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  • Writer's pictureolivershearman

Our Star - The Sun

Updated: Oct 28, 2023

The Sun has been a constant presence for billions of creatures over billions of years, but while it may look like a constant; the Sun is actually a very volatile object. The Sun has many dynamic processes that make up the apparently constant bright light in the sky. The Sun has sunspots, a changing surface, complex magnetic fields that twist, tangle and release in dynamic cycles. Further the Sun has its own seismic activity with Sun-quakes and vibrations as the Sun develops and changes over time. The Sun occasionally produces huge blowouts and explosions of plasma into space called Solar flares. Solar flares are accompanied by huge storms of electrically charged particles and bursts of radiation, which can light the Earth up, like a giant Christmas tree. We see these bursts of activity here on Earth through the beautiful lens of the Auroras at both poles! These Solar events can easily disrupt electronics and global communications, particularly satellites further up in our atmosphere.

Below is an approximate schematic of how the Sun works with regards to structure and the associated temperatures. I then explain how the diagram works with regards to structure underneath it.

Figure 1: The layers of the Sun.

Let’s consider outwards from the core of the Sun towards the surface and what layers there are in-between:

  1. The core makes up about from 0-20% of the interior of the Sun with temperatures of about 15 million degrees K. This is where Hydrogen being fused into Helium occurs and the energy that powers all of the Sun is produced.

  2. Then we have the radiation zone, which spans the realm of about 20-65% of the interior of the Sun which has temperatures of approximately 7 million degrees K towards the bottom and about 2 million K towards the top or surface of this zone. In this zone, there is what we call a random walk mechanism, which we discuss further on, in this section.

  3. The third layer we have is the convection zone 65%-90% of the interior of the Sun that contains temperatures of between about 2 Million degrees K at the base and about 6000K near the surface, which is a lot cooler compared to the interior of the Sun.

  4. The photosphere spans approximately 91-100% interior of the sun and has a temperature of around 5900 degrees K.

  5. The Chromosphere is part of the solar atmosphere, which is why in this diagram we include it in the Corona portion of the diagram and the temperatures can vary greatly with a range of 5000 Kelvin up to about 20,000 degrees K.

  6. The Corona is also part of the solar atmosphere and has incredible temperatures of roughly 1 million degrees K.

  7. The solar wind is out on the edges of the stellar environment and it is a constant presence, but can fluctuate wildly with coronal mass ejections and other stellar events and this is why it is hard to pin down a temperature for this area. It extends well beyond the extent of the other parts of the Solar atmosphere and extends beyond the planets of the Solar system which is why I did not include it on this diagram. Please remember that these numbers are conceptual and are there to give you a rough idea on all the information, I do not back them up with scientific papers like any proper scientist should do - this is all just to give a good idea of what we are looking at.

The Jaw-dropping fact of the Sun is that in the sci-fi movies, the spacecraft is able to go close to the Sun and pass right by, now how can this be? Since it should melt from the fact it is passing through a region with temperatures around 1 million degrees Celsius, right? Well as it turns out that as long as you stay far enough away from the chromosphere that the density is still small enough (as it is throughout the corona) then very little of the 1 million degree heat will be transferred to your spaceship and you will be able to sail on by, of course death by radiation must be taken into account but that is another matter entirely (bring on a radiation shield).

There is a constant stream of matter and radiation from the Sun at all times and the big occasional ‘blowouts’, this constant stream is called the Solar wind. While the Solar wind is neither as impressive or as destructive as solar flares and bursts it is a constant hazard to those wishing to travel in space. You can think about the Solar wind like a constant push that ripples out from the Sun in all directions like a rock on a calm pond. As in the below:

Figure 2: A set of ripples on a calm body of water representing the Sun in the centre and the Solar wind pushing outwards from the middle of the ripples.

As noted before, if we take a closer look within the Sun we find that there are three main layers: The core, the radiation zone and the convection zone, these are the most interesting pieces of the puzzle that is the Sun.

Starting from the inside towards the outside: the core is the place (in the centre of the Sun) where temperatures are hot enough and pressures high enough to allow fusion to occur and this is where the Suns energy is produced. Moving outwards we reach the radiation zone - the largest zone within the Sun. Within this zone, photons which are small packets of light energy move very slowly outwards from the core. The photons do move with the speed of light, however these photons are bounced around and deflected so much that it takes about 200,000 years to get to the convection zone. It can take much longer or shorter depending on the particular photon and its individual ‘random walk’.

Figure 3: Demonstration of a random walk/image of general radiation

Image link: (Original from Pearson Education 2004, Addison Wesley.)

Moving upward to the upper most layer of the Sun we find the convection zone, which is where - as its name suggests - convection of hot plasma takes place in the Sun. Now: what is convection you may ask?

We actually see it often and in many places in the world around us, one of the most common places is when boiling water, as we begin to boil the water it begins spreading via radiation, but as the bottom of the pan heats up this becomes too inefficient. As the water begins to churn and bubble as larger hot bubbles of water rise to the surface forcing cool water down to the bottom to be heated.

