Question

There are over 100 billion stars in our galaxy, the MIlky Way. Scientists estimate there are about 100 billion galaxies in the universe. Recall that 1 billion = 10⁹. If every galaxy has about 100 billion stars, about how many stars are in the universe?

Answer 1

$$\begin{gathered} \mathrm{Number}\mathrm{of}\mathrm{galaxies}\mathrm{in}\mathrm{the}\mathrm{universe}={10}^{11} \\ \mathrm{Number}\mathrm{of}\mathrm{stars}\mathrm{in}1\mathrm{galaxy}={10}^{11} \\ \mathrm{So},\mathrm{total}\mathrm{number}\mathrm{of}\mathrm{stars}\mathrm{in}\mathrm{all}\mathrm{galaxies}={10}^{11}\times {10}^{11}={10}^{11+11}={10}^{22}\left[\mathrm{as},{\mathrm{a}}^{\mathrm{m}}+{\mathrm{a}}^{\mathrm{n}}={\mathrm{a}}^{\mathrm{m}+\mathrm{n}}\right] \end{gathered}$$