Supernova 'standard candles' not so standard after all


They're used in astronomy to measure the expansion of the Universe and probe dark energy, but Ia supernovae aren't consistent across the board – and it seems to depend on their fuel. Belinda Smith reports.


Kepler’s supernova remnant, shown here in a combination of X-ray and optical wavelengths, is a remnant from a Type Ia supernova. Such supernovae are used to measure cosmological distances. – X-ray: NASA/CXC/SAO/D.Patnaude, Optical: DSS

A type of exploding star – Ia supernova – is known in cosmology circles as a "standard candle" for its consistent bright flash, letting astronomers calculate how far it lies from Earth.

But the candles, it turns out, are not so standard. Manuel Moreno-Raya from the Spanish Research Centre for Energy, Environment and Technology and colleagues examined the local environment of 28 Ia supernovae and found those with more elements heavier than helium, which astronomers call "metals", were dimmer than their less metallic counterparts.

The analysis was published in The Astrophysical Journal.

Ia supernovae were thrust into the spotlight when astrophycisists Saul Perlmutter, Brian Schmidt and Adam Reiss used them to show the Universe expanding at an ever-faster rate. The trio scored the 2011 physics Nobel prize for their observations and calculations.

So what gives the standard candle its apparent consistency?

Ia supernovae erupt when a white dwarf star, perhaps guzzling fuel from another star, reaches the critical mass of 1.4 times that of the Sun (called the "Chandrasekhar mass").

At that point, it's unable to support its growing girth and collapses on itself, with atoms smashed and mashed in its core.

The ensuing nuclear fusion blasts the star apart in an explosion five million times the brightness of the Sun – the standard candle.

So Moreno-Raya and colleagues asked: could the fuel being sucked in, as well as the white dwarf's local environment, affect how bright the supernova explosion is?

To find out, they examined the areas surrounding 28 Ia supernovae using Spain's William Herschel telescope.

Supernova brightnesses vs. the metallicity of their environments. Low-metallicity supernovae (blue shading) and high-metallicity supernovae (red shading) have an average magnitude difference of ~0.14. – Moreno-Raya et al. 2016

They found the supernovae with more metals were dimmer than those with a low metal ratio. They think this relationship of higher metallicity/lower luminosity, which has an 80% chance of not being random fluctuation, comes from the build-up of the supermova blast.

Luminosity is produced when nickel decays to cobalt, then iron. If a white dwarf already has a fair proportion of heavier elements, its bright blast will be shorter-lived and drop off faster than a white dwarf that must build up from the smaller carbon and oxygen.

This kind of "non-standard" standard candle has been shown before, but never with such accurate measurements of a supernova's local environment.

So now what? Do we throw out all papers that used Ia supernovae – even the Nobel prizewinning work?

Not necessary, the researchers write. There are plenty of other ways to determine distance. But as telescopes such as the Dark Energy Survey start scanning the heavens above, using distance markers in the Universe, they will need to be more precise than ever.

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  1. http://iopscience.iop.org/article/10.3847/2041-8205/818/1/L19/meta
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