Everything Comes from Stars

What gravity is and why it is, no one knows. Albert Einstein spent most of his life trying to figure it out, but the secret eluded him. it is simply a fact that everything in the universe with mass pulls on everything else. If it weren't for the initial outward impetus of the Big Bang, gravity would have caused the entire universe to collapse into a heap. (Indeed, someday the cosmic collapse may happen, if and when the initial impetus is expended, although the best evidence suggests that the expansion will go on forever.) According to present theories, the universe began about 15 billion years ago as an explosion from an infinitely small, infinitely hot seed of pure energy As the primeval fireball expanded and o. cooled, matter—hydrogen and helium—condensed from radiation. The first stars and galaxies were born as gravity pulled the hydrogen and helium gas together into massive spheres and eddies. As stars burn, they fuse hydrogen nuclei into helium and then into heavier elements, and when stars die explosively, they scatter those heavy elements into space. After many generations of exploding stars had seeded the universe with carbon, oxygen, silicon, and iron, our own Earth and Sun were squeezed into existence by gravity.

When atomic nuclei fuse at the core of a star, some of their mass is turned into pure energy, according to Einstein's famous equation, E = mc^2 (energy equals mass times the speed of light squared). Every second at the Sun's center, 660 million tons of hydrogen are fused into 655 million tons of helium, and the missing 5 million tons of matter ultimately appears at the Sun's surface as heat and light radiated into space. A tiny fraction of the Sun's energy falls upon the Earth's oceans and evaporates water molecules into the air. It takes about 1,000 calories of energy for the Sun to evaporate a thimbleful of water from the sea; each thimbleful of water in the atmosphere represents 1,000 calories of stored solar energy. The Sun does the heavy lifting on Earth, heaving tens of thousands of cubic miles of water up out of the seas and into the atmosphere each year. Most of this water precipitates back into the oceans, but some of it falls on land as rain or snow, from whence it makes its way downhill to the sea in a great recirculation called the water cycle. Around and around the water has cycled for 4 billion years. since the oceans were born, in trickle or torrent, eroding and shaping the land, bringing fresh water to land plants and animals, providing terrestrial habitats for life.

The Big Bang to the formation of stars, which formed the elements crucial to life. It takes 1,000 calories of sunlight to evaporate one thimbleful of water.