The sun is the nearest star to Earth, and the source of the energy that powers life on Earth. It is made mostly of hydrogen and helium, the predominant building blocks of the universe, glowing with inconceivable heat. The sun may be 5,600 degrees Celsius at the surface, and 15,000,000 degrees C. at the core.
Like all the stars, our sun was born in the collapse of a cloud of gas and dust. The Earth was made with it, and all the other planets were formed out of the same dust and gas. The cloud shrank, coming together under the influence of its own gravity, and as it coalesced the hydrogen molecules within it were shoved together. Their collisions began to heat the mass.
Then hydrogen began to be converted to helium in the heat and pressure, four atoms of hydrogen joining to become one of helium, losing energy as they fused. The lost energy was emitted as a photon, a gamma ray, and millions of gamma rays at once began to migrate to the outside of the proto-star. When they reached the surface, they became visible light. The sun began to shine.
The sun has been shining for five billion years. The fusion reactions that power it have continued all that time, and are likely to continue for five or six billion more years. Then the hydrogen fuel will be gone. Our ancient sun will swell, and engulf Mercury, Venus, and the roasted Earth. Our seas will dry, our atmosphere will boil away. By then our descendants, or whoever replaces them, will have had to find homes on outer moons or circling other stars.
But the red giant sun will not be dead yet. Within its red shell the core will again fall in upon itself. New reactions will begin in the density of the core, and new elements will be created. Carbon will be made from helium, and oxygen from carbon, in the slowly dying sun.
When the last fuel is gone, the sun will slowly pulsate, to beats that each last a few thousand years, and then spew its last atmosphere into space in shells of glowing gas called planetary nebulae. But, odds are, no planets will be left. Astronomers see many of these nebulae in their telescopes, quite near us in galactic terms; they are the last signals of former suns. Our sun will be an ultra-dense white dwarf then, cooling to its final extinction as a lightless black dwarf.
Meanwhile, the sun is the star that gives us life, and gives life to all of earth. The energy it liberates from the nuclear fusion of the lightest and commonest elements in existence powers virtually every reaction, grows every plant, and pulls the water up into the clouds to fall as cooling rain.
