Where did the Sun come from?
Astronomers think that the Sun was formed from a giant cloud which was rotating slowly, about five thousand million years ago, and that the Sun is slightly older than the Earth.
By studying radioactive decay of certain elements in rocks geologists can determine how old those rocks probably are. The oldest rocks they've found on Earth that I know of are between 3 and 4 thousand million years old, but the oldest meteorites that have been discovered turned out to be about 4.5 thousand million years old. Assuming that the Earth and Sun were formed at about the same time as the oldest meteorites and making allowance for some period of time that the meteorites were still too hot to freeze the evidence of their age, the age of the Earth is estimated at about 4.7 thousand million years.
Most of the cloud from which the Solar System formed was made of hydrogen gas, but there was also a little bit of other stuff, such as oxygen, silicon, and carbon. Because of gravity, all of the gas wanted to concentrate in one place, but when the gas got closer to the axis of rotation, it started to rotate faster around that axis, just like an ice skater rotates faster around her axis when she pulls in her arms. The fast rotation of the gas meant that it could not all be concentrated in one place, and what happened instead is that the gas concentrated in a flat disk (like a pancake) with most of the material in the center. The material in the center concentrated more until it got so hot and dense that it could start generating energy through nuclear fusion of the hydrogen. At that moment, the Sun became a star. The rest of the material clumped together and formed the planets, including the Earth.
Astronomers think that the hydrogen gas that is now in the Sun was created in the Big Bang, when the whole universe came into being. We're not sure exactly how long ago that happened. Most observations indicate that the universe is between ten and twenty thousand million years old. This hydrogen gas formed really giant clouds at that time, and those clouds concentrated and formed galaxies. Some of the hydrogen gas floated around in our own galaxy until something made it concentrate and form the Sun and the Solar System. We don't know what caused the gas to start concentrating. Perhaps it was the shock wave from a nearby supernova explosion.
In the Big Bang, only the very light elements hydrogen, helium, and a little bit of lithium were formed. All the other, heavier elements such as carbon, oxygen, nitrogen, and silicon were formed later, inside stars. When the some of the early stars exploded as supernovas, then these heavier elements got mixed with the hydrogen gas that was still floating around, and so these elements ended up also in our Solar System. The carbon, calcium, and oxygen atoms that are an essential part of your body; the silicon, iron, and oxygen atoms that form most of the Earth's inside; the oxygen atoms that are needed to form water; as well as the nitrogen and oxygen atoms that form most of the Earth's atmosphere: all of these atoms were formed inside very, very hot stars a long time ago. Of course, the hydrogen atoms that are part of your body and of water are even older: they were formed in the Big Bang.
Stars very seldomly form alone. Usually many dozens of stars are formed from a single big cloud of hydrogen gas. The Pleiades are one such group of stars. Such sister stars eventually move off in different directions, and after about a thousand million years you can't tell anymore that these stars ever belonged together. The Sun may have some sister stars out there, but we don't know where they are.
(c) Mr Sunspot's Answer Book
