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The rise and fall of the luminosity of a supernova detected in 2010 was typical for its class, but its apparent brightness was 30 times greater than similar events. Quimby et al. (p. 396) compared spectra from the time of peak brightness and after the supernova faded, from which they concluded that something was interfering with our line of sight to the supernova. A previously unknown foreground galaxy turned out to be acting as a lens, bending and magnifying the light from the supernova. Potentially, spacetime warping like this could allow direct testing of cosmic expansion.
Objects of known brightness, like type Ia supernovae (SNIa), can be used to measure distances. If a massive object warps spacetime to form multiple images of a background SNIa, a direct test of cosmic expansion is also possible. However, these lensing events must first be distinguished from other rare phenomena. Recently, a supernova was found to shine much brighter than normal for its distance, which resulted in a debate: Was it a new type of superluminous supernova or a normal SNIa magnified by a hidden gravitational lens? Here, we report that a spectrum obtained after the supernova faded away shows the presence of a foreground galaxy—the first found to strongly magnify a SNIa. We discuss how more lensed SNIa can be found than previously predicted.