The universe is believed to have originated about 15 billion years ago as a dense， hot globule of gas expanding rapidly outward. At that time， the universe contained nothing but hydrogen(氢) and a small amount of helium(氦). There were no stars and no planets. The first stars probably began to form out of hydrogen when the universe was about 100 million years old. This is how our Sun originated about 4.49 billion years ago. Many stars came into being before the Sun was formed; many others formed after the Sun appeared. This process continues, and through telescopes we can now see stars forming out of compressed pockets of hydrogen in outer space.