4. According to one astronomer, there are one hundred thousand million galaxies in the universe, each containing one hundred thousand million stars. How many stars is that altogether?
A $10^{13}$
B $10^{22}$
C $10^{100}$
D $10^{120}$
E $10^{121}$
Solution: $\mathbf{B}$
One hundred thousand is $10^{5}$ and one million is $10^{6}$. So one hundred thousand million is $10^{5} \times 10^{6}=10^{5+6}=10^{11}$. Therefore the total number of stars is $10^{11} \times 10^{11}=10^{11+11}=10^{22}$.