[填空题]
The Sun rotates around the center of the Milky Wy h4ottfxk.d ,9 b5rl6ay Galaxy (Fig. 4gx pdyal*l:r(,rd 2fiki)u 05-46) at a distance of about 30,000 light-years udlr),l*(4 iraif yg0:kxdp 2from 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?$\approx$ $\times 10^{11}$
