What are stellar atmospheres composed of?

The majority of a star's atmosphere is made up of hydrogen and a small amount of helium, with the actual composition of the star varying depending on the nebula from which it formed.

Fusion reactions that produce heavier elements occur in a star's core; the Sun's core makes up 20% to 25% of its radius. Fusion products can exit the core and enter the atmosphere through convection currents.

Increasingly heavier elements form layers in larger stars, which are effectively layered; the outermost layer is still mostly hydrogen. Larger stars produce heavier elements up to iron.

Large stars near the end of their lives when their core collapses and causes fast fusion in the surrounding layers, causing the star to explode as a supernova. The star's outer layers are still mostly made of hydrogen, and the explosion spreads out the star's atmosphere to form a nebula, which has enough hydrogen in it to support the formation of another star.

With trace amounts of heavier elements like carbon, oxygen, nitrogen, and others, hydrogen and helium make up the majority of the elements found in stellar atmospheres. These gases are heated by the star's interior, resulting in a variety of layers and temperature gradients within the atmosphere.