It may not appear so to us, but the space between the stars isn't completely empty. Tenuous and not-so-tenuous clouds of gas and dust drift in the darkness. A region of space some 700 light-years away is a fascinating exception. There, among the constellations of Perseus and Taurus, astronomers have found a large, spherical void over 500 light-years in diameter. Around its perimeter are the Perseus and Taurus molecular clouds – dense clouds of cold gas and dust where stars form. It's called the Per-Tau Shell, and it seems to be the product of at least one giant supernova explosion...