Every second, somewhere in the Universe, a star explodes.
And astronomers, being human, like to try and sort each of these exploding stars into categories, based on their observed properties so we can understand them better.
This flowchart shows how astronomers figure out which category most exploding stars belong in.
From this flowchart, it looks like there is far more variety in so-called ‘core-collapse supernovae’, or dying massive stars - type Ib/c and type II - than there is in type Ia or ‘thermonuclear’ supernovae. As a result, many astronomers mistakenly believe that type Ia supernovae are ‘well understood’. It doesn’t always help that we also use them as distance markers to measure how far away distant galaxies are. In fact, type Ia supernovae were used to show that the expansion rate of the universe is accelerating!
However, while we know that some type Ia supernovae - the so-called ’normal’ type Ia supernovae - make good distance indicators, we don’t know a lot about them. For instance, we know that they must involve the explosion of at least one carbon-oxygen white dwarf star that reaches a mass close to 1.4 solar masses because it gobbles up material from a binary companion, and that they glow brightly and then fade away because radioactive nickel is made when this explosion takes place. However, we aren’t sure exactly how the explosion happens, or what the exploding star system looked like before it died.
And that’s just the normal type Ia supernovae. Just like core-collapse supernovae, there is actually a whole zoo of varieties of thermonuclear supernovae that look slightly weird. One of these varieties is called the ‘SN1991bg-like supernova’, or 91bg-like SN for short. These supernovae can’t be used as distance indicators because the relationship between their maximum brightness and how fast they decline cannot be standardised in the same way as normal Type Ia supernova. They are much fainter, and fade away much faster. This means they make much less radioactive nickel.
We also know they make some elements that normal Type Ia supernovae don’t usually produce. The spectra of 91bg-like SNe show strong absorption lines that can only be due to the presence of titanium in the thermonuclear ash. (see the link for a definition of the term ‘ash’ in this context!). These two pieces of information - that 91bg-like supernovae don’t make a lot of nickel, and they make a lot of titanium - gives us an idea of what the stars may have looked like in the moments before they died, and during their deaths.
What we really want to know though, is what made the stars get to that point? From the supernova explosion alone, it is hard to tell how old the star system was when it finally exploded. Knowing the ages of star systems that explode as supernovae is important not only to understand how common different types of supernova explosions are, but also to inform other astronomers who model when and where the chemical elements are formed in galaxies throughout cosmic time. What’s more, there may be a relationship between the age of a system exploding as a supernova, and it’s intrinsic brightness, that can introduce biases in our distance measurements in cosmology.
So how do you measure how old a star system was that makes a particular supernova? One method is to look at the stars around the location the supernova exploded. On relatively large scales of a few thousand light years, stars in a spiral galaxy tend to stick in the groups they were born in. So if we can look at just the light from this population of stars, we can attempt to measure how old the stars are.
This is possible because the spectrum of light from stars of a particular age carries a specific fingerprint. We can try and match the fingerprint light of the stars close to the supernova to fingerprints of particular types of stars we have on file to calculate how old the stars are.
That’s what I did in my most recent paper! In this paper, published soon in Publications of the Astronomical Society of Australia, we used a special camera called WiFeS, attached to the ANU 2.3m telescope at Siding Spring observatory in New South Wales, Australia, to take pictures of galaxies where 91bg-like supernovae exploded. WiFeS is short for the ‘Wide Field Spectrograph’ - it’s a special camera that can take pictures where each pixel of the image contains the spectrum of light associated with that pixel. This is a technique astronomers call ‘Integral Field Spectroscopy’.
By only looking at the light in the pixels immediately surrounding the location in a galaxy where a supernova occurred, an area with a real radius of around a thousand light years in each galaxy, my co-authors and I were able to calculate that the average age of the stars that would have been born around the same time as the stars that exploded was greater than six billion years!
On its own, this may not sound that exciting, but it turns out that there are no other types of supernovae known to occur so long after star formation. Most stars that die as core-collapse supernovae die only a few million years after they form, and most thermonuclear supernovae occur about a billion years after star formation. So these are the oldest star systems found so far to explode as supernovae!
This also means that new chemical elements can be made long after stars form - in particular, titanium. Most simulations assume that only core-collapse supernovae produce titanium, and they do it only a few million years after massive stars form. While 91bg-like supernovae occur at a lower rate than core collapse supernovae, this is more evidence that new chemical elements can be formed long after most astrophysicists assume that nucleosynthesis has mostly switched off!
The work also confirmed the long-held belief that 91bg-like supernovae are associated with old stellar populations (they occur at a much higher rate in elliptical galaxies containing old stars, than in spiral galaxies), as well as quantifying just how old ‘old’ really is! And this might give us a hint at what sort of star systems give rise to these supernovae as they must be binary systems that live a really long time, bringing us just a little closer to understanding the mystery of what sort of stars really end their lives as thermonuclear supernovae.