Light and Dark: Two Side of Same Coin

The image on the left is the image of our Earth from space showing the difference between night and day from up there. So my dad was right after all: "There is no difference between night and day, except that bad people can hide in the cover of darkness to perpetuate evil deeds. Everything is the same in both light and dark," he'd say. So, from a distance everything is just the same and in harmony. It is just time that changes and even that change were described by Albert Einstein as a stubbornly persistent illusion. In this post, I would like to discuss the Sun, from what causes it to heat up to such extreme temperatures to how it causes night in some parts of the world and day in some other parts at the same time, and everything in between. This is a lot of stuff to cover so let us begin.

What is the Sun?

The sun is classified as a G V star or what is called a yellow dwarf star which has been around for about 4.5 billion years, but that does not really help, now does it? Apart from being the blazing hot and blinding object that supplies heat and light to the Earth, the sun is a massive ball of plasma and gas. In Biology, plasma is the liquid part of the blood that contains proteins that are essential for performing important functions in the body such as blood clotting. It is also called blood plasma, but that's not the plasma that is being referred to here. Plasma is a neutral gas that has been subjected to heat or electromagnetic fields that makes it become more electrically conductive. The main gases that make up the Sun are hydrogen and helium. 91.2 percent of the gases in the sun is hydrogen which is converted to energy in the sun's core and released to the sun's atmosphere and the solar system.

^{Wikipedia CC0Features of the Sun Drawn to Scale}

Wait! Here is an excellent time to ask: if the Sun is made up of light gases and electrically conductive gases, why doesn't it all just fall apart and evaporate? Well, to begin answering this question, you must understand Newton's gravitational law which says that every object exerts a force of attraction on other objects close to it. Now, the force a body would exert on another body is directly proportional to the mass of the attracting body. That means that a massive body would attract lighter objects to it, which is basically what the earth is doing to all of us and other objects close to it. The enormous force required to launch spacecraft out of the earth's atmosphere is necessary just to overcome the gravitational pull of all objects close to earth. Now, imagine the gravitational pull of an object so massive that its gravitational pull can keep all the planets in the Solar System suspended in their orbits. So, no, the gases aren't going anywhere. They must remain close to the sun due to the gravitational pull of the sun.

Sun's Gravitational Pull in Perspective

According to Sir Isaac Newton, the gravitational pull one object exerts on another is proportional to the mass of the of the object (say, G) and it weakens the farther away we get from the object such that the force exerted is inversely proportional to the inverse of the distance of the body from the core or center of the attracting body (that is 1/(r²), such that r is the distance from the massive body.

The mass of the sun, M has been determined as 1.989 x 10³⁰ with a radius, R of about 695,500 km. I wish I could say that this is the exact radius of the sun but we have already established that the sun is made up of gases. The above measurement is the radius of the sun from the centre to the photosphere which is the portion of the sun that is visible with white light.

The universal gravitational constant G, according to Sir Isaac Newton, is 6.7 x 10^-11. So a one kilogram object on the photosphere would experience a force,

F = G(M)/R² = 6.7 x 10^-11 (1.989 x 10³⁰)/(695,500,000)² = 275.496012

Therefore the force of gravity on the photosphere is about 276 N which is about 28 times stronger than the strength of gravity on the surface of the earth on a 1kg body, so it is easy to imagine this force being able to keep everything together.

Nuclear Fusion: Heat and Light

It is easy to think that hydrogen fuse together to form helium, releasing huge quantities of light and energy but hydrogen occur naturally on earth, but it does not fuse to form helium. The gravitational forces at the sun's core create extremely high pressure and temperatures. Recall one definition of temperature as a measurement of the kinetic energy of particles. When gases are heated, they vibrate more and tend to move faster in the container housing them, but when they are compressed and temperature allowed to fall, they could become liquid like in compressors. But at the core of the sun, the temperature is up to 15 million degrees Celsius and at these temperatures and pressure, atoms of compressed hydrogen can and do fuse to form helium in a process known as Nuclear Fusion. This process is responsible for about all the energy of the sun, and it takes place in the core. More specifically, the process begins with an atom of hydrogen and another atom of hydrogen (two protons), fuse to create a helium atom. Sometimes, one transforms to a neutron to form deuterium. Another proton collision with deuterium would result in a helium-3 nucleus. Two helium-3 collision would result in helium-4 and two extra neutrons. The helium-4 formed has a mass that is less than the four helium atoms that formed it.

According to Albert Einstein's mass-energy equivalence, mass and energy (for the sake of simplicity) are related by:
E = mc². The loss in mass during this fusion reaction is only 0.7 percent which is very little mass and is expected to result in the release of very little energy, but one must realise that the reaction takes place 9.2 x 10³⁷ times per second. Therefore the amount of mass that is converted to energy per second is about 4.26 x 10³ metric tonnes of matter. This translates to 3.8 x 10²⁶ joules of energy being released per second.

