No one noticed the death of Cassiopeia A. But this star's supernova remnant, the youngest known in the Milky Way, has since become an icon of the success of the Chandra X-ray Observatory. Chandra produced its first images of Cassiopeia A in 1999, soon after its launch. Five years later, these images have become exquisite in their detail.

Credit: NASA/CXC/GSFC/U. HWANG ET AL.

From the size and rate of expansion of the remnant, it is estimated that the light from Cassiopeia A's supernova explosion reached Earth more than 300 years ago. Over the course of history, firm sightings of other supernovae have been made, noted by contemporary astronomers as ‘new stars’ in the sky. Cassiopeia A, it seems, might have been spotted only by John Flamsteed, Britain's first Astronomer Royal. In 1680, he recorded a star close to Cassiopeia A's position — but subsequent star catalogues have tended to regard this as an error. Anyway, an explosion date in the 1660s would fit with the observed expansion better.

In all likelihood, Cassiopeia A just wasn't very bright on the supernova scale of things. But at radio wavelengths, it is now the brightest object in the sky. And in the X-ray emission picked up by Chandra, details of Cassiopeia A's structure and composition are clear. The shock wave created by the supernova (the outer green ring in this picture) now spans a diameter of 10 light years; bipolar jets of mostly silicon atoms (one of which can be seen, in red, on the upper left) suggest that such a feature might be more common in supernovae than had been thought.

The tiny yellow speck at the centre of this 1-million-second exposure is a neutron star — an incredibly dense mass of collapsed stellar material formed from the supernova. Cassiopeia A's neutron star is strangely quiet: it is not firing out pulses of radiation as other neutron stars do. The Chandra team suspects that this star might have a particularly strong magnetic field, even qualifying as a ‘magnetar’ — a class of neutron stars with magnetic fields a thousand trillion times stronger than Earth's.