I'm working with an optimization problem, and faced a problem when trying to understand the area/curve in the problem, it is the following:
$$ x^2+y^2-y=0$$
I was trying to determine whether this area is compact or not, to find out that this function is a circle with radius $\frac{1}{2}$ and base in origin!

Now I know that the equation of a circle is:
$$ x^2 + y^2 = r^2 $$

But I couldn’t really understand how $\sqrt{y}$ is considered as the radius $\frac{1}{2}$.

What is the best way to understand it?