Look up on a clear night and you can spot the distinctive 'W' shape of Cassiopeia with the naked eye. The middle star of that W, which has the catchy name of Gamma Cas, has been puzzling astronomers since 1866, and for the last fifty years it's been blazing with peculiar high energy X-rays that simply shouldn't be there. Now, thanks to a next generation space telescope with extraordinary precision, the mystery has finally been solved. The culprit is a hungry invisible companion, quietly feeding in the dark.