Suppose we have a Banach algebra A without unit. We can always embed $\displaystyle A$ into an algebra $\displaystyle A\oplus \mathbb{C} $ with unity such that the elements in $\displaystyle A\oplus \mathbb{C} $ are of the form $\displaystyle (a,\alpha),$ $\displaystyle a\in A ,\alpha \in \mathbb{C}. $

We know that under the norm $\displaystyle \|(a,\alpha)\| = \|a\|+ |\alpha|$ ,$\displaystyle A\oplus \mathbb{C}$ is a Banach algebra and with the involution $\displaystyle (a,\alpha)^*=(a^*,\bar{\alpha})$, $\displaystyle A\oplus \mathbb{C}$ ia a Banch *-algebra.

From the book, it is stated there that $\displaystyle A\oplus \mathbb{C}$ can be a Banach *-algebra under many norms. Can anyone give me any example of norms that can make $\displaystyle A\oplus \mathbb{C}$ a Banach *-algebra?