Final answer:
Early stars in the galaxy, known as Population II stars, have fewer heavy elements because the universe initially contained mostly hydrogen and helium. Subsequent generations, such as Population I stars, incorporated heavy elements produced and dispersed by older stars' supernovae. The star generation cycles enriched the interstellar medium, making newer stars metal-richer.
Step-by-step explanation:
Stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements because the universe initially had only hydrogen and helium. Heavy elements, which account for much of what makes up planets like Earth, are forged within the cores of stars and are spread into space when stars explode as supernovae. As such, the first stars, known as Population II stars, formed from this pristine material and therefore contained only trace amounts of heavy elements.
Over time, with each generation of stars, more heavy elements were created and released into the interstellar medium. Population I stars, like our sun, formed later, incorporating these elements into their compositions. This recycling process has been less efficient in small, less crowded galaxies like the Small Magellanic Cloud, resulting in even their youngest stars having a deficiency in heavy elements.
Therefore, as time passes, the proportion of heavier elements in stars increases because each subsequent generation of stars is born from material that has been enriched by the previous generations' end-of-life supernova explosions.