Without the sun, life as we know it on Earth would cease to exist. Solar radiation warms the planet, provides plants with energy to conduct photosynthesis, and powers the water cycle and other elemental cycles on Earth. In fact, the gravitational field of the sun holds together our entire solar system. How is that gravitational field, magnetic power, and solar radiation produced? It’s a long story.
In order to give off light and heat, a series of complex chemical reactions take place in our sun. These occur on the outer layer, the layer visible by telescopes, satellites, and space probes. More happens inside the sun. The sun’s core is the only place in our solar system where fusion reactions can occur.
The star’s gravitational field and magnetic power are generated by internal processes. Sunspots and coronal mass ejections change even have the power to change the Earth’s climate and precipitation patterns.
This article takes a deep dive into the anatomy of the sun. We’ll cover each layer in detail, starting with its location, chemical composition, temperature, and effect on the sun’s functioning. Continue reading to learn more about the mechanics that power our solar system’s very own star.
7 Layers of the Sun
The sun has 7 layers that are divided roughly in half into two categories. The first four are the outer layers and the last three are the inner layers. We’ll go in order from exterior to interior when explaining each layer.
Each concentric layer is similar to a gobstopper or an onion. However, each layer has different physical characteristics. They are different materials, densities, and thicknesses.
It’s crucial to remember that the sun is not solid like Earth. The delineations between the layers of the sun aren’t exact. This is for twofold reasons: the sun’s substance is plasma, not solid, and thus can’t be measured, and second, the sun is trillions of miles away and current estimates must be made with mathematical calculations, not direct observation.
Outer Layers
1. Corona
The corona is the outermost layer of the sun’s surface. It gets its name from the Latin word for crown because of its long-reaching arms that extend out like points on a crown. Instead of being part of the sun’s actual surface, the corona is really just part of the atmosphere.
Chemically speaking, the corona is burning-hot plasma that reaches up to 1.1 million° C. (2 million° F). It’s many times hotter than the sun’s surface, which is just under 5,537° C (10,000° F).
When there are solar flares, the corona is responsible. The corona sends out arms of energy and heat that register on telescopes and image capturing technology.
The corona is a dynamic and active part of the sun’s 7 layers. However, because the light of the sun is so bright, it’s hard to see.
The best chance of viewing the corona is during a solar eclipse. It looks like an orange halo around the sun. Sometimes, it may have uneven extent on a certain side of the sun.
2. Transition Region
The transition region between the corona and the chromosphere is more of a semi-layer because it is so narrow – just 60 miles in depth. However, it serves such an important role in separating the corona from the chromosphere that scientists placed it in its own category.
The temperature of the transition region decreases dramatically from millions of degrees to just under 5,537° C (10,000° F). This decline in temperature is just one of a multitude of chemical changes that differentiate the corona from the inner layers of the sun.
Helium gas is the star of the show in the transition region. Here, the temperature catastrophe occurs. Below the transition region, helium is partially ionized, making it good at radiating heat out, and thus cooling.
Above the transition region, helium ionizes completely. It holds onto heat, causing the astronomical rise in temperature from a few thousand degrees to over a million.
3. Chromosphere
The chromosphere is just below the transition region and right above the photosphere. On our sun, the chromosphere measures between 3,000 to 5,000 km (1,900 to 3,100 miles) thick. Even though this layer is very thin, it’s still thicker than the transition layer.
The term ‘chromosphere’ comes from Latin for “sphere of color” because it is red. Sometimes the red color can be seen during solar eclipses. The light is visually red because the wavelength given off by this layer of the sun is in the red section of the visible light spectrum.
Temperatures in the chromosphere average about 5,726° C (10,350° F), but can get as high as 100,000° C (180,032° F) and as low as 4,226° C (7,638° F). The temperature graph is not a simple gradient.
It moves in two directions. First, the inner boundary temperature is about 5,726° C (10,350° F). It decreases by about 1,726° C (3,138° F) while traveling outward, then increases to over 34,726° C (63,000° F) at the edge of the transition zone.
