<P> The blown - off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars . These heavy elements allow the formation of rocky planets . The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium . </P> <P> The post--main - sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass . If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other . When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common - envelope binaries, cataclysmic variables, and type Ia supernovae . </P> <P> Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust . A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (10) galaxies in the observable universe . In 2010, one estimate of the number of stars in the observable universe was 300 sextillion (3 × 10). While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered . </P> <P> A multi-star system consists of two or more gravitationally bound stars that orbit each other . The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found . For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars . Larger groups called star clusters also exist . These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars . Such systems orbit their host galaxy . </P>

Among the elements found on the earth agb stars are thought to be the primary source of