Stellar Evolution
Main Sequence
90% of a star's life — fusing hydrogen to helium in the core
The main sequence is the band on the Hertzsprung-Russell diagram where stars spend ~90% of their lives, fusing hydrogen into helium in their cores via the proton-proton chain or CNO cycle. Position on main sequence determined by mass: high-mass stars are hot and luminous; low-mass stars are cool and dim. Sun is a G2V main sequence star, 4.6 Gyr old, ~5 Gyr remaining on main sequence. Stars leave main sequence when core hydrogen exhausted — become red giants.
- Lifetime range~10 Myr (massive O) to >10¹³ yr (M dwarfs)
- Mass range0.08 M_sun to ~150 M_sun
- Sun on main sequence4.6 Gyr in; ~5 Gyr remaining
- Energy sourceHydrogen fusion (p-p chain or CNO cycle)
- Spectral types (hottest first)O, B, A, F, G, K, M (mnemonic: Oh Be A Fine Girl/Guy)
- Sun's spectral typeG2V (5778 K, yellow-white)
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Why main sequence matters
- Stellar life cycle. 90% of a star's existence.
- Habitability. Stable main sequence period crucial for life.
- Stellar populations. Most stars in galaxies are main sequence.
- Distance ladder. Main sequence calibration helps stellar distances.
- Galaxy ages. Stellar populations indicate galaxy ages.
- Sun-like stars. Where to look for habitable exoplanets.
- Energy source. Why stars shine.
Common misconceptions
- Stars move along main sequence. They sit at one position based on mass.
- Main sequence is one path. Wide band; mass determines position.
- Sun is unusual. Pretty average G-type main sequence star.
- All stars are main sequence. ~90% are; rest in evolutionary phases.
- Lifetime increases with mass. Decreases sharply (M⁻²·⁵).
- Main sequence lasts forever. Limited by hydrogen supply.
Frequently asked questions
Why is mass so important?
Core temperature scales with mass. Higher mass → higher core T → faster fusion (CNO cycle dominates above 1.3 M_sun). More fuel but burned much faster. Lifetime ∝ M⁻²·⁵ for high-mass stars. So massive O star (50 M_sun): lives ~10 Myr. Sun (1 M_sun): 10 Gyr. M dwarf (0.1 M_sun): >10¹³ yr.
What's the difference between p-p chain and CNO?
P-p chain: 4 protons → ⁴He via direct fusion. Dominates in low-mass stars (T < 1.5×10⁷ K). CNO cycle: carbon, nitrogen, oxygen catalyze 4H → He. Dominates above. Both produce same net reaction; CNO faster but needs heavier seed elements. Sun: ~99% p-p, ~1% CNO.
Where on HR diagram is main sequence?
Diagonal band from upper-left (hot, luminous, massive) to lower-right (cool, dim, low-mass). Slope reflects increasing luminosity with mass. Width reflects evolution within main sequence. Stars stay on this band while fusing hydrogen — leave it when fuel exhausted.
How long does Sun spend on main sequence?
~10-11 Gyr total. Currently ~4.6 Gyr in. ~5 Gyr remaining. Then expansion to red giant phase. Earth: not directly destroyed by main sequence end, but Sun's expansion will affect Earth long before that — habitable zone moves outward as Sun brightens.
How do main sequence stars vary?
Different masses populate different parts. M dwarfs (most numerous): low-mass, cool, faint. K stars: orange. G stars (Sun-like): yellow-white. F, A: white. B: blue-white. O (rare): blue. Stars don't move along main sequence — they sit at single position based on mass.
What's main sequence "lifetime"?
Time hydrogen burning sustains. Sun: ~10 Gyr. 10× heavier: ~50 Myr. 0.1 M_sun: trillion years. Lower-mass stars are longer-lived because they burn fuel slowly. Universe is only 13.8 Gyr old, so smallest M dwarfs haven't aged yet.
What ends the main sequence?
Core hydrogen exhaustion. Star contracts, heats up. Hydrogen shell burning begins around inert helium core. Star expands → red giant phase. For Sun: ~5 Gyr from now. Massive stars: shorter time to red supergiant phase, then supernova.