The density of the chromosphere decreases farther out from the center of the sun.
Our sun’s chromosphere is particularly active. Different types of activity occur on a regular basis. Spicules, tall spines of cold gas, stick out from the chromosphere into the chroma like slow-motion bolts of lightning. They only last a few minutes because they sink back down into the chromosphere.
4. Photosphere
Find the photosphere just outside the inner layers of the sun. This 400-km thick layer is the innermost of the sun’s outer layers. Scientists have photographic evidence of the existence of the photosphere thanks to probes and extremely detailed telescopes.
On the photosphere, temperatures are much cooler than they are deeper into the sun. The photosphere’s surface measures 4,500 to 6,000 K (4,226° C to 5,726° C). With each step deeper into the sun, the density and temperature increase.
Chemical composition of the photosphere is similar to that of the rest of the sun. This layer is about 75% hydrogen and 24% helium. Other elements, including iron, neon, carbon, and oxygen, make up just 1%.
Granulation is one characteristic of the photosphere. Granulation functions like a pot of boiling water. Cells of hot plasma rise and fall in a matter of minutes, similar to the varying pattern of a pot of boiling water. When people view the columns from above via telescope, they look like small grains of rice, thus the name ‘granulation.’
The photosphere of the sun is the visible surface people see if they watch a sunset or make the mistake to look directly at the sun. It isn’t delineated with a clear border because it’s made of plasma, which doesn’t have clear boundaries.
Inner Layers
5. Convective Zone
The outside layer of the sun’s inner layer is called the convection zone. It’s surrounded by the photosphere on top and the radiative zone on the bottom.
It’s the first of the interior layers, and is separate from the outer layers because of its temperature and heat transfer properties. This zone’s average temperature is about 2 million° C and starts about 200,000 km deep in the sun.
Our sun’s convection zone is always busy. It moves columns of hot plasma up from the radiative zone into the photosphere.
Here, takes much longer for energy to travel than in the photosphere. While the photosphere conveys energy in minutes, the energy created from a single fusion reaction takes over 170,000 years to get from the core to the edge of the photosphere.
Convection is the choice method for transferring heat in this zone because it’s the most energy efficient way to do so. Even though the convective zone is extremely hot, it’s not hot enough to use thermal radiation to transfer heat, as is done in the radiative zone.
6. Radiative Zone
The radiative zone is the second-most inner layer of the sun and as such, is privy to intense heat and pressure. Most of the sun’s radius is taken up by this layer.
On the inner edge, it’s about 7 million K (7 million° C), while at its outer edge, it’s 2 million K. (2 million° C). That pressure and heat, while extremely hot, is much less than the core.
The radiative zone is less adept at transporting energy than the convective zone even though it’s right next to the core. It’s also less efficient than the convective zone. Its lower efficiency is because it uses thermal radiation to transfer heat.
Light particles called photons play hopscotch between the ions in the radiative zone. This takes thousands of years.
Luckily, fusion reactions produce a lot of radiation and energy. The radiative zone has enough space and ions to pass along the photons. It’s also hot enough that thermal radiation is possible.
7. Core
The sun’s core takes up about 25% of its interior. It is the innermost layer of the star and the average temperature is 15 million K (15 million° C).
Just the core is large enough to fit a thousand planet Earths. Around the core is the radiative zone. Even though the core is composed of the light gases hydrogen and helium, it is 8 times more dense than gold!
What makes the sun so special is that it conducts nuclear fusion reactions. Such reactions power the intense light and solar rays that eventually make their way to Earth’s atmosphere. These fusion reactions shove the protons from hydrogen atoms together at such a high temperature and pressure that they combine and become helium.
Energy gets released in the form of photons, what scientists call particles of light. These photons aren’t visible to the naked eye. Instead, they are evidence of one of light’s properties: to act as a wave and a particle at the same time.
After its creation, the energy in photon form attempts to leave the sun’s core. That journey outward is repeated with immeasurable numbers of atoms. Based on the age of our sun and the type of star it is, scientists estimate that it will continue to fuse hydrogen into helium for another 5 billion years.
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