With a radius of 700000 km, the Sun is the largest object in the Solar System. The Sun is about 150 million kms from the Earth. This distance is called an astronomical unit AU and is used as a standard measure of measurement in astrophysics. It is classified as a G2V star, G2 for the second hottest star in the yellow G class and V for dwarf star. The Sun rotates and revolves too. The Sun rotates around its orbit in a counter clockwise way and it takes 25–35 days to do so. It also revolves around the centre of the nebula clockwise. Its orbit is between 24,000 and 26,000 light-years away from the galactic center. The Sun takes about 225 million to 250 million years to orbit once around the galactic center.
The Sun was born 4.5 billion years ago in a vast cloud of gas and dust called Nebula. Over a period of millions of years, these began to compress, and some regions of gas collapsed under their own gravitational pull to fall to a common centre. As the mass fell inward, it created a lot of temperature and pressure. As this increased it became hotter and hotter until at 1 million degrees its core ignited. Thus, it began to produce its own light, heat and energy. This process was called Thermonuclear fusion. This heat travels to the surface and radiates for a lot of distance beyond it. Through this stars burn a fuel called hydrogen to form helium. Stars do not burn like fire does, but they burn by a process called convection.
The Sun is made up of a number of gases in ionised or charged state. Such gases are called plasma. About 75% of this is hydrogen which is being ionised and is constantly converted to helium. Thus, most of the balance that is, about 25% is helium. The remaining 1% is iron, nickel, oxygen, silicon, sulphur, magnesium, carbon, neon, calcium, and chromium. Thus, it is not a solid mass and does not have fixed rocky boundaries. The surface viewed from the Earth is called the photosphere and most of the radiations are from this surface
From the Sun emanate solar winds. These are of two types – a fast, uniform and steady one at 800 km/s and a slow, sporadic and gusty one at half the speed. They originate at different places on the Sun. These winds drag magnetic fields out from the surface. Particles reach Saturn after approximately 27 days or one rotation. However, by then the source has moved to the opposite side. Thus, the magnetic field lines form a spiral. To reach Earth solar winds take four days. This has a continual effect on the outer atmosphere.
Sun spots are regions of extremely high magnetic field found on the surface of the sun and even though they are visible even to the naked eye, they need to be looked at through telescope. The origin of the sunspot cycle is not known. A mature sunspot has the form of a daisy with a dark central umbra, surrounded by a less dark ring of fibrils called the penumbra. The umbra appears darker because it is much cooler than the surrounding photosphere. The sunspot cycle lasts for eleven years and then shifts in direction and the sub spots are visible to the opposite side. The most spectacular phenomenon related to sunspot activity is the solar flare, which is an abrupt release of magnetic energy from the sunspot region.
Sun’s energy travels to the Earth at the speed of light in the form of electromagnetic radiation (EMR). These are waves of different frequencies and wavelengths. A majority of these are invisible to us. The most high-frequency waves emitted by the sun are gamma rays, X-rays, and ultraviolet radiation (UV rays). The most harmful UV rays are almost completely absorbed by Earth’s atmosphere. Less potent UV rays travel through the atmosphere, and can cause sunburn. Most of the heat of the Sun arrives on Earth as infrared radiation. In between the infrared and UV radiations is the visible spectrum of white light, which contains all the colours we see on Earth. The Sun itself is white but appears orange-yellow. This is because the blue light it emits has a shorter wavelength and gets scattered, just the same reason why the sky appears blue. A photon of light takes 8 minutes to travel to the Earth.