(a) Estimate the mass of the luminous matter in the known universe, given there are $10^{11}$ galaxies, each containing $10^{11}$ stars of average mass 1.5 times that of our Sun. (b) How many protons (the most abundant nuclide) are there in this mass? (c) Estimate the total number of particles in the observable universe by multiplying the answer to (b) by two, since there is an electron for each proton, and then by $10^{9}$ , since there are far more particles ( such as photons and neutrinos) in space than in luminous matter.