The upper left figure shows the spectra -- brightness versus wavelength -- for two supernovae. One is nearby and one is very distant. To measure dark energy, scientists need to measure the distance between them very accurately, but how do they know whether they are the same? The lower right figure compares the spectra -- showing that they are indeed "twins." This means their relative distances can be measured to an accuracy of 3 percent. The bright spot in the upper-middle is a Hubble Space Telescope image of supernova 1994D (SN1994D) in galaxy NGC 4526.