In this interactive, online activity, students practice estimation skills as they begin to explore the Hubble Deep Field image. Students first give a rough estimate of the number of objects in the image. They then use representative sampling techniques to improve upon their original estimates. Finally, they use their estimates to calculate the number of galaxies in the universe. Students can work through the activity independently or in groups. Detailed teacher pages, identified as Teaching Tips on the title page of the activity, provide science background information, lesson plan ideas, related resources, and alignment with national education standards. This activity is part of the online exploration "The Hubble Deep Field Academy" that is available on the Amazing Space website.