Sun

The Sun, also known as Sol, is a star at the center of our solar system. It is a white star that gives off different types of energy such as infrared energy (heat), ultraviolet light, radio waves and light. It also gives off a stream of particles, which reaches Earth as "solar wind". The source of all this energy is nuclear fusion. Nuclear fusion is the reaction in the star which turns hydrogen into helium and makes huge amounts of energy. It is a nearly perfect ball of hot plasma.

The Sun looks yellow to a person standing on the Earth, but it is actually white. It looks yellow from the ground because the Earth's atmosphere scatters sunlight, causing shorter blue and green wavelengths to disperse, making the Sun's remaining visible light appear more yellow.

The Sun is a star like many others in our Milky Way galaxy. The Sun is a type of star called a G-type main-sequence star based on its spectral class. 

The Sun has existed for a little over 4.5 billion years.

The Sun is about a hundred times as wide as the Earth. It has a mass of . This is 333,000 times the mass of the Earth. 1.3 million Earths can fit inside the Sun. The Sun fuses about 600 million tons of hydrogen into helium every second.

The Sun is the main source of energy for the Earth. This energy is made deep inside the Sun in a process called nuclear fusion. Four hydrogen atoms are fused together to make one helium atom. Some of the leftover matter turns into energy. This is the same way energy is released in a hydrogen bomb.

It can take between 10,000 and 170,000 years for the energy in the core of the Sun to escape.

No Sun, no life on Earth 
The Sun is the most important source of energy for life on Earth. Without the Sun, there would be no vegetation on Earth: every plant needs light to live and grow. Everything on Earth would freeze over without the Sun. The Sun gives the Earth heat as well as light. 

Plants make oxygen by photosynthesis. All humans and animals breathe oxygen. The Earth's position in the Solar System is just right for the development of life on Earth.

General characteristics 
The Sun is a G-type main-sequence star. It has about 99.86% of the mass of the Solar System. The Sun has an absolute magnitude of +4.83. It is brighter than about 85% of the stars in the Milky Way galaxy. The Sun is a Population I star,  i.e it  a metal-rich fairly young star.

The Sun is the brightest object in the Earth's sky. It has an apparent magnitude of −26.74. It takes 8 minutes and 19 seconds for light to travel from the Sun's surface to Earth's surface.

Physics of the Sun

Origin 
Scientists think that the Sun started from a very large cloud of dust and small bits of ice about 4.567 billion years ago.

At the center of that huge cloud, gravity caused the material to build up into a ball. Once this got big enough, the huge pressure inside started a fusion reaction. The energy this released caused that ball to heat and shine.

How it works 
 At its very center, hydrogen atoms collide together at great temperature and pressure and fuse to form atoms of helium. This process is called nuclear fusion. It was proposed by Hans Bethe just before World War II.

Orbit 
The Sun and everything that orbits it is in the Milky Way galaxy. The Sun orbits around the centre of the Milky Way. It takes along everything in the Solar System. The Sun moves at 820,000 km an hour. At that speed, it still takes 230 million years for a full orbit.

Visible features 
Since the Sun is all gas, surface features come and go. If the Sun is viewed through a special solar telescope, dark areas called sunspots can be seen. These areas are caused by the Sun's magnetic field. The sunspots only look dark because the rest of the Sun is very bright.

Some space telescopes, including the ones that orbit the Sun have seen huge arches of the Sun's matter extend suddenly from the Sun. These are called solar prominences. Solar prominences come in many different shapes and sizes. Some of them are so large that the Earth could fit inside of them, and a few are shaped like hands. Solar flares also come and go.

Sunspots, prominences and flares become rare, and then numerous, and then rare again, every 11 years.

Photosphere 
This is the surface of the Sun. The light that the Earth receives from the Sun is radiated from this layer. Below this layer, the Sun is opaque, or not transparent to light.

Composition 
The Sun is mainly composed of hydrogen and helium. All elements heavier than hydrogen and helium, account for less than 2% of the mass of the Sun.

The Sun's chemical composition was got from the interstellar medium. The hydrogen and most of the helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe. The heavier elements were produced by stars that died before the Sun was formed. The heavier elements were released into the interstellar medium when stars exploded as supernovae.

Atmosphere 
Five layers make up the atmosphere of the Sun. The chromosphere, transition region, and corona are much hotter than the outer photosphere surface of the Sun. It is believed that Alfvén waves may pass through to heat the corona.

The minimum temperature zone, the coolest layer of the Sun, is about  above the photosphere. It has a temperature of about . This part of the Sun is cool enough to allow simple molecules such as carbon monoxide and water to form. These molecules can be seen on the Sun with special instruments called spectroscopes.

The chromosphere is the first layer of the Sun which can be seen, especially during a solar eclipse when the moon is covering most of the Sun and blocking the brightest light.

The solar transition region is the part of the Sun's atmosphere, between the chromosphere and outer part called the corona.  It can be seen from space using telescopes that can sense ultraviolet light. The transition is between two very different layers. In the bottom part it touches the photosphere and gravity shapes the features. At the top, the transition layer touches the corona.

The corona is the outer atmosphere of the Sun and is much bigger than the rest of the Sun. The corona continuously expands into space forming the solar wind, which fills all the Solar System. The average temperature of the corona and solar wind is about . In the hottest regions it is . We do not understand why the corona is so hot. It can be seen during a solar eclipse or with an instrument called a coronagraph.

The heliosphere is the thin outer atmosphere of the Sun, filled with the solar wind plasma. It extends out past the orbit of Pluto to the heliopause, where it forms a boundary where it collides with the interstellar medium.

Eclipses 
A solar eclipse appears when the moon is between the Earth and Sun. The last total solar eclipse occurred on December 26, 2019, and was visible from Saudi Arabia, India, Sumatra and Borneo, with a partial eclipse visible in Australia and much of Asia. 

A lunar eclipse happens when the moon passes through the shadow of the Earth which can only occur during a full moon.The number of lunar eclipses in a single year can range from 0 to 3. Partial eclipses slightly outnumber total eclipses by 7 to 6.

Fate of the Sun 
Astrophysicists say our Sun is a G-type main-sequence star  in the middle of its life. In a billion years or so, increased solar energy will boil away the Earth's atmosphere and oceans. In a few more billion years, they think the Sun will get bigger and become a red giant star. The Sun would be up to 250 times its current size, as big as  and will swallow up the Earth.

Earth's fate is still a bit of a mystery. In the long term, the Earth's future depends on the Sun, and the Sun is going to be fairly stable for the next 5 billion years.  Calculations suggest that the Earth might move to a wider orbit. This is because about 30% of the Sun's mass will blow away in the solar wind. However, in the very long term the Earth will probably be destroyed as the Sun increases in size. Stars like the Sun become red giants at a later stage. The Sun will expand beyond orbits of Mercury, Venus, and probably Earth. In any event, the ocean and air would have vanished before the Sun gets to that stage.

After the Sun reaches a point where it can no longer get bigger, it will lose its layers and form a planetary nebula. Eventually, the Sun will shrink into a white dwarf. Then, over several hundred billion or even a trillion years, the Sun would fade into a black dwarf.

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References 

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