October 31, 2008

In Today's One Minute Astronomer...

Mira, The "Wonderful" Star

Mira, in the constellation Cetus, is a strange pulsating variable star that fades in and out of view every 11 months. The name Mira means “wonderful”; this star exhibits the largest change in brightness except for novae and supernovae. If you're keen, you can make your own observations of Mira and contribute to astronomical research.

The Basics

• Mira, also called omicron Ceti, was the first pulsating variable star to be discovered. At first, it was thought to be a nova… a star that brightens then fade from view forever. But in the early 1600's, astronomers discovered it changed periodically every 11 months or so.

• Mira is the brightest long-period pulsating variable in the sky. It varies from magnitude 2.0 to 10.0, roughly. That's a factor of 1,500 in brightness… a huge change. But it's not always completely regular in its extremes. For a few months every year you see it, then you don't. Here's how to find it.

• Why does Mira pulsate? The short answer is that it's going through a phase of evolution where the pull of gravity and the unsteady burning of fuel in its core fight against each other. Each dominates for a few months before the other takes over. It's a little of like a weight on the end of a spring bobbing up and down in the Earth's gravitational field: the spring pulls up, the Earth pulls down, again and again.

• The Hubble Space telescope has imaged Mira and shown it to be not spherical but asymmetrical… very strange:

An image of irregularly-shaped variable star Mira

A Deeper Look

• Mira happens to be a double star. It's companion star, Mira B, revolves about the variable star Mira A every 400 years. For many years, astronomers thought Mira B was a white dwarf. But recent observations cast doubt on this view.

• In 2007, a space-based telescope discovered Mira has a comet-like tail caused by the star's loss of material as it hurtles through space. Here's a spectacular image.

• If you're really keen, you can observe the variability of Mira for yourself and contribute useful data to professional astronomers who try to understand how such stars work. You can learn more from the American Association of Variable Star Observers (AAVSO).

Good To Know

The next maximum of Mira occurs late this year or early in 2009.

Personal View

You Star Trek fans out there might remember that Mira, also known as omicron Ceti, played a part in the episode “This Side of Paradise” in which Mr. Spock has a fling with Jill Ireland. The planet is called “Omicron Ceti III”. Given Mira's variability, it's not likely there are any habitable planets in orbit. Entertaining episode, however. If you like Star Trek.

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