The size of the Moon\'s orbit is about 450,000KM. How long does it take the Moon to orbit the Earth? Convert this number to seconds, and use this data on the Moon\'s orbit to find the mass of Earth. The Sun orbits the Mily Way every 300 million years. The size of the orbit is about 30,000 light years and there are 9.5 x 10^15m in a light year. Use this to estimate the mass in the center of Milky Way. Use this, and the answer above for the Suns mass, to estimate how many stars there are in the Milky Way Galaxy.