Sky watch: Mira the Wonderful's changing brightness

There's an extra star in the sky right now. Its name is Mira, and it's what astronomers call a long-period variable.

OK, so it's not really a "new' star, but it's one that we haven't seen for nearly a year. You see, Mira pulsates brightly over about 11 months, becoming easily visible in the night sky, then fading well beyond naked-eye visibility. Right now, it shines near its brightest and outshines all but one star in its celestial region.

The star was found about four centuries ago by German astronomer David Fabricius, who had been searching for the planet Mercury. Instead, he found this peculiar star that appeared nowhere on his star catalogs, atlases or globes.

A few months later, when he looked for the star in the sky, it wasn't there. Then, on Feb. 16, 1609, there it was again.

Not until 1660 did astronomers realize that this strange star had been there all the time, but that it varied in brightness over a period of 11 months. Mira, also known as Omicron Ceti, became the first star ever discovered to change its brightness, and soon became known as Mira the Wonderful.

Modern astronomers know that not only does Mira's brightness vary over time, but so does its size. Though we cannot see this with the naked eye or even a telescope, astronomers have calculated that its orb swells and contracts by about 20 percent. At its largest and brightest, the star is more than 300 times larger than the sun. This means that, if it replaced the sun in our solar system, its glowing atmosphere would swallow the orbits of Mercury, Venus, Earth, and Mars, and would extend part of the way to Jupiter.

Astronomers also have discovered that Mira has a strange comet-like tail about 13 light years long, possibly formed out of material ejected by the star over the centuries.

Late in the evening during the week beginning Oct. 25, you can discover this remarkable star for yourself. In the southeastern sky lies the constellation Cetus, the sea monster. Think of its head on the left and its body on the right, connected by a long neck. And there, in the middle of Cetus, shines the unusual star known as Mira the Wonderful.