Stellar Evolution Tracks
Stars spend their lives moving across the Hertzsprung-Russell diagram in predictable patterns called evolutionary tracks. These tracks depend critically on stellar mass, which determines temperature, luminosity, lifetime, and final fate. This toy shows how stars evolve from the main sequence to the giant branch, with different masses following dramatically different paths.
Mass-Dependent Evolution
A star's mass is its destiny. Low-mass stars (0.1-0.5 M☉) burn hydrogen slowly and can remain on the main sequence for trillions of years—longer than the current age of the universe. Solar-mass stars (like our Sun) spend about 10 billion years on the main sequence before evolving into red giants. Massive stars (>20 M☉) burn through their fuel in mere millions of years, ending in supernova explosions.
The Main Sequence: Hydrogen Fusion
Stars are born from collapsing molecular clouds. Once core temperatures reach ~10 million K, hydrogen fusion ignites: 4¹H → ⁴He + energy. This marks the start of the main sequence phase, where stars spend 90% of their lives. The position on the main sequence follows the mass-luminosity relation: L ∝ M³·⁵. Massive stars are hot (30,000 K) and luminous; low-mass stars are cool (3,000 K) and dim.
Post-Main Sequence: The Giant Branch
When core hydrogen is exhausted, the core contracts and heats while hydrogen burns in a shell around it. The outer envelope expands enormously, and the surface cools—the star becomes a red giant. For low-mass stars (<2 M☉), the core becomes electron-degenerate before helium ignition, leading to the dramatic "helium flash." More massive stars transition smoothly to helium burning.
The HR Diagram as a Map
The H-R diagram plots log(luminosity) versus log(temperature), with temperature decreasing left to right (hotter stars on the left). Stellar evolution tracks are not static—they're time-lapse photographs of a star's journey. The diagram's structure reveals the physics: the main sequence is where nuclear equilibrium balances gravity; the giant branch is where shell burning dominates; and the horizontal branch (for low-mass stars) marks stable helium core burning.
Lifetime Scales
Main sequence lifetime scales roughly as t_MS ∝ M/L ∝ M⁻²·⁵. A 25 M☉ star lives only ~7 million years on the main sequence; a 0.1 M☉ red dwarf would last ~10 trillion years. Our Sun, at 4.6 billion years old, is about halfway through its main sequence life. In about 5 billion years, it will swell into a red giant, engulfing Mercury and Venus, before shedding its outer layers as a planetary nebula.
Beyond the Giant Branch
Low- and intermediate-mass stars eventually lose their envelopes as planetary nebulae, leaving behind white dwarfs (visible in the lower-left of the H-R diagram). Massive stars continue burning heavier elements (C, O, Si) in nested shells, culminating in iron-core collapse and supernova explosions. The remnants become neutron stars or black holes—too faint to appear on optical H-R diagrams.