What unit is used for atomic mass

universal mass unit

The universal mass unit, abbreviated u (sometimes amu for atomic mass unit), is defined as one-twelfth of the mass of the 12C atom which has been defined to be exactly 12 u. The absolute mass of a 12C atom is obtained by dividing the value 12 by the Avogadro number (NA = 6.022 137 × 1023).

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.