Helium indeed has a different atomic mass compared to the Sun, primarily because the atomic mass of helium refers to an individual helium atom, whereas the mass of the Sun refers to the collective mass of all the matter within it, including various elements and compounds.
Helium has an atomic mass of approximately 4 atomic mass units (amu). This value represents the average mass of a helium atom, considering the different isotopes and their abundances found in nature. Helium-4, the most common isotope of helium, has a mass of exactly 4 amu.
On the other hand, the Sun is composed of a vast amount of matter, primarily hydrogen (about 74% of its mass) and helium (about 24% of its mass), along with smaller amounts of other elements. The atomic mass of the Sun takes into account the collective mass of all the atoms and particles within it, including the hydrogen and helium atoms that make up most of its composition.
The difference in atomic mass between helium and the Sun arises from the fact that the Sun consists of a mixture of various elements, with different atomic masses and abundances, while helium refers specifically to the mass of a single helium atom or the average mass of helium atoms.