How many atoms are in the universe?

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    Under the given assumptions
    (a) Assume that all of the atoms in the universe are hydrogen atoms in stars.
    (b) Assume that is a typical star composed of pure hydrogen with a density of ##1.4gcm^-3## and a radius of ##7xx10^8m##.
    (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
    Using CGS units
    Mass of sun##=DensityxxVolume##
    Inserting given values we obtain
    Mass of sun##=1.4xx4/3 pi(7xx10^10)^3=2.011xx10^33g##
    Average atomic mass of hydrogen atom##= 1.008am u##
    Avogadro’s number ##6.02 xx 10^23## of atoms in ##1## mole of Hydrogen
    Number of Hydrogen atoms in our sun##=Mass of sun/Molar mass of Hydrogen atomxxAvogadro’s number##
    ##=(2.011xx10^33)/1.008 xx6.02 xx 10^23=1.201xx10^57##
    Number of atoms in our milky way galaxy##=Number of starsxxNumber of atoms in sun##
    ##=1.00xx10^11xx1.201xx10^57##
    ##=1.201xx10^68##
    Number of atoms in observable universe##=Number of galaxiesxxNumber of atoms in our galaxy##
    ##=1.00xx10^11xx1.201xx10^68##
    ##=1.201xx10^79##

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