Nature & Evolution of Stars
This domain covers what stars are and how they change: spectral classification, the Hertzsprung–Russell diagram, energy generation, and the evolutionary paths set by mass. Questions test whether you can place a star on the H–R diagram, infer its mass, and predict its fate.
Classifying stars
- Spectral sequence OBAFGKM (hottest to coolest), subdivided 0–9; the Sun is G2. Temperature correlates with absorption-line patterns, not color alone.
- Luminosity classes Ia–V run from supergiants to main-sequence dwarfs.
- The H–R diagram plots luminosity against temperature or spectral type, with temperature decreasing to the right. About 90% of stars fall on the main sequence.
- Mass–luminosity relation L ≈ M³·⁵ on the main sequence; mass is the single parameter that determines a star's entire life.
Birth, maturity, death
- Stars form in cold molecular clouds when a fragment exceeds the Jeans mass; the protostar contracts, heats, and reaches the main sequence when core hydrogen fusion balances gravity.
- Fusion: proton–proton chain dominates in the Sun; the CNO cycle dominates above about 1.3 solar masses.
- After core hydrogen exhaustion: red giant expansion, helium flash (low mass), horizontal branch, asymptotic giant branch, then ejection of a planetary nebula and a white dwarf below the Chandrasekhar limit of 1.4 M☉.
- Massive stars fuse heavier elements up to iron, then collapse: Type II supernova, leaving a neutron star or black hole. The Schwarzschild radius is 2GM/c².
Worked example
Estimate the main-sequence lifetime of a 3 M☉ star. Luminosity scales as 3^3.5 ≈ 47 L☉, and lifetime scales as M/L, so t ≈ 10¹⁰ yr × (3/47) ≈ 6 × 10⁸ yr. The star is far more luminous but lives only about 6% as long as the Sun. This is why young clusters still contain massive blue stars while old clusters do not, and why cluster turnoff points date stellar populations.
Traps that cost marks
- More massive stars are hotter, bluer, and shorter-lived - never "bigger stars live longer."
- Red giants are cool but highly luminous because of their enormous radius.
- White dwarfs are supported by electron degeneracy pressure, not fusion.
- In the H–R diagram, temperature increases to the left; students routinely reverse the axis.
- The Chandrasekhar limit applies to white dwarfs; exceeding it triggers a Type Ia supernova, not a neutron star.