The Swan

We, and men and women of all, time have seen the Summer Triangle. Always the same stars.But the patterns, the constellations, are different.

Below Deneb, the northern corner of the triangle, we see a great cross, the Northern Cross, pointing along the summertime Milky Way.

But those living centuries before Christianity saw, in these same stars, a giant swan. Its head was the star Albireo - a beautiful double - midway between the brilliant stars Vega and Altair. Deneb was its tail. The horizontal of the cross were the wings of a giant swan. This is Cygnus.

As a swan, Zeus came to the beautiful Leda, who conceived....two eggs! (Apparently, nothing surprised the Greeks!) In one egg were the children of Leda's mortal husband, Tyndarius. These were Castor and Clymnestra. In the other were Helen of Troy and Pollux, the immortal children of Zeus. That's why Helen, the daughter of Zeus, was so beautiful. Castor and Pollux became the Gemini twins, in the winter sky. And Clymnestra -- just the wife of Agmemmnon, king of Sparta.

To moderns, though, Cygnus has even more reason to take center stage.

Deneb itself is a giant fifteen hundred light years away. The light we see tonight has been traveling since the fall of ancient Rome, six trillion miles a year for fifteen hundred years. Nine thousand trillion miles away, and Deneb still shines as a bright star. It is one of the brightest stars in the entire galaxy, sixty thousand times as luminous as the Sun.

The Milky Way through Cygnus is one of the richest star regions in the sky. Through binoculars, the Milky Way seems to disappear. In its place we see thousands and thousands of stars.

But the real treasure of Cygnus cannot be seen, even if we used the largest telescopes. A constant stream of X-rays comes to us from a point about halfway between Deneb and Albireo. There is no star visible there, but there is one nearby. It is thought that the gasses from this star leak onto an invisible companion, and as these gasses spiral in, they heat up and emit X-rays. This invisible companion is thought to be...a black hole.

(06/25/08)