What do elements of the same period have in common? number of valence electrons
O number of shells
O number of protons
O atomic mass

its actually the number of shells

Explanation:

valence electrons are the same down any group. atomic orbitals, or shells, are the same across periods

answer

fusion: the energy source of stars. the energy released from the collapse of the gas into a protostar causes the center of the protostar to become extremely hot. when the core is hot enough, nuclear fusion commences. fusion is the process where two hydrogen atoms combine to form a helium atom, releasing energy.

hope it