66. Stars in the Universe Astronomers frequently say that there are more stars in the universe than there are grains of sand on all the beaches on the earth. (a) Given that a typical grain of sand is about $0.2 \mathrm{~mm}$ in diameter, estimate the number of grains of sand on all the earth's beaches, and hence..

 66. Stars in the Universe Astronomers frequently say that there are more stars in the universe than there are grains of sand on all the beaches on the earth. (a) Given that a typical grain of sand is about $0.2 \mathrm{~mm}$ in diameter, estimate the number of grains of sand on all the earth's beaches, and hence the approximate number of sand stars in the universe. It would be helpful to consult an atlas and do some measuring. (b) Given that a typical galaxy contains about 100 billion stars and there are more than 100 billion galaxies in the known universe, estimate the number of stars in the universe and compare this number with your result from part (a).