The Hubble Deep Field is a long-exposure image taken in 1995, which revealed thousands of galaxies, all found in one very small part of the sky. In 2004 a similar, even longer exposure was taken called the Ultra Deep Field. These images have been used to determine that there are about 100 billion galaxies in the observable universe. Assuming that these galaxies are all roughly similar to the Milky Way, estimate the total number of stars in the known universe.