But that's not the end of it.

The gases in the core get heat up, and their atoms break apart into charged particles, turning the gas into plasma. Then, the energy from the fusion is carried into the radiation zone, in the form of gamma-ray photons and neutrinos and according to NASA, these photons can bounce around in for up to a million years before it can travel to the convective zone which is considered the upper layer of the sun's interior. Large bubbles of gas form hot plasma that moves onto the photosphere which is the visible part of the sun that contains radiation that is visible to white light and has a temperature of about 5,000 degrees Celsius. Above the photosphere is the chromosphere which appears red due to enormous hydrogen content within this zone and is has a temperature of about 4350 degree Celsius. Next is the corona which extends thousands of kilometres above the photosphere. Incidentally, this area is much hotter than the surface of the sun with temperatures that could rise from one million to a few million degrees Celsius.

The corona is the outermost part of the sun's atmosphere so one would expect it to be cooler than the inner parts but the fact is that the core of the sun is very hot, but the temperature falls and then rises dramatically in the corona from where the energy and light of the sun reach the solar system. It takes eight minutes for light from the sun to reach the earth, and this forms the basis of measurement of the distance from the sun to the earth - 8 light-minutes

Night and Day

A joke goes as follows:

All the world leaders met in a single room, and each stood up to address the group on key issues from the perspective of their country. One leader stood and proclaimed: "We have figured out a way to land a manned spaceship on the sun," he said. Everyone had a good laugh. "I know what you all are thinking, but we have found a way: we shall land in the night!"

In case you didn't realise why this is funny as hell, may I point out that first, night and day does not exist in the sun. The stuff has been burning for billions of years and more importantly, spacecraft can only land on solid surfaces of which the sun has none since it is made of gases. Don't even get me started on the temperatures close to and the surface of the earth.

So nights only occur as a result of the rotation of the earth.

As already mentioned, the gravitational pull of the sun is enough to keep the planets in the solar system in an elliptical orbit and depending on the distance of each planet from the sun; it takes a certain number of days or years for each planet to complete a revolution. It takes our Earth 365.26 days to complete a revolution, and this is what we count as one year. But apart from its revolution, the earth also rotates about its axis once every 23.93 hours, and because of its almost spherical shape and opaque nature, light from one side cannot be seen on the other end especially since light usually travels in a straight line. As a result, the Sun appears to rise in the East and set in the West, creating about twelve hours each of light and darkness. This description is not always so because the earth's axis is inclined at an angle of 23.4° (that is, the earth's axis is not perpendicular to the orbital plane).

For people that live in the North and South pole, this inclination is quite significant. It means that some people in the South Pole may not see daylight for several days during the Southern Hemisphere winter. Therefore they experience complete darkness for this period. Whereas some areas in the North Pole would not experience darkness for several weeks during the Northern Hemisphere's summer because the North Pole is facing the sun permanently during this period.

Conclusion

The sun is the closest star to the earth which has been classified as a yellow dwarf star. Like other stars, the light that comes from the sun comes from gases that continually burn on the surface of the sun. Our sun has been burning for about 4.5 billion years, and stars have been known to burn for as long as 10 billion years. The sun is made up of mostly hydrogen gas which atoms fuse to form helium in a process called nuclear fusion in the sun's core. Before the light and energy of the sun which gets to the earth reaches us, it goes through layers of the sun which can take up to one million years but when the light leaves the sun, it could get to us within ten minutes which has become a means of measuring the distance of the earth to the sun. Night and day are as a result of the rotation of the earth about its axis and the days do not equal the nights in the North and South Poles at certain times of the year due to the inclination of the earth's axis at an angle of 23.4° to the orbital plane. The sun could get very hot in certain parts of the world and relatively gentle on other parts, but one thing is certain: there would be no life here if we did not have the sun and the sun would not burn if we did not have hydrogen.

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Question:

So the hydrogen in the sun keeps fusing to form helium? Won't it be exhausted one of these days and our sun would die, ending all life on earth? Or is there a means by which the sun replenishes its supply of hydrogen to keep it burning forever?

Thank you for reading :)

References

1. Stanford University | Sun's Corona
2. Space | Sun's Hear Source Explained
3. Universe Today | What Kind of Star is the Sun?
4. Energy Education | Mass-Energy Equivalence
5. Enotes | Gravitational Force: Earth and Sun
6. Almanac | Gravitational Pull
7. University of Illinois | Gravitational Pull of the Sun
8. How Stuff Work | How the Sun Works
9. Grifolsplasma | What is Plasma?

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