The Janice Emens McAdam Department of Physics

Twin Suns: Binary Stars and Supernovae

19 April 2016


Dr J J Eldridge
Dr JJ Eldridge

Supernovae are the explosive death-throes of massive stars that have burnt all their available nuclear fuel.

Supernovae create and disperse most of the heavy elements in the Universe. In fact, many of the atoms in our bodies were created by supernova explosions. They might have then drifted as star dust forever if it weren’t for another supernova – whose shock waves triggered collapses in the densest parts of the galactic dust cloud. This led to the formation of stars and eventually planetary systems, including our own.

Astrophysicists at the University of Auckland are building a unique set of computer codes to understand the evolution of binary stars, single stars, and their supernovae. To confirm our models, we check that these theoretical stars reflect reality, matching them to rapidly growing observational databases of stars, galaxies, and supernovae.

The details

Star-forming regions in the Eagle Nebula
Star-forming pillars in the Eagle Nebula © NASA, ESA, and The Hubble Heritage Team (STScI / AURA)

We are one of very few groups in the world considering binary star systems in all our work and calculations. Approximately 70% of all stars have at least one companion, and 25% have two or more. Stars that live out their lives in close proximity to another star (or stars) evolve differently from singletons. And every time we compare our models to observations we find that including binary stars is key to understanding the Universe. 

When exothermic (heat-producing) nuclear reactions cease in the core of a very massive star, there is no longer an outward force to counteract the immense gravitational pull of the star’s own mass. The star collapses inwards and then blows itself apart in a cataclysmic outburst.

Type II supernovae

Half of all supernovae are hydrogen-rich explosions, and classified as Type II. A number of their progenitor stars are relatively well understood, since they have been detected in images taken before they exploded. This is the best way to understand the stars that give rise to supernovae – but it relies on us being lucky enough to have captured a pre-explosion image of the galaxy in question. A classic example is Supernova 1987A, captured in before and after images below.


Before and after images of Supernova 1987A
Before and after Supernova 1987A, which occurred at the edge of the Tarantula Nebula in the Large Magellanic Cloud (a nearby dwarf galaxy) © AAO, photographs by D. Malin

Type I supernovae

The stellar progenitor of a Type Ib/c supernova has lost all hydrogen from its surface, becoming a helium star. This category is particularly obscure, as there is currently only a single observed progenitor star. However, it appears to confirm the growing suspicion that most of these events come from binary systems where the paired stars have interacted. It is already widely accepted that for Type Ia supernovae, a carbon-oxygen white dwarf attracts so much matter from its companion star that its swollen mass causes it to explode. (White dwarves are stellar remnants of extreme density, being around the size of the Earth but the mass of the Sun.) 

Stellar populations and galaxies

To model the death of a star we must calculate its entire life history. The more we understand how stars evolve, the better we can model stellar populations in galaxies and indeed the evolution of galaxies themselves. 

Galaxy M51a, which hosts unusually frequent supernova events
Galaxy M51a hosts unusually frequent supernova events, left 2005 and right 2011. Supernovae are indicated with yellow lines. Copyright © R Jay GaBany,

We can also work in the other direction – using our expanding knowledge of galaxies to better understand the supernovae within them. For example in the images below, two supernovae are seen in the spiral arms of nearby galaxy M51a. They’re located in middle-aged stellar populations. 

PhD student Lin Xiao is studying very young stellar populations and modelling their emission nebulae (clouds of ionised gas) to estimate how many stars are being formed. She is then cross-referring to the observed supernova rate in order to check whether the theoretical models are correctly predicting the number of stars being born and dying. This research will provide a sensitive test for our ideas about the lives of stars and galaxies.

Further links

JJ Eldridge's staff profile

JJ Eldridge on Twitter

The Binary Population and Spectral Synthesis (BPASS) website

Postgraduate research topics in physics

Funded PhD opportunities in physics

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