Define pure substance and give an example

In the more general sense, a pure substance is any homogeneous mixture. That is, it is matter that appears uniform in appearance and composition, no matter how small the sample size. Examples of pure substances include iron, steel, and water. Air is a homogeneous mixture that is often considered to be a pure substance. (got the answer from google hehe)

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.