Tour: Cassiopeia A, Bubbles With Titanium Trigger Titanic Explosions

Astronomers using NASA's Chandra X-ray Observatory have announced the discovery of an important type of titanium blasting out from the center of the supernova remnant Cas A, a result that could be a major advance in understanding how some massive stars explode.

Cas A is located in our Galaxy about 11,000 light-years from Earth, and it is one of the youngest known supernova remnants, with an age of about 350 years.

When the nuclear power source of a massive star runs out, the center collapses under gravity and forms either a dense stellar core called a neutron star or, less often, 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 heat from this cataclysmic event produces a shock wave — similar to a sonic boom from a supersonic jet — that races outwards through the rest of the doomed star, producing 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 suggest that neutrinos — very low mass subatomic particles — made in the creation of the neutron star drive bubbles that speed away from the center of the explosion. These bubbles continue driving the shock wave forward to trigger the supernova explosion.

This new Chandra study reports that finger-shaped structures pointing away from the explosion site, to the lower left, contain titanium and chromium, coinciding with iron debris seen in orange. The titanium found by Chandra is a stable isotope of the element, meaning that its atoms do not change by radioactivity into a different, lighter element. 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 stable titanium isotope found by Chandra is not shown in the figure.

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.