Giants in the Sky

Brilliant Vega shines in the east, the brightest of the three stars forming the Summer Triangle. Further to the east, and only half as bright, is one of the greatest giants in the sky: the star Deneb. It is the "lucida" -- the brightest star, in the constellation of Cygnus the swan.

The light we see now left Deneb over fifteen hundred years ago. (By contrast, the light from Vega left just twenty-six years ago.) For us to be able to see it as a bright star after a journey of nine thousand trillion miles it must be truly tremendous. At that distance our sun would be visible only in the largest telescopes. Deneb is a supergiant star, sixty thousand times as bright as our sun, and one of the brightest stars in the galaxy.

But there's more in Cygnus: one of the most beautiful double stars in the sky and an invisible giant.

The star is Albireo, with topaz and sapphire components shining like two jewels. Modern people see the great swan Cygnus as the Northern Cross, with Deneb at its head, Albireo at its foot.

The invisible giant is called Cygnus X-1. It's not a star at all. It's a black hole. We receive a constant stream of X-rays from a point midway between Deneb and Albireo. We explain it this way:

Once there was a double star, like Albireo, whose two components were giants, each more than ten times as massive as the sun. Exhausting its fuel, one of the components collapsed. Even light could not escape from it. This is the invisible black hole.

But the other component continues to circle its mate. Gas from this star, attracted by the black hole, races faster and faster as it approaches it. Friction heats the gas to millions of degrees. This superheated gas emits X-rays. It is those X-rays that we receive, and that are the evidence for this complex system.

Does this prove that X-1 is a black hole? Since we can never see a black hole, we can only infer its existence from what we can detect -- the stream of X-rays. The clearest sight isn't always with our eyes -- itıs often with our minds.

(05/30/07)