Standard candle without standard
To measure distant distances in the Universe, astronomers often use specific stellar explosions of roughly equal magnitude. However, a supernova of the sought-after Type Ia is stepping out of line and once again admonishes astronomers to be careful when determining cosmic distances.
Estimating the distance of a distant ship on the high seas can be difficult if you don't know the type of ship or have any reference points. Is it big and far away or closer and small? The twinkling stars in the night sky probably posed similar puzzles to astronomers a few hundred years ago. Luckily, there are now the distance ladders.
What sounds strange is actually necessary to explore the infinite expanses. Without them, scientists would not know, for example, that the expansion of the universe is accelerating. The rungs of this ladder are made up of different methods of measuring distance, allowing you to measure a little more space, step by step.
Starting with signal propagation times to other planets, measuring the angles of the nearest stars, it builds up to the observation of so-called standard candles that shine in galaxies billions of light years away. Their advantage over other stars: a well-known absolute brightness. It gives astronomers a measure of the actual radiant power of the celestial bodies.
For a terrestrial observer, the cosmic candles appear much weaker than from up close, because the initial luminosity decreases according to strict physical regulations with the distance covered. Astronomers know about this and can therefore infer the distance of the source. Type Ia supernovae in particular have proven themselves as range finders in this way.
Such stellar explosions occur in white dwarfs, which form a close binary star system together with a red giant. Matter constantly flows from the companion to the surface of the dwarf and is deposited there. In principle, no reason to burst right away. When the stressed star reaches the so-called Chandrasekhar limit, however, this changes: At around 1.4 solar masses, the white dwarf collapses under its own gravity.
The enormous energy triggers fusion processes and finally the dwarf explodes as a supernova. In the process, the entire star rips apart and disperses into space, lighting up with the brightness of an entire galaxy. The maximum intensity of this radiation should be about the same due to the same initial mass and a similar composition, i.e. the same ignition substance. Astronomy has relied on this theory for decades.
But astronomers led by Andrew Howell from the University of Toronto have now found a Type Ia supernova that doesn't fit into the scheme. Four billion light-years from Earth, in the middle of a galaxy, SNLS-03D3bb presented twice as bright as its siblings. In addition, the speed of the ejected matter was much lower than usual. Howell and his colleagues could only explain these variations by estimating that the original star was about twice the mass of the sun. It should have been well above the Chandrasekhar limit and, according to theory, should not have held together at all.
Possibly it was a rapid rotation that saved the overweight dwarf from gravitational collapse, the researchers suspect. On the other hand, the explosion may have involved two ordinary white dwarfs that had previously merged. In order to be able to make more precise statements, however, more detailed model calculations are required. But one thing is already certain: Supernova SNLS-03D3bb is unusable as a standard candle.
This raises the question of whether less strong and therefore less conspicuous type Ia supernovae might change the basic results of the distance determinations carried out so far. No - the authors believe, but in order to continue to be able to use them as reliable standard candles, they should be examined in detail and extreme representatives excluded from cosmological studies.
