“This is the kind of question that we’re only just getting sufficient data to take a crack at”
Understanding supernovae is the focus of a growing fraction of the world’s professional astronomers. Fascinating and confusing in their own right, these dramatic explosions – which can outshine all the stars in a galaxy – are now the yardsticks with which we measure the expansion of the Universe. But it’s hard to understand supernovae without thinking about the environment they exist in. Few stars sit in isolation: their formation and evolution is controlled to some extent by their surroundings. These effects can be dramatic, or rather subtle.
Most supernovae, for example, are caused by the collapse of a massive star that has used up the nuclear fuel available at its heart. A rapid contraction is followed by an explosive rebound, but this only happens when the star is bigger than about eight solar masses to begin with. Such massive stars live for only a short time, and so these ‘core collapse’ supernovae are only found in galaxies where stars have been forming in the last few hundred million years. For the most part, that means spiral galaxies.
The class of supernova of interest to cosmologists, Type Ia supernovae, are more complicated. Most form, we think, when material from a hot star is
The features of spiral galaxies – bulges, bars and discs – all have different star-formation histories, which in turn influence which type of supernova can occur accreted onto a companion white dwarf, a star in the later stages of its life. Eventually the dwarf builds up sufficient mass to reignite. Because you need time for one star to live out its entire life as a normal star before becoming a white dwarf, the link between Type Ia supernovae and star formation is less clear, and they’re seen in both spiral and elliptical galaxies.
A paper led by Artur Hakobyan of Armenia’s Byurakan Astrophysical Observatory takes this sort of analysis one step further by looking at where within spiral galaxies supernovae take place. This is the kind of straightforward question that we’re only just getting sufficient data to take a crack at, and the paper uses data on more than 500 supernovae discovered over the last few decades to attempt an answer.
The fact that there might be differences isn’t too surprising; a spiral galaxy’s bulge, disc and bar (if there is one) all have different star-formation histories. A bulge, for example, is like a little elliptical galaxy plonked in the middle of the disc, and so it hosts Type Ia supernovae but not core collapses.
Bars, the paper explains, also make a difference to core collapse supernovae, though not to Type Ia supernovae. That makes sense too: the relationship between bars and star formation is complicated, but in galaxies with tightly wound arms and a bar then it seems likely that barred galaxies form fewer massive stars. In galaxies with looser arms, a bar seems to make no difference to star formation, and indeed there’s not much difference in the pattern of supernovae. More subtle differences exist, and it seems there is a detectable link between the disturbance of a galaxy by a merger and an increase in the supernova rate.
If this is all sounding a bit convoluted, then I think that’s the point. Galaxies are complex places, and not all supernovae are the same. After hundreds of years of study, we’re finally getting to the point where we can disentangle their lives, and make more sense of what we’re seeing.