Helium is the second most abundant element in the Sun by mass, after hydrogen. It makes up roughly one-quarter of the Sun's mass, while hydrogen accounts for about three-quarters.
The Sun formed about 4.6 billion years ago from a cloud of gas and dust whose composition was dominated by hydrogen and helium. In its core, enormous pressure and temperature allow hydrogen nuclei to fuse into helium. This nuclear fusion produces the energy that makes the Sun shine, while gradually increasing the amount of helium in the core.
The Sun's composition was discovered without sampling it directly. In the 19th century, astronomers identified an unknown spectral line in sunlight and named the new element helium after Helios, the Greek god associated with the Sun. Helium was later found on Earth.
A frequent mistake is choosing oxygen or carbon because both are important elements in the universe. They are present in the Sun, but in far smaller amounts than hydrogen and helium. The exact percentages vary slightly depending on whether the core or the Sun's outer layers are measured.