Artists illustration (left) of the inner layers of Cassiopeia A's predecessor star, just before it exploded. Chandra image (right) of the Cassiopeia A supernova remnant today. Iron is shown in blue, other elements are sulphur (green) and magnesium (red). Credit: {link url=}NASA/CXC/M.Weiss; X-ray: NASA/CXC/GSFC/U.Hwang & J.Laming{/link}

Astronomers have taken a fresh look at an old supernova and found that it was turned inside out during its explosion. Iron, which forms during the stars death, is usually in the centre of the supernova remnant. But in Cassiopeia A they found it on the outside instead.

This analysis has also shed some light on a phenomenon called 'neutron star kick', in which the neutron star formed in a supernova recoils during the explosion.

Cassiopeia A (or Cas A for short) is the result of a core-collapse supernova, a type of stellar explosion that only really massive stars go through. It is located about 11 thousand light years from Earth and exploded 330 years ago, making it the second youngest supernova remnant in our galaxy.

Stars run on hydrogen. When they have used it up, something has to give. The star's core begins to collapse, heating up as it goes. This increase in temperature means that the star can start to fuse helium instead. All main sequence stars (like our sun) will eventually reach this 'red giant' stage.

But what happens next depends on how big they are. Really massive stars, over eight times the mass of the sun, begin to fuse heavier elements. They burn their way through carbon, oxygen, neon and silicon, the core collapsing more each time and outer layers cooling and expanding. Eventually, a core of iron is left. Fusing iron uses up more energy than it makes, so fusion stops.

Now there is no outward pressure from the fusion, gravity takes over and the star collapses. In about a second, the core of the star collapses down from something roughly Earth-sized to a neutron star (about 15km across) or a black hole (theoretically, 0km across). The subatomic particles that made up the core are crushed together. Protons and electrons turn into neutrons and neutrinos.

These neutrinos rush outwards, pushing infalling layers of the star back out into space. This neutrino ‘bounce’ gives the infalling layers, now a shock front moving outwards, enough energy to fuse even heavier elements. Gold, silver, platinum, and even uranium, form in supernovae.

The neutrinos that formed during the star’s collapse reach the Earth before we see any light – they get a head start while the shock is still battling through the outer layers of the dying star. Light is emitted as a result of expanding shock wave crashing into gas and dust on its way out of the supernova. By studying this light, astronomers can identify elements present in the supernova remnant.

Cas A, at somewhere between 15 and 25 times the mass of the sun before it exploded, followed this path. But something strange happened.

The iron that formed during its death was ejected out from the centre of the supernova remnant, according to a new paper by Una Hwang and J Martin Laming, of the Goddard Space Flight Centre, Maryland, and the Naval Research Laboratory, Washington, respectively, published in the The Astrophysical Journal.

Hwang and Laming studied X-ray data from Cas A collected by NASA’s Chandra X-ray Observatory. They looked at how various elements were distributed across the supernova remnant. All of the iron they saw was well outside the central region of the remnant. “It is surprising that we see basically all the iron that we expected, but it is on the outside, with apparently nothing in the centre,” said Laming.

We are seeing Cas A 330 years after it exploded. Normally, we wouldn’t be able to see the inner ejecta – the bits of the exploded star – so early on in the remnant's evolution. But thanks to a companion star that ‘stole’ some of its material, the remnant’s predecessor lost a lot of mass. This means that we can have a look inside Cas A a lot earlier than we otherwise would have been able to.

“[The mass loss] allows the shocks that light up the ejecta in X-rays to have made it nearly entirely through the star in just over 300 years, so that much of the ejecta can be observed,” said Hwang. “Even though Cas A is actually the second youngest supernova remnant in our galaxy, it's virtually the 
only core-collapse supernova remnant that shows significant emission from iron ejecta.”

This stroke of luck meant that Hwang and Laming could investigate another phenomenon associated with core collapse supernovae: neutron star kick.

It has long been known that neutron stars, left behind after supernovae explode, recoil from the centre of the explosion. This is called the neutron star kick. Hwang and Laming think that this kick is generated by instabilities in the core of the supernova. If momentum is conserved (and it should be if the neutron star kick comes about this way) then the ejecta would move in the opposite direction to the neutron star – and that’s exactly what they saw with Cas A. The ejecta, as a whole, moved in the opposite direction to the neutron star. But they didn’t see the iron moving in the opposite direction, which they would have expected, too.

But this analysis is only the first attempt at a detailed, comprehensive view of Cas A's X-ray emitting ejecta. Hopefully it will not be the last. “Believe it or not, these data can
 probably support at least a 4 times more detailed study, but it takes a lot of man and computer power,” said Hwang. “We hope that some theorists who are working on simulations of core-collapse explosions will take
 notice and realize that there are data out there that can begin to test their theories.”

NuSTAR, the first high energy X-ray observatory, is due to launch later this year. It should be able to provide better data with which astronomers can investigate how Cas A came to be and how its neutron star came to be kicked.

In particular, it will help pin down the location of titanium-44 in the supernova remnant. This radioactive nucleus is produced in the same process that makes pure iron, so should get distributed in the same way that the iron does. Laming says that hints from the current data suggest that it is not located with the iron on the outskirts of the remnant, but is in fact in the centre. But, he cautioned, that data is noisy and inconclusive. NuSTAR will be able to image the titanium-44 and hopefully provide more definitive answers.

“If it turned out to be true, it would be a major surprise,” said Laming.

Titanium-44 in the centre of the remnant could say something about the presence of ‘invisible’ (that is, unshocked) iron that might also exist in the 
centre, or it could give clues about the details of the explosion or the nature of the neutron star, said Hwang.


Una Hwang, & J. Martin Laming (2011). A Chandra X-ray Survey of Ejecta in the Cassiopeia A Supernova Remnant ApJ arXiv: 1111.7316v1