Before the Space Age, astrophysicists were limited to studying the universe via ground-based telescopes, and so could only use information from the parts of the electromagnetic spectrum that penetrated the Earth's atmosphere. Many of the most interesting phenomena are best studied at frequencies that are best or only accessible from space—satellite telescopes have been critical to understanding phenomena like pulsars and black holes as well as measuring the age of the universe. The Hubble Space Telescope is arguably the most valuable astronomical tool ever built