Question

1.

a) What happens in a star when it follows the Hayashi track?

b) Estimate (calculate) how long a 2 Msun star will take to reach the “bottom” of the Hayashi track. Be sure to explain, and justify, any assumptions that you make. To answer this question, you should perform a calculation, do not just read off the time from a figure.

Answer 1

1(a) The Hayashi track is a nearly vertical path of stellar evolution on the Hertzsprung-Russell diagram down which an infant star progresses on its way to the main sequence. While on the Hayashi track, a star is largely or completely in convective equilibrium; as it progresses, its luminosity, initially very high, decreases rapidly with contraction, but its surface temperature remains almost the same. The sequence runs in reverse for stars leaving the main sequence to become giants.

1(b)

It is approximately true that The diagram to the right shows how Hayashi tracks change with changes in chemical composition. Z is the star's metallicity, the mass fraction not accounted for by hydrogen or helium. For any given hydrogen mass fraction, increasing Z leads to increasing molecular weight. The dependence of temperature on molecular weight is extremely steep—it is approximately

.

Decreasing Z by a factor of 10 shifts the track right, changing by about 0.05. thus it takes about 10^7year .