A ball of gas that was left over after the Big Bang along with the very first star. Star is not a static object, it actually evolves with time to a red giant and is getting much bigger.

What happens in the evolutionary here, there are two possibilities:

• If a star has strong stellar winds, it can lose mass from its surface and the lost gets put back into the reservoir. They don’t explode in supernova and do not contribute to any chemical evolution cycle.

• If a star keeps evolving until it explodes as a giant supernova, the outer and inner portions of the star get spilled around and put back into the reservoir again. This introduces new elements from the core of the star that are being put into the reservoir. After the death of these stars, the gas cloud is chemically enriched which enables the next generation of star forms from this enriched material, and they evolve. The massive ones contribute to the creation of new elements. All the massive stars with the new generations contribute to a successive build up of all the elements with time.

Comments:

• Old stars contain little of heavy elements (> H, He).
• Younger stars are more enriched.