Quadrillions and Quadrillions of Stars

An eagle-eyed sky gazer could count more than 2,000 stars on a good clear night, but Simon Driver, an astronomer at Australian National University, can do a lot better. Using two large telescopes in Australia and the Canary Islands, he has determined that the visible universe contains about 70,000 million million million stars—that’s a 7 followed by 22 zeros. Driver arrived at this number by quantifying the brightness of a sample strip of sky containing about 10,000 galaxies. “What we’ve actually measured is an estimate of the blue-wavelength photon density of the universe—how many particles there are per unit volume in a certain wavelength range,” he says. He multiplied this figure by current estimates of the volume of the universe. Since the brightness of galaxies is directly related to the number of stars they contain, Davis was able to derive a reasonably accurate star count. His estimated total is far more than all the grains of sand on all of Earth’s beaches, yet he says there are probably many more stars out there. “The universe is so big that light from its farthest regions hasn’t reached us yet,” he says.