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Supernovae mystery solved

Supernovae are gigantic stellar explosions that can be seen across the entire universe. Type Ia supernovae are a relatively homogeneous class of stellar explosions, which researchers use as ‘standard candles’ to observe the acceleration of the universe.

It has long been known, however, that they exhibit considerable variation in their spectra and the origin of the differences has been unknown. Now scientists have shown that supernovae explode asymmetrically and the difference in their appearance is simply due to the directions from which the supernovae are being observed.

There is now broad consensus that the relative homogeneity of type Ia supernovae is due to their having the same origin, namely a white dwarf in a binary system, absorbing material thrown out from its companion star, and when it reaches 1.4 solar masses it explodes as a supernova.

Supernovae that fade in the same way may exhibit quite different behaviour in how fast their expanding material slows down (the so-called velocity gradient). With new detailed studies, scientists have now shown that the velocity gradient is closely associated with these supernovae exploding asymmetrically, where the ignition takes place away from the centre. So the different appearances of the supernovae simply depend on the point of view they are observed from.

Type Ia supernovae have played a crucial role in cosmology because they can be used to measure the distances across the universe. The results give new insight into how these stars explode and solving the problem with their different appearances, and also brings good news for the use of ‘standard candles.’

Image: A schematic picture of the structure of a supernova Ia. The ashes of the first phases of the explosion, right after ignition (yellow), are offset with respect to the centre of the ejected material. Depending on where we view the supernova from, it will demonstrate different spectral properties.

Source: Niels Bohr Institute | See also: SPACE.com