from Nature 520, 411 (23 April 2015) doi:10.1038/520411d
Exploding stars grouped in one family because of their similarities actually form two distinct groups. This may have important cosmic implications because the explosions, called supernovae, are the primary evidence that the Universe’s expansion is accelerating.
Half of type Ia supernovae seem to have similar intrinsic brightnesses when seen in the visible spectrum. But when Peter Milne of the University of Arizona in Tucson and his team analysed data from the Hubble Space Telescope and NASA’s Swift satellite, they found that the supernovae fell into two subfamilies, each brighter than the other in a different part of the ultraviolet spectrum.
The relative abundances of the two subfamilies seem to have changed over the past several billion years, a fact that could complicate their use as markers of cosmic expansion, the authors say.