After its main sequence existence, a medium-sized star, like our Sun, will become a

A. protostar

B. red dwarf

C. white dwarf

D. red giant

E. red supergiant

The answer is D. The life cycle of a star is dependent on its mass. Stars by forming at nebulas as protostars. When the sun exhausts hydrogen fuel at its core, the core begins to fuse helium to carbon and the core becomes hotter but smaller. The outer layers are pushed outwards and the sun becomes larger and paler. Later, the helium fuel begins to run out and the sun loses the outer expanding layers into a planetary nebula. The remaining core is a white dwarf.

Stars that are 10 times our sun's solar mass just explode to supernovas when their hydrogen fuel is exhausted.

The correct answer is A. Stars, like our sun, will turn into protostars after their main existence before moving on to the next phase of their lives. This happens for medium-sized stars. Larger stars have a slightly different life cycle. They can burn hotter and longer.