Supernova explosions are studied in the framework of stellar evolution and are important factories for heavy chemical elements. Moreover, they also play an important role in modern cosmology: supernovae of type Ia (SNe Ia) are among the most precise distance indicators in the Universe. Astronomers have used these supernovae over the past years to map the expansion history of the Universe. This led to the surprising finding that the expansion of the Universe is accelerated. One possible explanation is ”Dark Energy“, which accounts for about 75 percent of the energy density of the Universe. The principle behind such distance measurements with supernovae is simple: A well-established relation exists between their peak luminosity and the rate of their fading from the peak, which means that the intrinsic luminosity of SNe Ia can be obtained directly from their light curve shapes. By comparing the apparent brightness to the inferred intrinsic luminosity, the distance from which the supernova is observed can be calculated.

Obviously, the success of this method depends critically on the relation used to calibrate the peak luminosities of the supernovae, and hence on the assumption that SNe Ia with the same luminosity all look the same. However, this paradigm has been challenged as more and more supernovae have been observed in detail. First signs of diversity were noticed already in the late 1980s, and quantified beyond any doubt in 2005. It turned out that SNe Ia with the same luminosity do show differences in their spectra, which evolve differently with time. So far the origin of this diversity has been unclear, giving rise to several concerns: Is there only one uniform type of progenitor system? Are SNe Ia really good distance indicators?

The differences appear mostly in the supernova spectra, which are a good tracer of the chemical composition of the matter ejected in the supernova explosion. Different elements leave an imprint on the spectra due to characteristic absorption or emission lines. Over time the cloud of ejected matter expands and becomes increasingly transparent, so that deeper and deeper layers of the ejecta are examined. Differences in the spectral evolution of various SNe Ia thus point to differences in chemical composition, ionization and excitation conditions, or velocities of the ejected material.

The progenitors of type Ia SNe are believed to be white dwarfs — old, burnt-out stars made from carbon and oxygen. Hydrodynamical simulations show that exploding white dwarfs can indeed produce SNe Ia, although it is difficult to reproduce the diversity seen in observations, as white dwarf stars have pretty well constrained properties. One possibility to change the outcome of the explosion is to modify the way the white dwarf is ignited — in a single or in multiple sparks, in the middle or off-centre. Off-centre ignition may lead to asymmetric explosions; the supernova appearance would then depend on the direction from which the event is observed. In theory this is straightforward, but it was unclear up to now whether or not asymmetries do actually play a role in real supernovae.

With their new results, the international research group around Keiichi Maeda from the IPMU at the University of Tokyo now took an important step towards a unified picture of SN Ia explosions. They found that different spectral properties that were observed in supernovae and had been studied independently thus far are actually strongly correlated. In addition, these properties can all be explained in a straightforward manner if SNe Ia are asymmetric, off-centre explosions. The observed diversity is then merely a consequence of the random directions from which the supernovae are viewed.

With these results, the scientists can kill at least three birds with a single stone: They explain not only the origin of the spectral diversity but eliminate also a major concern in using SNe Ia as distance indicators for cosmology by rescuing the idea of a uniform progenitor system for the majority of events. Moreover, the results are the first strong observational indication of how supernova explosions are ignited. The finding points to asymmetric, off-centre explosions.

”Type Ia supernovae with a nearly identical photometric properties can exhibit appreciably different spectral evolution, which has been a nagging concern for cosmologists so far,“ says Keiichi Maeda. ”Our study now strongly indicates that these supernovae do not have intrinsic differences, but their diversity arises solely from a viewing angle effect. The model unifies recent advances in both theoretical and observational studies of type Ia supernovae, supporting the idea of asymmetric explosions as a generic feature.“

Original publication:

K. Maeda, S. Benetti, M. Stritzinger, F. K. Röpke, G. Folatelli, J. Sollerman, S. Taubenberger, K. Nomoto, G. Leloudas, M. Hamuy, M. Tanaka, P. A. Mazzali and N. Elias-Rosa, "An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae", Nature, Vol 466, p82; doi:10.1038/nature09122