Chapter 6, Problem 129E

you can use the concepts in this chapter to obtain an estimate of the number of atoms in the universe. These steps will guide you through this calculation.

a. Begin by calculating the number of atoms in the sun. Assume that the sun is pure hydrogen with a density of $\text{1}{\text{.4 g/cm}}^3$. The radius of the sun is $7\times {10}^8\text{ m}$, and the volume of a spheres is $V\text{ = }\frac{\text{4}}{\text{3}}\pi {\text{r}}^{\text{3}}$.

b. The sun is an average-sized star, and stars are believed to compose most of the mass of the visible universe (planets are so small they can be ignored), so we can estimate the number of atoms in a galaxy by assuming that every star in the galaxy has the same number of atoms as our sun. The Milky Way galaxy is believed to contain $1\times {10}^{11}$ stars. Use your answer from part a to calculate the number of atoms in the Milky Way galaxy.

c. Astronomers estimate that the universe contains approximately $1\times {10}^{11}$ galaxies. If each of these galaxies contains the same number of atoms as the Milky Way galaxy, what is the total number of atoms in the universe?

Our sun is one of the 100 billion stars in the Milky Way galaxy. The universe is estimated to contain about 100 billion galaxies.