Life cycle of a sun-like star, illustration. The cycle begins with a gas cloud (top) that contracts to form a globule. Rotation and contraction causes this to form into a disk called the solar nebula, with the Sun at the centre. Around 6 billion years the later, the Sun expands to become a subgiant. This is followed by the red giant stage (second from bottom) that loses it outer layers to form a planetary nebula (bottom centre) leaving behind a white dwarf (bottom left), the core of the original star.