An $05$ type star releases $5\mathrm{.}1$x$10^49$ high energy photons per second. If the density of hydrogen in the interstellar medium surrounding the star is $1500$cm$^-3,$ make an estimate of the radius of the stromgren sphere around the star in parsecs. Assume a$($alpha$)=3$x$10^13$cm$^3$s$^-1$ for the interstellar medium.