When a massive star runs out of fuel, it collapses and explodes, scattering the material from its interior across space. The remnant of such an explosion, Cassiopeia A, has long been known to astronomers for its lopsided appearance: bright clumps of material are scattered unevenly. Now, Japan's X-ray telescope XRISM has peered into these clumps and seen that the heavy elements inside them are not distributed randomly either.

In the southeastern part of the remnant, manganese is noticeably more abundant than in the northwest. And the difference is not random: the manganese signal is detected with a confidence above four sigma, meaning the probability of error is less than one in ten thousand. Moreover, the ratios of titanium to iron, manganese to chromium, and nickel to iron vary from region to region. This means the star's explosion was not a uniform sphere expanding equally in all directions. Rather, it resembled a burst balloon, with gas escaping more strongly in one direction than another.

Such unevenness could have arisen for several reasons. Perhaps during the explosion, layers of the star that had previously been separate, like pages in a book, got mixed together. Or, in the dying star itself, the amount of neutrons was unevenly distributed before the explosion. Or, streams of elusive particles, neutrinos, irradiated the material differently in different places. Which of these reasons is correct remains unclear, but future observations with even more sensitive instruments will help untangle this knot.