THE STRUCTURE OF THE SUN
The Sun is a vast, layered structure, comprising all but a small fraction of the mass of the entire Solar System.
The sun is a ball of gas about 865,000 miles
(1,392,000km) in diameter. If you could put the Earth at its center, the Moon would orbit about halfway to the Sun's surface. It would take 109 Earths, lined up side by side, to span the Sun's diameter. The Sun contains about 333,000 times as much matter as Earth does, and comprises more than 99.99 percent of the mass in the entire Solar System. The remaining 0.01 percent makes up the rest of the Solar System—that is, the nine planets and all the moons, comets, asteroids, and dust.
The Sun is a medium-size, medium-hot star in the middle of its life, which began some 4.6 billion years ago. We can be thankful that it is such an ordinary star. If the Sun were 10 times more massive, it would already have lived its life and exploded as a supernova. And had the Sun been 10 times smaller, it would not produce enough light and heat for life to exist in the Solar System.
A single element, hydrogen, accounts for 92.1 percent of all the atoms in the sun. The next most common element is helium, which amounts to 7.8 percent. All the other elements together, make up the remaining 0.1 percent. The Sun is fueled by thermonuclear reactions in its core, in a process where hydrogen atoms are fused to make heavier helium atoms, which results in an enormous release of energy.