Astronomers now believe that the universe began at least 15 billion years ago, when the first clouds of the elements hydrogen and helium were formed. Gravitational forces collapsed these clouds to form stars, such as those shown in the upper center of the illustration. These stars converted hydrogen and helium into heavier elements, including those such as carbon, nitrogen, and oxygen, which are necessary for life. These elements were returned to interstellar space by explosions of some of these stars to form clouds (note nebula in illustration) in which simple molecules such as water, carbon monoxide, and hydrocarbons were formed. These clouds then collapsed to form a new generation of stars and solar systems. In at least one solar system, our own, a variety of objects were formed, including comets (believed to be the most primitive objects in our solar system), meteorites, asteroids, and planets (represented by Saturn in the illustration). One of the planets, the Earth, formed at a distance from the sun where conditions were favorable and the necessary chemical ingredients were available (note illustration's infalling comet and dust) for the origin of life.