Hayashi track
Hayashi track describes early evolution of low-mass stars.
The Hayashi track describes how young stars with less than three times the Sun's mass change in brightness and temperature during their pre-main-sequence phase. It appears as a nearly vertical line on the Hertzsprung–Russell diagram, which charts luminosity against temperature. The track is named for Japanese astrophysicist Chushiro Hayashi (1920–2010). Once a protostar finishes its rapid contraction and becomes a T Tauri star, it is very bright. The star then contracts more slowly, moving downward along the Hayashi track. As it does so, its luminosity drops by several times, but its surface temperature stays roughly the same. This continues until either a radiative zone forms—at which point the star shifts to the Henyey track—or nuclear fusion ignites, marking the star's arrival on the main sequence.
The track's shape and position on the diagram depend on the star's mass and chemical makeup. For a star like the Sun, the track sits at about 4000 K. Stars on the Hayashi track are almost fully convective, and their opacity is dominated by hydrogen ions. Stars under 0.5 solar masses remain fully convective even on the main sequence, but after fusion begins, Kramers' opacity law takes over, moving them off the Hayashi track. Stars between 0.5 and 3 solar masses develop a radiative zone before reaching the main sequence. Stars between 3 and 10 solar masses are fully radiative from the start of the pre-main-sequence phase. Heavier stars appear to be born directly onto the main sequence, with little observable pre-main-sequence evolution.
At the end of a low- or intermediate-mass star's life, it follows a reverse analogue of the Hayashi track: it grows brighter, expands, and holds roughly the same temperature, eventually becoming a red giant.
**History**
In 1961, Chushiro Hayashi published two papers that established the concept of the pre-main-sequence and form the foundation of modern early stellar evolution theory. Hayashi realized that the existing model—which assumed stars were in radiative equilibrium with no significant convection zone—could not explain the shape of the red-giant branch. He replaced that model by incorporating the effects of thick convection zones in a star's interior.
A few years earlier, Osterbrock had proposed deep convection zones with efficient convection, analyzing them using the opacity of H⁻ ions (the dominant opacity source in
- born
- 1920
- died
- 2010
- field
- Astrophysics
- nationality
- Japanese
- known_for
- Hayashi track, pre-main-sequence stellar evolution
Lore & Background
In 1961, Professor Chushiro Hayashi published two papers that led to the concept of the pre-main-sequence and form the basis of the modern understanding of early stellar evolution. Hayashi realized that the existing model, in which stars are assumed to be in radiative equilibrium with no substantial convection zone, cannot explain the shape of the red-giant branch. He therefore replaced the model by including the effects of thick convection zones on a star's interior. A few years prior, Osterbrock proposed deep convection zones with efficient convection, analyzing them using the opacity of H− ions (the dominant opacity source in cool atmospheres) in temperatures below 5000 K. However, the earliest numerical models of Sun-like stars did not follow up on this work and continued to assume radiative equilibrium.
Reader's Guide
The Hayashi track is a fundamental concept in stellar astrophysics, defining the nearly vertical path on the Hertzsprung–Russell diagram that pre-main-sequence stars of less than 3 M☉ follow as they contract slowly while maintaining roughly constant surface temperature. Its significance lies in establishing the forbidden zone—a region on the HR diagram to the right of the track where no star can be in hydrostatic equilibrium. Newborn protostars start in this zone and rapidly move toward the Hayashi track. The track's shape and position depend on a star's mass and chemical composition; for solar-mass stars, it lies at roughly 4000 K. Stars on the track are nearly fully convective with opacity dominated by hydrogen ions. The work of Hayashi in 1961 replaced earlier radiative-equilibrium models and correctly predicted the ages of solar-type stars in the young cluster NGC2264, identifying them as rapidly contracting T Tauri stars. Subsequent numerical models in 1965 by Iben and Ezer & Cameron realistically simulated pre-main-sequence evolution, including the Henyey track that stars follow after leaving the Hayashi track. At the end of a low- or intermediate-mass star's life, the star follows an analogue of the Hayashi track in reverse, increasing in luminosity and expanding to become a red giant.
Did You Know?
- The Hayashi track is named after Japanese astrophysicist Chushiro Hayashi (1920-2010).
- Stars on the Hayashi track are nearly fully convective and have their opacity dominated by hydrogen ions.
- The forbidden zone is the region on the HR diagram to the right of the Hayashi track where no star can be in hydrostatic equilibrium.
- At the end of a low- or intermediate-mass star's life, the star follows an analogue of the Hayashi track in reverse, eventually becoming a red giant.
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