© High-Z Supernova Search Team, NASA/ESA Hubble Space Telescope.
The bright object in the lower-left quadrant of this Hubble Space Telescope image is supernova (SN) 1994D. Discovered on the outskirts of the galaxy NGC 4526 by the High-Z Supernova Search Team, this supernova was part of the data set that first indicated the existence of what we now call "dark energy." SN 1994D is a type Ia supernova. Type Ia supernovae are special in astronomy because it is possible to determine their intrinsic brightness by monitoring how their light output changes over time. This allows astronomers to infer how far away they are. By making an independent measurement of how fast the supernovae are moving, the High-Z Team and their competitors were able to explore the relationship between distance and velocity for a large number of supernovae throughout the history of the universe. When they compiled their results, they were shocked to find that the universe appeared to be expanding at an accelerated rate, pushed apart by dark energy. (Unit: 11)