Hayashi track

PMS stars with M < 0.35 M0 have a simple structure - they are fully convective balls of gas all the way to the ZAMS. As the star contracts along its Hayashi track the core heats up, but the temperature gradient stays very close to adiabatic except in the surface layers. Li begins to burn in p, a reactions when the core temperature, Tc reaches c 3x 106 K and, because the reaction is so temperature sensitive (oc Tc16-19 at typical PMS densities) and convective mixing so very rapid, all the Li is burned in a small fraction of the Kelvin-Helmholtz timescale (see Fig. 1). 

The B-N object may be considered to be a zero-age main sequence star that evolves with increasing surface temperature and luminosity at optical wavelengths. The descent from the right-hand upper quarter of the HR diagram, along with what has been called the Hayashi track or birth lines, precedes the entrance onto the... 

The Hayashi track (Hayashi 1961, 1966) is the locus of fully convective stars in the HR diagram, dependent on the mass, luminosity and chemical composition. However, it is mainly the surface properties - luminosity and radius or luminosity and effective temperature - that govern the entire stellar structure by fixing the adiabatic constant. A sketch derivation of its properties is the following. 

Supposing that the atmospheric opacity of a fully convective star is proportional to the abundance of easily ionized metals (which at low temperatures replace hydrogen as the source of free electrons to make H ), correct Eq. (5.65) for the Hayashi track by finding an appropriate power of Z to multiply by. 

Hayashi track for a star with mass about twice that of the Sun... 

Hayashi track The line on a Hertzsprung-Russell diagram during which a star s luminosity stays relatively constant, while its temperature continues to decrease, helium burning A series of nuclear reactions in which helium nuclei fuse to make larger atomic nuclei, helium flash A period of very rapid helium burning that occurs in the core of a star with low mass. 

From this short summary about star formation we learn that stars are born from large molecular clouds, they are always born in clusters (also called stellar associations or open clusters—they disintegrate over time). In the Hertzsprung-Russell diagram (H-R diagram) the process of star formation from the collapses of a protostar to the point where it reaches the main sequence where nuclear fusion ignites is also called Hayashi track. 

Cloud-collapse and stellar evolution depend on stellar mass. The higher the mass, the faster the collapse and the subsequent evolution. To descend the Hayashi track it takes... 

Figure 3 Comparison between observations and evolutionary tracks applicable for extremely helium-rich stars. The helium main sequence (HeMS) is labelled with stellar masses, HL is the Hayashi limit. The hatched line indicates the Eddington limit for pure helium composition. Evolutionary tracks are from Paczynski (1971 dashed lines labelled with M/MQ), Schonberner (1977, M = 0.7 MQ, full drawn line) and Law (1982 M = 1 Mq, dotted line). Stellar symbols are the same as in Fig. 1. 

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