The Ophiuchi Hotline by John Varley Book

After the effortless capture of Earth by vastly superior aliens, humanity is forced to fight for existence on the Moon and other lumps of airless rock. The invention of the Hotline — a constant stream of data from a star in the constellation Ophiuchus — facilitates survival and enables the development of amazing new technologies. Then, after 400 years, humanity’s unknown helpers send a bill for their services…and suddenly everything is threatened once again.