Solution
= Solution
At closed-string level $N=\widetilde N$, the <bosonic string mass spectrum> is
$$
M^2=\frac4{\alpha'}(N-1).
$$
The state on the leading <Regge trajectory> uses only level-one oscillators and has maximal spin $J=N+\widetilde N=2N$. Eliminating $N$ gives
$$
\boxed{J=2+\frac{\alpha'M^2}{2}}.
$$
Because $N$ is unbounded, the spectrum contains infinitely many particles of increasing mass and spin. The intercept $J=2$ at $M^2=0$ includes the graviton.