Main Sequence
The main sequence is the stage of stellar evolution during which a star fuses hydrogen into helium in its core in hydrostatic equilibrium; on the Hertzsprung–Russell diagram, such stars form a diagonal band from hot, luminous, massive stars to cool, dim, low-mass stars, with position determined primarily by mass (and secondarily by composition and age). About 90% of the stars observed are on the main sequence.
Curious
When a young star becomes hot enough in its core to start fusing hydrogen, it settles into a stable state and joins the main sequence. In this phase the star's brightness and surface temperature are fixed mostly by one thing: its mass. The Sun is a middle-sized main-sequence star that has been burning hydrogen for about 4.6 billion years and has roughly as long again to go.
Massive stars live fast and die young. A star ten times the Sun's mass shines thousands of times brighter, using up its fuel in only tens of millions of years, while small red dwarfs burn so slowly that they can last far longer than the current age of the universe. You can see the band on the Hertzsprung–Russell diagram.
Exploring
| Mass (M☉) | Luminosity (L☉) | Approx. lifetime |
|---|---|---|
| 0.5 | ~0.09 | ~57 Gyr |
| 1 | 1 | ~10 Gyr |
| 2 | ~11 | ~1.8 Gyr |
| 10 | ~3,200 | ~32 Myr |
Deep Dive
Why mass sets everything. In hydrostatic equilibrium (see Stellar Structure), the central temperature scales with M/R. For stars whose energy is carried by radiation and whose opacity is dominated by electron scattering, L ∝ M³ (Eddington); for lower-mass stars with other opacities, the exponent is closer to 4, and for very massive stars it flattens towards 1 as radiation pressure dominates. Hence the commonly quoted 3.5 is an interpolation, not a law of nature. The relation has been checked empirically with masses of eclipsing binary stars.
Fusion channels. Below roughly 1.3 solar masses the proton–proton chain dominates; above it the CNO cycle, whose rate depends steeply on temperature (roughly T16–20), takes over. This gives massive stars convective cores and low-mass stars radiative cores with convective envelopes; stars below about 0.35 solar masses are expected to be fully convective. The lower mass limit for hydrogen fusion is about 0.08 solar masses (brown dwarfs lie below it). Details of these thresholds vary with composition and the model used.
Leaving the band. When the core hydrogen is exhausted the star moves off the main sequence (see Stellar Evolution). The “turnoff” point of a star cluster on the HR diagram measures the cluster's age, since the most massive stars still on the main sequence set a clock, a method used to date globular clusters and to cross-check the age of the universe. Eventually, many stars end as white dwarfs, and massive ones as supernovae.
Concept Relationship Map
Requires: stellar structure, nuclear fusion
Enables: stellar evolution, HR diagram interpretation
Commonly confused with: an evolutionary track (stars sit on the main sequence; they do not travel along it)
Sources
- Eddington, A.S. (1926). The Internal Constitution of the Stars. Cambridge University Press.
- Kippenhahn, R., Weigert, A., Weiss, A. (2012). Stellar Structure and Evolution (2nd ed.). Springer.
- Carroll, B.W., Ostlie, D.A. (2017). An Introduction to Modern Astrophysics (2nd ed.). Cambridge University Press.