Why are algebraic tori affine?

Question

Working with the definition that an algebraic torus $T$ is an algebraic group over a general field $k$ which, over $\bar{k}$, is isomorphic to a product of finitely many copies of $\mathbb{G}_m$. So we know that $T \times_{Spec(k)} Spec(\bar{k})$ is isomorphic to $Spec(R)$ for a particular $R.$ I'm not sure how to prove this means $T$ was Spec of something to begin with.