Friday, August 17, 2007

A new comet? No, an old star.

Photo Credit: JPL-Caltech/C. Martin (Caltech)/M. Seibert (OCIW)

A news story about the above ultraviolet photograph of the star Mira has been making the rounds the last few days. It is a neat discovery, although the physics behind what is causing the light in the picture is reasonably well understood.

The star Mira is a red giant star, a star nearing the end of its life, and is found in the constellation Cetus, the whale. Don't ask me to point this constellation out -- it's big but pretty obscure. Mira varies in brightness from a medium-brightness star to naked-eye invisibility every 330 days (roughly, the time varies a little). It represents a star in the final stages of its life -- its nuclear core is out of hydrogen fuel, but there is still hydrogen around the star's center that can burn and keep the star shining. The sun will go through a phase like this in a mere 5.5 billion years or so.

When stars are red giants, they tend to start losing a lot of matter. Winds from the star's surface blow matter off into space. The sun does this now, but at a tiny rate -- about one million tons a second (compared to the sun, that's tiny!), or about one earth-mass of material every 100 million years. By comparison, Mira is losing mass at a rate of one earth-mass every ten years.

Mira is moving through space very fast, about 80 miles per second. But space is not completely empty; it has very tiny amounts of gas in it. So, as Mira rockets through space, shedding lots of material, that material collides with the slow-moving gas in space and slows down, so Mira's material gets left behind as the star plows ahead. Think of it like a poorly-covered garbage truck going down the freeway, with bits of trash flying out along the way. Yes, most of the garbage will make it to the dump, but a tiny stream of debris marks out the path the truck took.

What is interesting about Mira's comet-like tail is that it hadn't been seen before. The tail is only visible in light from hydrogen atoms, which glow brightest in ultraviolet light. Thanks to ozone, ultraviolet light doesn't reach Earth's surface, which is good for us, but bad for astronomy. So, it took the launch of an ultraviolet telescope called "GALEX" in order to discover this tail.

No comments:

Post a Comment