Figure 4: What we see in this pan example is that convection is what drives the bubbling mechanism that we are all so used to.

A more obvious example is in a ‘lava lamp’ if any of you - like me - remember or have even owned a lava lamp then you will recall seeing the warm blobs being dragged to the top by their temperature and the cooler ones drifting slowly back down to the bottom to be heated again. While it is not technically correct to think of a lava lamp as convection, it does give a nice, approximate visual illustration of the process.

Convection occurs largely due to the difference in temperature from one side of a material to the other is too large to allow other mechanisms to move the heat effectively enough and that is when convection starts up. It is often pointed out that convection is not a heat transfer mechanism, but more accurately a material movement mechanism though the effects are to cause a variance in temperature so it can be considered both.

In the below example we can see a rough diagram of how the surface of the Sun looks. The rough circular shapes are called granules; these are like material cells of certain density and temperature (in reality they won’t be regular circles but much looser shapes). The yellow line across the middle of the image shows where we begin looking below the surface. As you can see in the image below, these hotspots on the surface tend to be warmer than the surrounding material. Convection is the mechanism that feeds the warmer material up and the cooler material back down in the surface and convection zone of the Sun.

Figure 5: A rough approximation of the surface of the Sun complete with yellow granules on the surface and the feedback mechanism with the arrows below.

The lava lamp example below shows another way of thinking about convection, the lava lamp is not an exact match for convection, since it works with different materials, but it does give a good visualisation of the concept of convection. It does also help to understand why the lava lamp works in terms of changing densities of the materials for the wax-like substance based on temperature, basically warmer materials tend to be less dense as a general rule. Less dense means that it will float and rise within the lava lamp fluid.

Figure 6: A diagram of how a lava lamp works

Now you know that when you heat up something it gets bigger as the molecules and atoms become more energetic and vibrate and move more rapidly. For example: when a balloon is warmed up or cooled down, it tends to get bigger or smaller or a soft drink can exploding on a hot summers day because the pressure becomes too high. So, you might ask: why is the Sun not expanding outwards, but rather staying at an approximately stable size?

The answer lies in Hydrostatic equilibrium - this should impress your friends! - while Hydrostatic equilibrium is not truly an entirely stable state, it does keep the Sun hovering around approximately the same temperature and size. Essentially what this means is that there is pressure from the core of the Sun pushing outwards and there is gravity from outer layers of the Sun towards the core pushing inwards. The figure below shows approximately how this works.

Figure 7: Solar Hydrostatic equilibrium

What this graphic shows is that if we take the left path, from the thermometer upwards, there is a slight cooling of the Sun and this will mean fewer fusion reactions in the Sun’s core. This will lead to a slight contraction of the Sun’s outer layers, which is shown by the black arrows towards the average size of the Sun (with the dotted line). We can see that the force of gravity pulls the outer layers towards the middle (the green arrows) and the radiation pressure from the Sun’s core pushes outwards (the red / brown arrows). Then we can see in the bottom left where the Sun has now contracted below its average size since it is smaller than the average size of the Sun as shown by the dotted line. Then finally the left path comes back to the thermometer.

So we return once again to the average temperature and size of the Sun. If we now take the right side loop and goes up and to the right from the thermometer - the Sun has an increase of fusion reactions. After there is an increase of reactions, then the radiation pressure will increase. Once again the red /brown arrows show radiation pressure and the Sun will push upwards on its outer layers while fighting gravity shown by the green arrows. We can then see on the bottom right image that the Sun will then be larger than its usual average size (the dotted line). Finally, it will return to the thermometer closing the right loop. In this way, the Sun is constantly swinging back and forth through the middle average size and temperature.

It is through this process – Hydrostatic equilibrium - that all stars regulate their core temperature and internal dynamics. While it should be noted that this is a simplification for what goes on in the Sun, it does give us insight into how the Sun keeps its size, temperatures and pressures roughly constant. It is very much like comparing it to a pendulum or a beat-keeper such as pianists use to keep time. In this example, the object swings out one way, back past its central point (where it spends most of its time) and then back out to the other side, then back once again through the centre. The idea being of course that while stars spend most of their time in their usual state of size, pressure and temperatures they are constantly correcting themselves as they go back and forth through the average values.

A very important fact about the Sun is that it is made of the fourth state of matter, as are all stars: plasma. Let’s take a moment here to stop and look at plasma since it makes up a lot of what we are looking at; plasma is the highest temperature state of matter and has largely free ions, meaning charged particles, electrons and radiation within its confines in stars.

The reason plasma is this way is simple, more heat means more motion and what happens when matter reaches the plasma state is that the electrons become so energetic that many of them abandon their atoms and become free electrons. If electrons leave their protons then neither are charged balanced since now the electrons are flying around on their own leaving protons with nothing to balance them out. So plasma is largely charged particles with very high internal speeds and very high temperatures. It can be thought of as an insanely hot charged particle soup where many electrons have left their respective atom - also called ‘disassociation’ - shunning their attraction due to the excess of energy from temperature.

