Final answer:
Stars are primarily made up of hydrogen, which makes up about three-quarters of their mass, while helium accounts for almost one-quarter. Cecilia Payne-Gaposchkin's research in 1925 confirmed their abundance in stars.
Step-by-step explanation:
The main element that stars are made of is hydrogen. Hydrogen is the lightest and most abundant element in the universe, comprising about 70-75% of the universe by mass and about three-quarters of the mass of most stars. On the other hand, helium is the second most abundant element in stars, making up almost a quarter of a star's mass. Together, hydrogen and helium make up from 96 to 99% of a star's mass. Larger stars can also fuse hydrogen into heavier elements like carbon and oxygen. The pioneering work of Cecilia Payne-Gaposchkin, who in 1925 was the first woman to earn a PhD in astronomy in the United States, demonstrated the abundance of hydrogen and helium in stars despite the initial skepticism from the scientific community.