stars undergo nuclear fusion, that is to say that the nuclei of 2 (or more) atoms literally combine. in stars it's hydrogen, and by combine i dont mean a chemical reaction. i mean that you take one proton, add another proton, and out comes a nucleus with 2 protons. so take 2 hydrogens (both 1 proton each) and use nuclear fusion to turn it into the atomic element with 2 protons known as helium
the nuclear fusion of hydrogen into helium
A "Star" can be anything from the beginning of a stars life (Protostar) up until it has died and only it's remnants remain (White dwarf, neutron star, black hole). However, for this question, we will assume that a "star" is when it is fusing hydrogen into helium, which is what most stars we see, including the Sun are doing. A protostar is the first stage in a stars life (See related question) whereas a "star" in this question is when there is enough pressure and thus temperature for the star to star fusing hydrogen into helium and it joins the main sequence - thus becoming a star (See related question)
mostly hydrogen and helium
A star begins its life as a ball of gas and dust. Gravity pulls the gas and dust into a spere. As the sphere becomes denser, it gets hotter and eventually reaches temperature of about 10,000,000 Celsius in its center. As hydrogen combines into helium, energy is released in a precess called neclear fusion.
Hydrogen and helium. Mostly hydrogen.
The nuclear fusion of hydrogen into helium
the nuclear fusion of hydrogen into helium
Mostly hydrogen (with some helium, deuterium, and lithium). Other elements (all other elements) are added as the date of formation of the protostar comes closer to the current date.
A protostar has 100% Hydrogen. A young star has a Helium core.
Hydrogen and helium
The chemical symbol for helium is "He."