The answer to that is as follows : see , imagine a number $N$, and write it as its prime factors $N = p_1^(n_1) p_2^(n_2) ... p_k^(n_k)$ where the $p_i$ are distinct primes. What is $alpha(N)$? It is $k$ . Now, the point is that all the $p_i$ are at least $2$. In particular, $p_1>= 2, p_2 >= 2, ... , p_k >= 2$. So trivially, we have $N >= 2^(n_1) 2^(n_2)...2^(n_k) = 2^(n_1+n_2+...+n_k)$. Now take the logarithms and we get $log_2 N >= n_1+n_2+...+n_k$, but all the $n_k$ are at least $1$.