The Sun is a huge ball of incandescent plasma at the center of our Solar System. It accounts for more than 99% of the mass of the Solar System, and provides all the energy we need for life here on Earth.
The Sun’s diameter is 1,392,000 kilometres or 865,000 miles; that’s 109 times the diameter of the Earth. You could fit 1.3 million planets the size of the Earth into the Sun. Everything in the Solar System orbits around the Sun, including 8 planets and their moons, many dwarf planets, asteroids, comets and dust.
The surface of the Sun that we can see is called the photosphere, and it has an average temperature of about 5,800 kelvin. This is the point at which photons generated inside the Sun finally reach the vacuum of space. But if you could descend down into the Sun, you would find the temperature and pressure increasing until you reached the core of the Sun. At the core, the temperature is 15.7 million kelvin and the pressure is sufficient to support nuclear fusion. This is where protons are merged together to form atoms of helium, releasing tremendous amounts of energy.
Because the Sun is made of plasma, it’s also highly magnetic. It has north and south magnetic poles like the Earth, and the magnetic field lines create the activity we see on the surface. The darker sunspots are created when magnetic field lines pierce through the Sun’s photosphere. Coronal mass ejections and solar flares occur when these magnetic field lines snap and reconfigure. The amount of activity on the Sun rises and falls over an 11-year cycle. At the low point, called solar minimum, there are few, if any sunspots. And then at the high point of the cycle, solar maximum, there are the most sunspots and the greatest amount of solar activity.
The Sun takes about 1 month to rotate once on its axis; however, this is a rough estimate because the Sun is a ball of plasma. Some parts of the Sun rotate faster than other parts, so it’s hard to say when it has completed a full rotation.
The Sun is composed almost entirely of hydrogen (74%) and helium (25%), with other trace elements. The innermost layer of the Sun is the core, where the nuclear fusion reactions are taking place. Outside that is the radiative zone, where photons of gamma radiation created in the core are emitted and absorbed by hydrogen atoms. A single photon can take 100,000 years to finally get through the radiative zone. Outside the radiative zone is the convective zone, where bubbles of plasma rise and fall like a lava lamp.
The Sun has been around for 4.6 billion years, and it’s expected to last for another 7 billion years or so. Once all the usable hydrogen in the core runs out, the Sun will expand outward into a red giant, consuming the inner planets (maybe even Earth). Then it will slough off its outer layers and collapse inward to become a white dwarf star.
Only about 5% of stars in the Milky Way are larger than the Sun; the vast majority are smaller red dwarf stars. Some of the biggest stars can be 100,000 times brighter and contain 100 times more mass. The Sun is also relatively young, a member of the Population I group of stars. Older stars, which formed billions of years before the Sun are Population II stars and have less heavier elements in them. The oldest stars are Population III stars, formed just after the Big Bang, but these are purely theoretical.
Sources : http://www.universetoday.com/47725/about-the-sun/