Questions: Stellar Evolution: From Main Sequence to Stellar Death

Question 1

Approximately what minimum initial mass must a star have to end its life in a core-collapse supernova rather than as a white dwarf?

• A. 1 solar mass
• B. 4 solar masses
• C. 8 solar masses
• D. 20 solar masses

Answer: (C)

The threshold is approximately 8 solar masses. Below this, stars cannot ignite carbon fusion; electron degeneracy pressure halts collapse after the helium-burning phase, and the star ends as a white dwarf surrounded by a planetary nebula. Above this threshold, successive fusion stages proceed until iron accumulates in the core, at which point degeneracy pressure fails and the core collapses catastrophically.

Question 2

The Sun will eventually explode as a supernova because most stars end their lives in a supernova explosion.

• T. True
• F. False

Answer: False

Only stars above roughly 8 solar masses end in supernovae. The Sun (1 solar mass) will exhaust its core hydrogen, expand into a red giant, expel its outer layers as a planetary nebula, and leave behind a white dwarf. This is a common misconception; the term 'stellar death' is dramatic but the Sun's fate is comparatively gentle.

Question 3

Why do more massive stars have shorter main-sequence lifetimes despite containing far more hydrogen fuel?

Model answer: Stellar luminosity scales much more steeply with mass (roughly L ∝ M^3.5) than the fuel supply scales with mass (proportional to M). The fuel-consumption rate therefore vastly outpaces the larger fuel reserve, giving massive stars lifetimes of millions of years versus billions for solar-mass stars.

This is a non-obvious consequence of how pressure and temperature scale in stellar interiors. Higher-mass stars need far greater core temperatures to maintain hydrostatic equilibrium, which drives fusion rates up enormously. A 10-solar-mass star is roughly 10^3.5 ≈ 3,000 times more luminous than the Sun, so it burns through its ~10× larger fuel supply ~300 times faster.