bounce back and forth in the dense radiative zone, which is why it takes a photon nearly a million years to emerge out of the radiative zone and into the convective zone.

Because no fusion occurs in this radiative zone, the temperature falls to 12.5 million °F (6.9 million °C).

The Convective Zone

Before we get into the details of the convective zone, it is worth mentioning that the core and radiative zone rotate differently than the convective zone. The boundary between these two different types of rotation zones is called tachocline.

The convective zone begins at a depth of 200,000 km and extends up until the visible surface. At its base, the temperature is around 3.5 million °F (2 million °C). 

This temperature is not very hot in astronomical terms, so elements such as carbon, oxygen, calcium, and iron can resist fusion and hold onto their electrons. However, this causes a dense environment as these elements crowd together, blocking the heat arising from the core. 

This trapped heat eventually increases the convective zone’s temperature and leads to ‘boiling’. We see these boiling motions as granules at the outer edge of the convective zone (see the image below).

Granules seen as part of the convective motions
Granules seen as part of the convective motions (Source)

The Photosphere

The Photosphere is the deepest layer of the Sun’s visible surface that we can observe directly with telescopes.

NEVER LOOK AT THE SUN WITH YOUR NAKED EYE, BINOCULARS, OR A TELESCOPE. YOU WILL BLIND YOURSELF! ONLY EVER USE APPROPRIATE EQUIPMENT AND TECHNIQUES FOR STUDYING THE SUN.

The photosphere begins at the visible surface and extends up to 250 miles (400 km) above that. The temperature here fluctuates between 1,100°F and 6,700°F (600°C – 3,700°C), which is as hot as Earth’s core

Much of the photosphere is crowded with granulation. It is also the source of solar flares and where we see sunspots on our Star’s surface.

The Chromosphere

The chromosphere extends from 260 to 1,300 miles (400 to 2,100 km) above the photosphere.

The temperature here rises with height, up to 44,000°F (24,400°C), which is hotter than the photosphere. This is counterintuitive because the temperature usually decreases as we move away from the core.

However, the science makes sense: the rising heat from the core causes this temperature increase. In fact, the chromosphere glows red when seen in the absence of the bright photosphere (this is only seen during a total solar eclipse because light from the chromosphere is too weak to be seen otherwise).

This red glow is appropriate because chromosphere literally translates to ‘sphere of color’. 

The Corona

The corona is the Sun’s outermost layer. Like the chromosphere, this layer is only visible during eclipses and is otherwise engulfed in the photosphere’s bright light. 

The Sun's corona is visible during a total solar eclipse
The Sun’s corona during a total eclipse (Source)

The corona begins at 1,300 miles (2,100 km) and extends into space, but it is 300 times hotter than the photosphere at 900,000°F (500,000°C). 

The latest research suggests that this may be due to the presence of millions of ‘nanoflares‘. The Sun’s magnetic field ‘loops’ are rooted in its photosphere, but they move around because each layer in the Sun rotates at a different speed. 

This differential rotation causes the loops to twist and turn, sometimes so much so that they break. Upon breaking, they release energy that’s then transferred to the corona as heat, which causes the corona to be hotter than the photosphere.

Why Does The Sun Shine?

The Sun has been shining for 4.5 billion years. On Earth, life has thrived for more than 3 billion years of that time. This indicates that the Sun has been shining steadily for a long time and that its process of generating light has been relatively consistent. 

The secret to its shine lies deep inside the Sun’s core, where the scorching temperature and immense pressure makes nuclear fusion possible. 

Hydrogen atoms of which the Sun has plenty, fuse to form helium and other heavier elements. This process releases heat, which is then transferred to the surface as electromagnetic radiation, later seen as light. 

Through this process, the Sun generates 3.9 x 1026 joules of energy every second (one joule is the energy needed to light a 1-watt bulb for 1 second). This is a staggering amount of energy and comfortably more than the whole Earth consumes in a year!

The Magnetosphere

Just like Earth, the Sun too creates its magnetic field deep in its core. The flow of electric currents generates this field. 

Because the Sun rotates once every 27 days, its magnetic field is a spiral shape. At times when the magnetic field is intense, it forms sunspots out of which solar flares, the sudden bursts of energy, are generated. 

Solar winds carry the magnetic field throughout the solar system. The area covered by the magnetic field is called the ‘heliosphere’. It extends well beyond the solar system’s planets and protects them, including Earth, from harmful intergalactic radiation from outer space.

Showing the protective effect of the Sun's heliosphere
See how vast the heliosphere is. Voyagers are only just leaving the solar system (Source)

What Are Sunspots?

Sunspots are dark spots that are seen on the Sun periodically. Their temperature is only 6,500°F (3,593°C) because the magnetic fields in these locations are so strong that they prevent heat from the Sun’s interior from reaching the surface. 

Sunspots on the surface of the Sun
Sunspots are dark because they’re cooler than the rest of the Sun (Source)

The field at these sunspots sways, twists and turns as the Sun rotates, and when it gets tangled and breaks, solar flares are created.

The Sun functions on an 11-year solar cycle. This cycle includes solar maximum and minimum when the sunspots and consequent solar flares are high and low respectively. 

It is also the time when the Sun’s magnetic field reverses: the magnetic fields weaken, their strength drops to zero and they reverse their polarity and increase in strength again.

Sunspots, especially the large ones, can practically be seen without a telescope. But, sunlight is so bright that you should never look at the Sun directly or focus a telescope or binoculars to view it.

Remember, to escape partial or complete blindness, 99% of the sunlight needs to be eliminated. A solar filter does this work for you. 

Another way to safely observe a sunspot is to build your own sunspot watcher. Project the Sun’s image onto a white screen or a wall and focus it until the Sun’s shape and its sunspots come into view. 

Coronal Flares

We’ve learned how the Sun’s magnetic field twists and sometimes breaks to result in solar flares. The same process sometimes creates another kind of explosion called a Coronal Mass Ejection (CME).

CMEs are similar to solar flares in how and why they are created but are different in other ways. They look, travel, and affect planets differently.

Unlike solar flares, CMEs travel in the direction in which they were initially emitted and impact only that targeted area

Image of a coronal mass ejection
A coronal mass ejection with Earth shown for scale (Source)

When they reach Earth, CMEs encounter the planet’s magnetic field and are funneled by it towards the poles where the field is weaker. This where the CME enters the atmosphere.

Once inside the atmosphere, they interact with its elements and give rise to auroras, or the Northern and Southern lights as we know them. 

CMEs and solar flares can occur at the same time from a dense sunspot. In these cases, they can affect the electrical grids on Earth and cause a communication blackout.

Summary

The Sun has been shining steadily and has been a reassuring object in the sky for 4.5 billion years. 

Up close, the Sun is nothing like the calm and steady force we believe it to be. The celestial body that had once formed out of just gas and dust has now turned into a complex, turbulent structure. Each of its 6 layers has a role to play in transferring heat and light created deep inside its core and out into the solar system. 

The Sun’s influence extends beyond its surface as well. Its magnetosphere is carried by solar winds throughout the solar system. This protects the entire solar system from harmful radiation coming in from outer space. 

However, the Sun is not always the protector of its planets. The 11-year solar cycle has a maximum and a minimum, which are times when the sunspot activity increases and decreases. 

At its maximum, the Sun bursts out powerful flares that when accompanied by CMEs have the power to destroy electric circuits on Earth and cause communication blackouts. 

Even though the Sun is scorchingly hot, it is a ‘cool’ star to know and see (with all necessary precautions of course)!


Written by Adam Kirk