A Ghost in the Sky

Right below the magnificent Beehive star cluster in Cancer is a small group of four stars representing the immortal head of Hydra, the water snake. (Hydra also had many mortal heads.) One of the first labors of the hero Hercules was to kill this snake, whose heads reappeared as soon as they were cut off. How he did this is not a tale for the squeamish, and will be told at the end of this column.

The showpiece of Hydra is the "Ghost of Jupiter". It is one of the finest planetary nebulae in the sky. It can be seen easily with a pair of binoculars. To find it, start at the bright star Alphard -- a second magnitude star ten degrees (a handspan) to the east of Procyon. Then, follow the windings of the snake --- a trail of dim stars to the east. The third of these is "mu hydrae," and just below this you'l see a tiny disk. This is the "Ghost of Jupiter," so-called because of its resemblance to that planet when seen throuh a small telescope.

A planetary nebula has nothing to do with the planets. This unfortunate name came from the fact that it looks like a planet when seen through a small telescope (or binoculars). There are thousands of these nebulae in the sky. They are the expanding atmospheres of stars ejected in their death throes. In a large telescope they resemble smoke-rings, with the core of the star, its nuclear fires extinguished, appearing as a glowing spark at the center of the ring.

This will be the fate of our own sun when, five billion years from now, it will have exhausted the hydrogen fuel in its core. The atmosphere will be ejected to become part -- a very important part -- of new stars. The core, like an ember in a fireplace, will glow for a while as a white dwarf and then will wink out.

(And just how did Hercules kill the Hydra, this almost immortal monster? With a torch he burned out the root of each of the heads as he cut them off so that new heads couldn't form.)

(04/09/03)