Atmospheric

Part 3 - Sun

The Sun has continuously emitted radiation with remarkable consistency, since it formed and this has kept the surface of Earth warm enough so that most of the water remained in a liquid state most of the time. 

 Although there have been Ice Ages and periods when the entire planet was largely frozen. 

 The Sun produces energy from the nuclear fusion of hydrogen nuclei (protons) to form helium nuclei. In the process, some of the mass is lost as it is converted into energy according to Einstein's famous law, Energy equals Mass times the Velocity of Light squared. 

 Earth receives a tiny fraction of the Sun's energy which is delivered mostly in the form of radiation ranging from infrared, through visible light frequencies to ultraviolet, x-rays and gamma rays. This steady output of solar radiation not only provides the energy for most of life on Earth but it is also the main driver of the Earth's climate and weather.

The Sun is about 1.39 million km in diameter (864,000 miles) and its mass is about 2×1030 kg, which is 99.86% of the total mass of the Solar System and 330,000 times greater than Earth's mass. About 73% of this mass is hydrogen (91% atom count) which is currently being converted into helium at a rate of 600 million tons of hydrogen every second. About 25% of the Sun's mass is helium (8.9% atom count) and the remaining 2% of the mass is mostly oxygen, carbon, neon, and iron with traces of other elements.

But not to worry. The sun has been doing this for the past 4.6 billion years and has enough hydrogen to last for another 5 billion years. 

 Complex processes automatically balance the energy produced in the Sun's core, and the energy radiated away from its surface. These involve the temperature, density and gravitational compression, the Sun's magnetic field and the composition of the plasma. (Plasma is a state of matter where electrons are stripped from the nuclei of the elements).

In the Sun, gravitational compression increases the density of plasma in the core to about 150 g/cm³ (grams/cubic centimetre); far greater than any natural compound found on Earth. (Osmium, an element with a density of 22.59 g/cm³, has the highest density of any material on Earth).

The Sun's energy output is remarkably stable. If the core temperature increases, more helium is produced, and the extra radiation pushes out against gravitational compression thus lowering the pressure and density of the core so that the rate of fusion is decreased.

The radiation, produced in the core, moves through the Sun's layers by radiative diffusion; the absorption and re-emission of photons of radiation through the hot plasma. This can take thousands to millions of years before the radiation reach the Sun's surface and the energy is released into space.

 From there, travelling at the speed of light in a vacuum, the Sun's output reaches Earth in about 8 minutes. 

The Sun's total energy output is approximately 3.828×10² watts (or 3.828×10²³ joules) per second but this radiates out in all directions so that only a tiny fraction (about 0.0001%) of this reaches the Earth's surface. Despite the small fraction, it is more than enough energy to power all life on the planet.