= Solution
Once swept-up matter dominates the ejecta and <radiative cooling> is negligible, the <Sedov-Taylor blast wave> has its <shock> radius fixed by <dimensional analysis>:
$$
R_s=\xi\left(\frac{Et^2}{\rho_0}\right)^{1/5}.
$$
Here $\xi$ is a dimensionless constant of order unity fixed by the <adiabatic index>. Treating the ambient matter as <hydrogen> gives $\rho_0\simeq10^6m_H\simeq1.67\times10^{-21}\,\mathrm{kg\,m^{-3}}$, while $t\simeq3.16\times10^{10}\,\mathrm s$. Thus
$$
\boxed{R_s\simeq1.43\times10^{17}\,\mathrm m\simeq4.6\,\mathrm{pc}\quad(\xi\simeq1).}
$$
For a monatomic gas, the usual similarity normalization $\xi\simeq1.15$ instead gives about $5.3\,\mathrm{pc}$. An estimate of a few <parsecs> is therefore appropriate. Negligible ejecta <mass> does not by itself guarantee this energy-conserving stage: substantial radiation losses or a strongly nonuniform ambient <mass density> would invalidate the assumed similarity law.
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