Photo: NASA

Satellite with Danish technology solving supernova riddle

Friday 21 Feb 14
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Allan Hornstrup
Head of Astrophysics and Atmospheric Physics
DTU Space
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NASA has been using the NuSTAR satellite to study the remains of an exploded star, thus obtaining new knowledge about supernovas.

By Christoffer Muusmann

What actually happens when a violent supernova explosion marks the end of an ageing star? An international team of space researchers has now come a step closer to the answer—using technology developed by DTU Space. The ground-breaking findings are published today in the international journal Nature.

NASA has trained the x-ray telescopes mounted on the NASA satellite NuSTAR on the Cassiopeia constellation. These telescopes make it possible to study the remains of an exploded star—a supernova—which are to be found among the other stars in the constellation. The remains are known as Cas A, and consist of an immense cloud of gas and other matter.

The new study has pieced together the first image of how radioactive material is distributed through the gas cloud. By studying the distribution of—in this case—the radioactive isotope 44Ti, researchers have firmly established that supernova explosions follow an asymmetric pattern. This means that the star does not simply implode in a single motion and subsequently detonate in a gigantic explosion. The process is more complex than that. Scientists have suspected this for years, but have not been able to prove the hypothesis until now.

Allan Hornstrup, an astrophysicist at DTU Space, explains the Danish contribution to the discovery:

“DTU Space developed the special reflective coating that enables the x-ray telescope on the NuSTAR satellite to focus and take sharp, accurate pictures of the hard x-ray radiation. We have every reason to be proud that our knowledge and technology are helping to improve understanding of the mechanisms behind one of the most violent and most complex phenomena in the universe.”

The observations from NuSTAR’s x-ray telescopes show in detail how 44Ti disseminates through the cloud. 44Ti is a radioactive isotope with a relatively short decay span or around 60 years. This means that 44Ti must have been created in the explosion itself. The distribution of 44Ti in the cloud therefore provides information about how the explosion occurred.

It was originally believed that a supernova explosion arose through the entire core collapsing in a single motion. This implosion then created an outward-moving pressure wave that blasted a large section of the outermost part of the star away in an almost symmetrical spherical shape. The only problem was that it was not particularly easy to produce symmetrical simulations and make them explode in the same way as authentic supernovas. Simply put, there was something wrong with the models. It turned out that a degree of asymmetry is required for the explosion actually to take place. The problem thus far has been that no observations have been able to confirm or refute this asymmetry directly.

The new observations of 44Ti by NuSTAR show precisely where the newly formed material is distributed in Cas A, therefore illustrating much more precisely how the explosion happened. It is clear to see that 44Ti is unevenly distributed, which must mean that, as mentioned previously, the explosion was asymmetrical. The in-depth observations likewise establish that the asymmetry was not caused by the core starting the explosion a little more in one area than in another—nor was it caused by a strongly rotating core. In other words, the new observations suggest that much more advanced models must now be made, and that they should probably include some kind of mixing of materials deep inside the exploding star.

More information on NASA's website.

NuSTARsatellitten. Illustration: NASA/JPL-Caltech