[填空题]
The Sun rotates arountzwul ( 4:d em;t6eo-nd the centera-4xg3o+4 sxb qbv9p b of the Milky Way Galaxy (Fig. 5-46) at a distance of about 30,000 li4q bbsxa gv-+4pxo 3b9ght-years from the center $ \left(1 \mathrm{ly}=9.5 \times 10^{15} \mathrm{~m}\right)$ . If it takes about 200 million years to make one rotation, estimate the mass of our Galaxy. Assume that the mass distribution of our Galaxy is concentrated mostly in a central uniform sphere. If all the stars had about the mass of our Sun $\left(2 \times 10^{30} \mathrm{~kg}\right)$ , how many stars would there be in our Galaxy?$\approx$ $\times 10^{11}$
