Very old stars often have very few heavy elements, while very young stars have much more. What does this difference imply about the chemical evolution of the universe?

Heavy elements, also known as 'metals' in astronomical terms, are elements heavier than hydrogen and helium. Stars are the factories where these elements are created.
When a star is born, it primarily consists of hydrogen and helium. During its life, it undergoes nuclear fusion, where hydrogen nuclei fuse to form helium. As the star ages, it begins to produce heavier elements like carbon, oxygen, and iron through additional nuclear fusion processes.
The presence of these elements depends on the star's age and the generation of stars it belongs to. Younger stars, formed from remnants of older ones that have already exploded as supernovae, contain more heavy elements. In contrast, older stars have fewer of these elements because they formed when the universe was less enriched.

Nuclear fusion is the process that powers stars, including our Sun. In the cores of stars, extreme pressure and temperature cause atomic nuclei to overcome their repulsive forces and fuse together. This fusion process releases tremendous energy, which makes the stars shine.
Initially, stars fuse hydrogen atoms to form helium. As the hydrogen gets depleted, stars start to fuse helium atoms into heavier elements. This process continues, generating energy and forming elements like carbon, oxygen, and eventually iron.
Once iron is formed, fusion cannot proceed further to release energy. Thus, the core may collapse, leading to a supernova explosion.

Supernovae play a crucial role in dispersing heavy elements throughout the universe. When a massive star ends its life in a supernova explosion, it ejects its outer layers into space. These layers are rich in heavy elements formed during the star's life.
The distribution of these elements from supernovae enriches the surrounding interstellar medium. This enriched material becomes the building blocks for new stars and planets.
Over successive generations of stars, the amount of heavy elements in the universe increases. This process contributes to the observed difference in the elemental composition of young and old stars.

The age of a star can often be inferred from its elemental composition. When stars form, they inherit the chemical make-up of the gas clouds from which they were born.
In the early universe, the gas clouds had fewer heavy elements because fewer supernovae had exploded to enrich them. As a result, older stars contain fewer heavy elements.
Conversely, younger stars, which formed later in the universe's history, emerged from gas clouds enriched by many generations of supernovae explosions. These stars have a higher proportion of heavy elements.
This difference in elemental composition indicates that the universe has chemically evolved over time, becoming richer in heavy elements as generations of stars live and die.