We are asked to estimate for the number of atoms in the universe.

Calculate volume of a star:

$\overline{){\mathbf{V}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\frac{\mathbf{4}}{\mathbf{3}}{\mathbf{\pi}}{{\mathbf{r}}}^{{\mathbf{3}}}}\phantom{\rule{0ex}{0ex}}\mathbf{V}\mathbf{}\mathbf{=}\mathbf{}\frac{\mathbf{4}}{\mathbf{3}}\mathbf{(}\mathbf{4}\mathbf{.}\mathbf{13}\mathbf{)}{\mathbf{(}\mathbf{7}\mathbf{\times}{\mathbf{10}}^{\mathbf{8}}\mathbf{}\mathbf{m}\mathbf{)}}^{\mathbf{3}}$

**V = 1.44 x 10 ^{27} m^{3}**

Use the concepts in this chapter to obtain an estimate for the number of atoms in the universe. Make the following assumptions:

(a) Assume that all of the atoms in the universe are hydrogen atoms in stars. (This is not a ridiculous assumption because over three-fourths of the atoms in the universe are in fact hydrogen. Gas and dust between the stars represent only about 15% of the visible matter of our galaxy, and planets compose a far tinier fraction.)

(b) Assume that the sun is a typical star composed of pure hydrogen with a density of 1.4/cm^{3} and a radius of 7 10^{8} m.

(c) Assume that each of the roughly 100 billion stars in the Milky Way galaxy contains the same number of atoms as our sun.

(d) Assume that each of the 10 billion galaxies in the visible universe contains the same number of atoms as our Milky Way galaxy.

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