Actinium is a silvery-white, radioactive metal with the symbol Ac and atomic number 89. It was discovered in 1899 by the German chemist Friedrich Oskar Giesel. Actinium is part of the actinide series and is found in trace amounts in uranium and thorium ores. Due to its radioactivity, it glows faintly in the dark and is used in some types of radiation therapy for cancer treatment. Actinium is highly reactive and can tarnish quickly when exposed to air. It has a relatively short half-life, with its most stable isotope, Ac-227, having a half-life of about 21.8 years. Because of its radioactivity, actinium must be handled with care in specialized facilities.