Supernovae mystery solved

June 30, 2010, Niels Bohr Institute
A schematic picture of the structure of a supernova Ia. The ashes of the first phases of the explosion, right after ignition (yellow), are offset with respect to the centre of the ejected material. Depending on where we view the supernova from, it will demonstrate different spectral properties.

( -- Supernovae are gigantic stellar explosions that can be seen across the entire universe. Type Ia supernovae are a relatively homogeneous class of stellar explosions, which researchers use as 'standard candles' to observe the acceleration of the universe. It has long been known, however, that they exhibit considerable variation in their spectra and the origin of the differences has been unknown.

Now researchers, including scientists from the Niels Bohr Institute, have solved the mystery. They have shown that the supernovae are exploding asymmetrically and the difference in their appearance is simply due to the directions from which the supernovae are being observed. The results have just been published in the scientific journal, Nature.

have played a crucial role in cosmology because they can be used to measure the distances across the universe. Even though they are not perfect ‘standard candles' (their can vary by up to 50 %), they can still be standardized based on the knowledge that the brightest supernovae fade more slowly, while the weaker fade more quickly.

There is now broad consensus that the relative homogeneity of type Ia supernovae is due to their having the same origin, namely a white dwarf in a binary system, where the two stars circle each other. A white dwarf is the type of star that our sun will become in the end of its life when it has run out of . The white dwarf absorbs material thrown out from its and when it reaches 1.4 solar masses it explodes as a .

The true story, however, is more complicated. Supernovae that fade in the same way may exhibit quite different behaviour in how fast their expanding material slows down (the so-called velocity gradient). This fact has raised doubts about whether type Ia supernovae could be used as cosmological standard candles.

"With new detailed studies we have now shown that the velocity gradient is closely associated with these supernovae exploding asymmetrically", explains astrophysicist Jesper Sollerman, Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.

New detailed studies by an international team including Keiichi Maeda, University of Tokyo and Giorgos Leloudas, Jesper Sollerman and Max Stritzinger from the Dark Cosmology Centre have shown that the velocity gradient is closely associated with an asymmetrical character of the explosions of these supernovae.

"What we could see was that the varying natures of the supernovae could be explained by an asymmetric explosion, where the ignition takes place away from the centre. So the different appearances of the supernovae simply depend on the point of view they are observed from", explains Giorgos Leloudas from the Dark Cosmology research group.

While there had already been indications from previous observations of type Ia supernovae that they might explode asymmetrically, this is the first time that it has been convincingly shown by observational studies probing the supernova central regions. The researchers observed the supernovae in the late stages where it is possible to see deep into their interior and like this confirm that they really explode asymmetrically.

"Besides giving us new insight into how these stars explode and solving the problem with their different appearances, the results are also good news for the use of supernovae as standard candles. If we just observe enough supernovae, the differences from the angles will even out", point out the three researchers from the Niels Bohr Institute, Giorgos Leloudas, Jesper Sollerman and Max Stritzinger.

Explore further: Astronomers simulate how white dwarf stars merge and become a supernova

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1.7 / 5 (6) Jun 30, 2010
I'd expect, this asymmetry may be related to CP-symmetry breaking - it means it would be related to rotational symmetry of supernova. It would be interesting to correlate the direction of rotation of supernovas and the direction in which jets are emanated preferentially. It's well known, massive black holes are exhibiting an asymmetric jets too.

After all, we know, even Earth is not completely spherical - the geoid has a pear shape. It would be nice to correlate this asymmetry with direction of rotation. too.

5 / 5 (6) Jun 30, 2010
Simulations show asymmetry which is caused by several parts of the white dwarf star reaching criticality nearly simultaneously. Each separate seed then induces criticality in adjacent areas, which all merge together within a very short interval.

Its very difficult to imagine how the explosion COULD be symmetrical given those simulations. Using the principal of Occam's razor, more complex origins should be excluded without a lot more evidence that more is going on.

The observed asymmetries are huge compared with the rotational bulge of cosmic objects.

Its good to see theory confirmed by observation!
not rated yet Jun 30, 2010
"Besides giving us new insight into how these stars explode and solving the problem with their different appearances, the results are also good news for the use of supernovae as standard candles. If we just observe enough supernovae, the differences from the angles will even out"

This is great news indeed. A deluge of type Ia supernovae will be available through upcoming mega-surveys like LSST. A preprint of the Nature paper has been posted over at arXiv:
not rated yet Jul 01, 2010
The sun will NOT become a white dwarf after it's burned all of its hydrogen. It will become a white dwarf after it's burned all of its HELIUM.

When I saw that gaff I stopped reading the article.
5 / 5 (2) Jul 01, 2010
So, in essence, they convinced themselves that they were observing supernovas that were always the same brightness, then had doubts, now they have convinced themselves that now they really do know what is going on.

I have never really trusted the idea of a standard type 1a supernova. These are the questions I have:

Luminosity with respect to rotation of the object. Meaning is it much brighter looking at the pole vs. the equator, especially if the accretion disk is from an object that does not orbit in the plane of rotation?

Luminosity with respect to location of a trigger event. This is a complex question because it isn't even know what a trigger event looks like, but if a trigger event starts at a specific location, how might it affect luminosity as shock waves travel through the star, perhaps like a shaped explosion?

Can a trigger event be initiated by a collision that could create a very lopsided luminosity event?

I have others but these are the major ones.
not rated yet Jul 07, 2010
Let me try a fairly simple model. Since the donor star is expected to be fairly close to the white dwarf at the time of the explosion, thinking of the WD as spherical is wrong. The distortion may be smaller than for an egg, but think of a WD as an egg with the yolk near the wider end, and the egg pointing at the donor star. The egg whiteis mostly helium*, and the yolk is metals, mostly carbon, oxygen, and silicon.

Now what happens is that either hydrogen burning near the surface, or Helium burning near the core causes a pressure spike and...Boom! If I'm right, the explosion will always start at the surface of the core, and the location will be near the axis between the two stars, but lagging a bit. The infall will cause occasional pressure spikes that are good triggers.

* Helium, huh? Sure, the outer layer is mostly from the donor star. There are lots of indications that novas, rather than supernovas are caused by hydrogen burning to helium or WD on neutron stars.

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