The Sky at Night: the constellation Cygnus
High up in the eastern and southern night sky in the summer months is the constellation of Cygnus, the Swan. It is fairly easy to recognise, because the stars in it are in the shape of a cross, as the attached diagram (using SkyMap Pro software) shows. Legend has it that the god Jupiter used to change himself into a swan when he wanted to pay a visit in disguise to one of his many lady friends. In memory of the episode, he later placed a swan in the sky.
Its brightest star is Deneb, which is in the tail of the swan. Deneb is roughly 1,500 light years away from us – in other words, it takes light, which travels at 300,000 kilometres per second, about 1,500 years to reach us. So we are viewing Deneb not as it is now, but as it was 1,500 years ago. It is a very massive star, and such a star eventually ends its life in a colossal explosion (known as a supernova), in which the star becomes as bright as a whole galaxy of stars for a few days or weeks. Deneb will do this some time in the next few million years.
At the other end of the constellation, in the Swan’s head, is the star Albireo, which is an attractive double star when seen through a small telescope. And close to Cygnus is the bright star Vega, the fifth brightest star in the whole of the night sky, and a mere 26 light years away. Of the stars visible from Devon, only Sirius and Arcturus are brighter.
Cygnus has a particular interest to modern astronomers because a black hole lies in that direction, about four times as far away as Deneb. Black holes are extremely dense concentrations of matter with such strong gravitational fields that nothing, not even light, can ever escape from them. This means that they cannot be observed directly, and we only know about this particular black hole because it is one of the strongest sources of X-rays in the sky.
David Love
