Sun

The star at Solar System's center is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. It was formed 4.6 b years ago. 

The Sun radiates this energy mainly as light, ultraviolet, and infrared radiation, and it's the most important source of energy for life on Earth.

Sun's radius = around 695,000 km (432,000 miles), or 109 times of Earth. 

Its mass = around 330,000 times that of Earth. 

Comprising about 99.86% of total mass of Solar System. 

Roughly 3/4 of Sun's mass = hydrogen (~73%); 

Rest = mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The sun is a G-type main-sequence star or a "yellow dwarf". 

Each second, Sun's core fuses about 600 million tons of hydrogen into helium, and the process converts 4 million tons of matter into energy. This energy, which can take between 10,000 and 170,000 years to escape the core, and is the source of Sun's light and heat. 

When hydrogen fusion in its core has diminished to the point at which Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually turning the Sun into a red giant. 

It's calculated the Sun become large to engulf Mercury and Venus' current orbits, and render Earth uninhabitable in 5 b years. 

After it'll shed its outer layers and become a dense type of cooling star known as a "white dwarf", and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.

The atmospheric of the sun is:

• hydrogen 73.46

• Helium 24.85%

• Oxygen 0.77% 

• Carbon 0.29%

• Iron 0.16%

• Sulfur 0.12%

• Neon 0.12%

• Nitrogen 0.09%

• Silicon0.07%

• Magnesium 0.05%

What the Sun is made of

Cecilia Payne wrote the most important astronomy PhD thesis of the 20th century. She discovered that the Sun is 98% by hydrogen and helium, gases essentially absent on Earth. It was her supervisor Henry Norris Russel who got the credit. It was believed that the Sun was made of iron. German scientists found that atoms glowed with light of colors or wavelengths when in heat. Atoms give out/absorb light when electrons jump form one orbit around a "nucleus" to another. Payne's insight is that, at the Sun's high temperature, some atoms lose most or all of their electrons, with atoms flying and colliding violently, the case for hydrogen and helium, having one and two electrons respectively, thus hit far lower their weight in the light of the Sun. By contrast, iron, with 26 electrons, hardly ever loses all of them, which is why it's crucial in sunlight.

Its source of heat

It's hot as it contains a lot of mass. Mass weight down on the solar core. In a bicycle pump, squeezed air gets hot, like the interior of the Sun. So much matter is crushed down on the center of the Sun that the temperature there is about 15 million degrees Celcius. At such temperature, any matter dissolves into an electrically charged gas/"plasma". The Sun has tons of mostly hydrogen gas. If billions of of ovens were fused, something like a Sun would be produced. The more electrons in an atom, the more effectively heat is bottled up in the Sun.

The Sun stays hot by continually losing heat into space with something constantly keeping the temperature dicated by the Sun's mass. Nuclear energy was discovered in the 20th century. At the high temperature in the Sun, the cores/nuclei of the light est element, hydrogen, collide, and stick gradually building nuclei of the nextheaviest element helium. It's 

The lack of a surface

The Sun is roughly 300,000 times bigger than Earth. The sun is a ball of gas with tremendous gravity of all matters squeezing the deep interior so that it's denser/thicker than any solids, but is a ball of gas regardless and is defined by light. 

Free electrons get in its way, so the light never travels more than a centimetre before being deflected by zigzagging its way out. 

The Sun's photosphere/surface is where light working its way out of the Sun goes from walking to flying. Once outside the surface, sunlight can fly in a straight line and take about 8 minutes to reach Earth. Light would take 2 seconds, not 30k years to get out if it travelled in a straight line.

This is the time it takes "neutrinos", which is also created in the energy-generating nuclear reactions to make sunlight. 

Solar wind

Solar winds are a million-mile/hour hurricane blowing out from the sun and past the planets. It consists mostly of hydrogen nuclei and carries with it the Sun's magnetic field. The wind's origin isn't completely understood. Although the surface temperature of the Sun is less than 6k degrees Clecisu, the Sun is surrounded by an atmosphere or ocorona at a temperature of millions of degrees. This is Coronal gas particles are moving so fast, that they can easily escape from the Sun's gravity. 

We are actually inside the Sun since its wind is its outer atmosphere and it extends to Earth and beyond.

Solar wind takes roughly 4 days to reach Earth. But Earth has a magnetic field called "magnetosphere", a shield protecting the planet so that the solar wind passes harmlessly around it like a stream around a boulder.

Death of the Sun

The Sun gets hotter as it loses heat. In its core the hydrogen nucleus and assembling it into nuclei of helium, the by-product is sunlight. Helium is heavier than hydrogen, so it falls to the center of the core. Its self-gravity squeezes the gas, making it hotter. The Sun is currently 30% hotter since its birth. 

The sun's red giant phase will be shot. In roughly 5 billion years, it will become a "white dwarf" after spending all its fuel.