If we begin from star birth, the next key stage in the star-gas-star cycle occurs when a(n) returns gas and new elements into space. The expelled gas forms a(n) that is very hot and expands in size. As the gas cools and merges with other interstellar gas, it forms a(n) . Further cooling allows the gas to become a(n) , in which star formation occur.

1. A Supernova

2. A Bubble

3. Atomic hydrogen cloud

4. Molecular cloud

Explanation:

If we begin from star birth, the next key stage in the star-gas-star cycle occurs when a Supernova returns gas and new elements into space. The expelled gas forms a Bubble that is very hot and expands in size. As the gas cools and merges with other interstellar gas, it forms an Atomic hydrogen cloud. Further cooling allows the gas to become a Molecular cloud, in which star formation occur.

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