Estimation based on critical density As noted in the previous section, since the universe seems to be close to spatially flat, this suggests the density is close to the critical density, estimated above at 9.30×10 −27 kg/m 3 . Multiplying this by (A) the estimated volume of the visible universe (3.38×10 80 m 3 ) gives a total mass for the visible universe of 3.14×10 54 kg, while multiplying by (B) the estimated volume of the observable universe (3.60×10 80 m 3 ) gives a total mass for the observable universe of 3.35×10 54 kg. The WMAP 7-year results estimate that 4.56% of the universe's mass is made up of normal atoms,  so this would give an estimate (A) of 1.43×10 53 kg, or (B) 1.53×10 53 kg, for all the atoms in the observable universe. The fraction of these atoms that make up stars is probably less than 10%.  Estimation based on the measured stellar density One way to calculate the mass of the visible matter which makes up the observable universe is to assume a mean stellar mass and to multiply that by an estimate of the number of stars in the
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