Hayashi line

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The Hayashi line is an almost vertical line in the color-brightness diagram , which separates areas in which stable hydrostatic equilibrium is possible from those in which this is not possible. It is around an effective temperature of 4000 Kelvin, but its exact position depends on the mass of the star being considered : the more mass, the higher the effective temperature at which the line lies. It was named after Chushiro Hayashi (* 1920, † 2010), who showed in 1961 that no stable stars can exist to the right of this line. Stars that are directly on the Hayashi line are fully convective and in hydrostatic equilibrium.

The Hayashi line plays an important role in star formation. If one looks at the development of the collapsing matter, from which the star is formed, in the color-brightness diagram, it approaches the Hayashi line from the right. The collapse of the cloud in free fall ends when it reaches the Hayashi line. This point in time can in a certain sense be seen as the birth of the star. The further evolution of the star is down the Hayashi line, i.e. In other words, if the surface temperature remains the same, the contraction reduces the surface and thus also the luminosity, see also star formation .

This line also plays a role in the evolution of the stars in the post-main sequence phase. Giant stars cannot cross this line and their development in the color-brightness diagram therefore turns upwards before reaching the Hayashi line.

Development paths of young pre-main sequence stars (blue lines)

Development paths of young pre-main sequence stars:

  • The development paths (blue) from the birth of a star (black line top right) to entry into the main sequence (black line bottom left) are shown.
  • The end of each path is labeled with the star's mass in units of the solar mass.
  • The red curves (labeled in years) are isochrones, i.e. H. Lines of constant age whose intersections with the evolutionary paths indicate the respective age of the stars.
  • The nearly vertical sections are hayashi lines.
  • Light stars with stay on the Hayashi Line until they reach the main sequence.
  • Heavier stars turn onto the almost horizontal Henyey line .
  • Very heavy stars with are born directly on the Henyey line.

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