On an average a galaxy has stars

Our galaxy is

Our galaxy is

Define galaxy.

Define galaxy.

Express the number appearing in the following statements in standard form In a galaxy there are on an average 100,000,000,000 stars.

The Sun, which is 2.2 xx 10^20 m from the center of the Milky Way galaxy, revolves around that center once every 2.5 xx 10^8 years. Assuming each star in the Galaxy has a mass equal to the Sun's mass of 2.0 xx 10^30 kg, the stars are distributed uniformly in a sphere about the galactic center, and the Sun is at the edge of that sphere, estimate the number of stars in the Galaxy.