What elements are stars mostly made of? how do we know this?

Mostly made up of Hydrogen and Helium. Spectral analysis of the star light is one of the techniques used to identify the elements.

Explanation:

Stars are made up of lighter elements like hydrogen and helium. The light and heat given out by the stars is due to the thermonuclear reactions that involve the proton proton cycle. Hydrogen fusion reactions forming helium , accompanied by the release of energy happen as a cycle in the core of the star giving it the immense heat , light and temperature.Heavier elements like Carbon, Nitrogen , Oxygen , and iron are also found in some stars.

The elements found in the star are detected using many techniques.. Spectral analysis of the star light is one of the methods that can be used to identify the elements found in the stars.  

it is easily magnetized.