The event that marks the end of the main-sequence lifetime of a star is the
A) formation of a nova.
B) formation of a planetary nebula.
C) end of hydrogen fusion in the core.
D) helium flash.

Question

Quizplus · Accepted Answer

The end of hydrogen fusion in the core, also known as hydrogen exhaustion, marks the end of the main-sequence lifetime of a star. At this point, the star will begin to evolve off the main sequence and eventually form a red giant or supergiant. This typically occurs when the hydrogen fuel in the core is depleted and the core contracts and heats up, causing the outer layers of the star to expand and cool. Eventually, the star will begin to fuse helium in its core, but this marks a new chapter in its evolution. Neither A, B, nor D mark the end of the main-sequence lifetime. A nova is a sudden increase in brightness due to a thermonuclear explosion on the surface of a white dwarf in a binary star system, B is the result of the shedding of the outer layers of a low-mass star at the end of its life, and D occurs in low-mass stars when helium fusion ignites but it is not catastrophic like in massive stars.

The Event That Marks the End of the Main-Sequence Lifetime