Figure 4.8: Plasma state of matter compared to the other forms of matter

Since the Sun is made of Plasma the central rotation at the equator of the Sun is slightly faster than that at the poles since the equator is where the greatest amount of centrifugal force (spinning force) is felt. This effect is very important for both the magnetic fields in the Sun and what we call the sunspot or butterfly cycle. Imagine if you tried to spin a ball of water, I know we cannot do that, but imagine what might happen? What would likely happen is that the middle would probably spin faster than the top and the bottom.

This is not far from what happens with the Sun since it is made of plasma, which is vaguely between a liquid and a gas in behaviour, but very different internal structure. We call this different rate of rotation – somewhat unsurprisingly - differential rotation and the below figure shows how this works with the Sun.

Figure 9 (on the right): The Sun showing differential rotation where the rotation is less near the poles and is faster near the equator.

It should be noted that this rotation occurs in the convection zone near the top of the Sun. Further down in the Sun at the radiation zone, the Sun rotates in what we call ‘solid body rotation’ such as a ball or spinning top will do.

Figure 10 (on the right): shows approximately how this will work. Now you can go impress people with your knowledge of rotation!

Coming back to the sunspot or ‘butterfly cycle’ I mentioned earlier, it is caused by the twisting of magnetic field lines in the Sun. As the sun rotates it gradually twists its magnetic lines due to the difference between rotation at the poles and rotation at the equator. This happens relatively slowly and it is this twisting and eventual rapid untangling by the Sun that causes a ‘pole shift’ every 11 years and gives a solar maximum activity. It is often called the Sunspot cycle because the increase in magnetic strength means a greater number of sunspots and in different locations seems to move change in number up to about 300 sunspots at maximum.

Figure 11: Shows the sunspots diagram (also called the butterfly diagram) and shows the approximate location of sunspots on the solar surface. Note that early in the cycle the sunspots are more towards the poles and tend to appear closer to the equator as the cycle progresses.

Figure 12: Is a diagram of the twisting of magnetic fields lines shown in pink, imposed over the top of the rotation arrows showing the spin in dark red.

The magnetic poles switch at the peak of the solar maximum in the cycle and while 11 years is all that it takes to flip the poles, the full solar cycle is 22 years. Sunspots are darker spots on the Sun's surface and are slightly cooler than the surrounding plasma; they can last from hours to months in their lifetime and are also regions of greater magnetic strength. Sunspots generally appear in pairs allowing magnetic field lines out of one of these spots and into the other much as a magnet needs both a North and a South Pole. This pairing of sunspots means that there are magnetic field lines running from one to another and trails of plasma can be sent or guided along these lines. This channelling of plasma is called solar prominences, which can last a few months and reach many times the diameter of Earth.

Figure 13: Sunspot pole example and example of solar prominences.

Figure 14: An example of how a Solar prominence looks compared to the size of the Earth.

Figure 15: An image of the Sun showing actual Sunspots towards the centre of the image.

The Sun is absolutely amazing; its magnetic field actually reaches beyond our solar system and the entire solar system is contained within its influence, even out there in the frozen landscape of Pluto. The sun causes compression of the planetary magnetic fields and is the reason that we see the auroras that we see on Earth. The following figure is where we show that the planetary magnetic fields of both Jupiter and Earth.

Figure 16: Shows how the magnetic field of Earth interacts with the Sun’s solar wind. The bow shock is the closest purple region which is Earth’s magnetic field close to the Sun.

If we go back to considering the solar prominences, if the plasma in a prominence becomes too much or unstable then tonnes and tonnes of plasma can be ejected. This will leave the solar surface into space in what is known as a coronal mass ejection (CME). Large prominences are impressive events and can use huge amounts of energy reaching up to about 1025 Joules, almost a tenth of what the core of the Sun produces each second. For comparison the most powerful Hydrogen bomb ever denoted released only about 1017 Joules of energy, which is about 40 million times weaker than the energy used up in a large solar prominence. If we take this a step further and consider a modern warhead packs a punch of only 1015 Joules of energy (less than one megaton) or about 4 billion times less than that used in a large solar prominence. It shows that we truly are still far behind the universe in terms of our technology and our ability to produce and use energy.

Solar flares which are often mentioned in popular cultures as being dangerous and violently unpredictable are the largest of these solar events. Solar flares usually come from groups of sunspots meaning very strong and probably unstable magnetic fields resulting in a huge explosion and ejection of plasma into space. Solar flares occur only once every week or two when the Sun is in its less active stage, but when it is more active and towards its cycle maximum flares can occur as often as several per day.

Figure 17: This figure shows the Sun in all its glory through various wavelengths at the peak of an active cycle.

In summary, this blog post has covered:

• The solar layout (what the inside of the sun looks like)

• How radiation and convection works (roughly)

• Hydrostatic equilibrium

• The magnetic field of the Sun and the Sun-spot cycle

• Ejections from the Sun (prominences, solar flares and mass ejections)

If you like a teaching resource that can link well with learning about the Sun, consider this science article.

I hope you have found it an enjoyable and useful read for learning a lot about our star - The Sun.

Thanks for reading.

Cheers and stay curious

Oliver - The Teaching Astrophysicist

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