The brightness $B$ of a star, called its "apparent magnitude" $m$, is expressed on a logarithmic scale: $$B = a^{-m} B_0$$ where $$a=\root 5 \of 100 \approx 2.511886432$$ and $B_0$ is a reference brightness. Notice the minus sign in the exponent, the smaller $m$ the brighter is the object! For example, the star Vega has an apparent magnitude close to zero. The dimmest stars visible to the naked eye have an apparent magnitude close to six. The brightest star in the sky, Sirius, has an apparent magnitude of $-1.4$ and Venus may be as bright as $-4.4$ (depending on its distance and phase).
The full moon has an apparent magnitude of $-12.6$. It is times as bright as Sirius.
The sun is 449,000 times as bright as the full moon, thus its apparent magnitude is .