Black Holes

Abstract

About 6,000 light years away, in the constellation of Cygnus the Swan, lies the blue supergiant star HDE 226868. Its mass exceeds that of our sun by a factor of about thirty, and its radius by a factor of nearly twenty-five. This in itself is nothing especially unusual. Many other stars of a similar nature are known. But once every five and one-half days, HDE 226868 circles in orbit about an invisible companion. It is this mysterious companion which concerns us here — with a mass one-half that of HDE 226868 but utterly tiny, its radius apparently being only about fifty kilometres. The companion of HDE 226868 is now believed by many astronomers to be a black bole — a bizarre consequence of the physical laws embodied in Einstein’s theory of general relativity. The identification of the companion of HDE 226868 as a black hole is not yet quite certain, but looks highly probable at the present time.