Life cycle of a sun-like star, illustration. The cycle begins at left centre with a supernova. This detonation compresses a nearby gas cloud (top) which begins to contract to form a globule. Rotation and contraction causes this to form into a disk called the solar nebula, with the Sun at the centre. Eventually the material in the disk agglomerates to form planets (right centre). Around 6 billion years the later, the Sun expands to become a subgiant (planet no longer shown for clarity). This is followed by the red giant stage (bottom), and then a planetary nebula as the red giant expands and ejects its atmosphere into space. The material is thrown back into the interstellar medium, and the cycle begins again.