Final answer:
Stars are primarily composed of the light elements hydrogen and helium, resulting from nuclear fusion. The universe is approximately 75% hydrogen and 25% helium by mass. Cecilia Payne-Gaposchkin's work in 1925 was pivotal in establishing the composition of stars.
Step-by-step explanation:
Objects composed mainly of very light elements such as hydrogen (H) and helium (He) are stars, including our Sun. These celestial bodies are massive accumulations of hydrogen gas undergoing nuclear fusion at their cores to produce vast amounts of energy. During this fusion process, hydrogen atoms collide and merge to form helium along with some other light elements like nitrogen and carbon, which are essential for life. In our universe, about 70%-75% of the mass is hydrogen, making it the most abundant element. Helium constitutes about 25% of the universe's matter, with other elements constituting a minor fraction. Cecilia Payne-Gaposchkin's groundbreaking work in 1925 revealed that stars have similar compositions predominantly of hydrogen and helium, challenging the scientific consensus at the time.
Furthermore, the abundance of heavier elements can vary among stars. For example, population I stars like the Sun hold 1-4% of heavy elements by mass, while population II stars have much lower concentrations. Understanding these compositions helps explain how elements beyond lithium were formed later within stars rather than during the Big Bang, revealing the ongoing process of nucleosynthesis in the cosmos.