The bright-line spectrum of sodium is produced when energy is

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.

space exploration has led to the development and technological improvement of many everyday items. (if this answer is wrong i will find the right answer) : )

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.