March 2001 December 2006 Astronomers used supernovae and Cepheids in distant galaxies, including several Cepheids in the spiral galaxy NGC 4258 (circled in inset boxes), to calculate the universe's expansion rate. This rate, called the Hubble parameter, constrains the nature of dark energy. 24 June 2015 sky & telescope RS PUPPIS: NASA / ESA / HUBBLE HERITAGE TEAM (STSCI / AURA)-HUBBLE / EUROPE COLLABORATION; SN 1987A: NASA / ESA / P. CHALLIS AND R. KIRSHNER (CFA); QUASARS: JOHN BAHCALL (IAS) / MIKE DISNEY (UNIV. OF WALES) / NASA; NASA / ESA /A. RIESS (STSCI / JHU) The Space Telescope Revolution