It can be shown that if an object orbiting a star of mass $M$ in a circular orbit of radius $R$ has speed $v$, then $$M=\frac{R v^2}{G}.$$ Suppose a star orbits the center of the galaxy it is contained in with an orbit that is nearly circular with radius $R=2.4\times 10^{18}$ and velocity $v=220\ \text{km/s}$. Use the result above to estimate the mass of the portion of the galaxy inside the star's orbit (place all of this mass at the center of the orbit).

Mass =