The Horton-Strahler number, also known as the register function, provides a tool for quantifying the branching complexity of a rooted tree. We consider the Horton-Strahler number of critical Galton-Watson trees conditioned to have size $n$ and whose offspring distribution is in the domain of attraction of an $α$-stable law with $α\in [1, 2]$. We give tail estimates and when $α\neq 1$, we prove that it grows as $\frac{1}α\log_{α/(α-1)} n$ in probability. This extends the result in Brandenberger, Devroye \& Reddad [6] dealing with the finite variance case for which $α=2$. We also characterize the cases where $α=1$, namely the spectrally positive Cauchy regime, which exhibits more complex behaviors. Our proofs are new and probabilistic; they relate the Horton-Strahler number with other shape parameters such as the height or largest degree.