  We compute the high energy entropy and the equation of state of a gas of open
superstrings in the infinite volume limit focusing on the calculation of the
number of strings as a function of energy and volume. We do it in the fixed
temperature and fixed energy pictures to explicitly proof their equivalence. We
find that, at high energy, an effective two dimensional behavior appears for
the number of strings. Looking at the equation of state from a ten dimensional
point of view, we show that the Hagedorn behavior can be seen as correcting the
Zeldovich equation of state ($\rho=p$) that can be found from the two
dimensional part of the entropy of the system. By the way, we show that, near
the Hagedorn temperature, the equilibrium state obtained by sharing the total
energy among open (super)strings of different length is stable.
