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REMARKS Notice the approximate nature of the computation, which uses 4π/3 ~ 1 on two occasions and 14~ 10 for the number of galaxies in the local group. This is completely justified: Using the actual numbers would be pointless, because the other assumptions in the problem-the size of the observable universe and the idea that the local galaxy density is representative of the density everywhere-are also very rough approximations. Further, there was nothing in the problem that required using volumes of spheres rather than volumes of cubes. Despite all these arbitrary choices, the answer still gives useful information, because it rules out a lot of reasonable possible answers. Before doing the calculation, a guess of a billion galaxies might have seemed plausible. QUESTION About one in ten galaxies in the local group are not dwarf galaxies. Estimate the number galaxies in the universe that are not dwarfs. 12 X Take the local group to be representative. What fraction of galaxies are not dwarfs? Use that fraction and the estimated total of galaxies to estimate the number that are not dwarfs.
EXERCISE Convert 4.85 x 10³ kg/m³ to g/cm³. 4.85e3 X There are 100 cm in each meter (100 cm = 1 m). g/cm³ Need Help? Read It HINTS: GETTING STARTED | I'M STUCK!