The discovery of an important type of titanium blasting out from the center of the supernova remnant Cassiopeia A (Cas A) in the form of bubbles has been reported by astronomers using NASA’s Chandra X-ray Observatory, so that could be a major advance in understanding how some massive stars explode. Cas A is one of the youngest known supernova remnants, with an age of about 350 years, and is located about 11,000 light-years from Earth in our Galaxy. When a massive star’s nuclear power supply runs out, the center collapses under gravity, forming a dense stellar nucleus known as a neutron star or a black hole.
When a neutron star is created, the inside of the collapsing massive star bounces off the surface of the stellar core, reversing the implosion. The shock generated by the heat from this cataclysmic event was similar to a sonic boom from a supersonic jet racing outwards through the rest of the doomed star, forming new elements by nuclear reactions as it goes. However, in many computer models of this process, energy is quickly lost and the shock wave’s journey outwards stalls, preventing the supernova explosion.
Recent three-dimensional computer simulations indicate that neutrinos, which are very low mass subatomic particles generated during the formation of neutron stars that drives bubbles that accelerate away from the explosion’s core. These bubbles continue to force the shock wave forward, causing the supernova to explode. According to a new Chandra analysis, finger-shaped structures pointing away from the explosion site, to the lower left, contain titanium and chromium, which corresponds to orange iron debris. The titanium discovered by Chandra is a stable isotope of the element, which means that its atoms do not transform into a separate, lighter element as a result of radioactivity. The titanium previously detected in Cas A with NuSTAR, seen in light blue, is an unstable isotope, which transforms over a timescale of about 60 years into scandium then calcium. The conditions required for the creation of the chromium and stable titanium in nuclear reactions, such as the temperature and density, match those of bubbles in three-dimensional simulations that drive the explosions. This new study strongly supports the idea of a neutrino-driven explosion to explain at least some supernovas.
It was previously observed by NASA’s NuSTAR telescope at higher X-ray energies. These Chandra and NuSTAR X-ray data have been superimposed on a Hubble Space Telescope optical-light image (yellow).
The different colours in this new image mostly represent elements detected by
Chandra in Cas A:
| COLOUR | ELEMENT |
| Orange | Iron |
| Purple | Oxygen |
| Green | Silicon compared to magnesium |
| Light blue | Titanium |
Reference: NASA-gov
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