segunda-feira, 18 de outubro de 2010

A Movie of Mira, The Wonderful Star

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Rui Moio
A Movie of Mira, The Wonderful Star

via One-Minute Astronomer by admin on 10/15/10
The remarkable variable star Mira approaches peak brightness this month, making it easily visible with the unaided eye from anywhere in the world.  The name Mira means "wonderful".  And it is wonderful, exhibiting the largest change in brightness of any star except for novae and supernovae. If you're keen, you can make your own observations of Mira and contribute to astronomical research. Ler mais

Mira, also called omicron Ceti, is the brightest pulsating variable star, and the first of its kind to be discovered. Early astronomers first thought it was a nova… a star that brightens then fades from view forever. But in the early 1600′s, astronomers noticed it changed periodically every 11 months or so.
Mira 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. At its brightest, it's easy to find in the neck of the constellation Cetus, the Sea Monster, towards the southern horizon.  It's on the left side of this image…
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 (see image above).
Mira happens to be a double star. Its 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 the space-based Spitzer telescope discovered Mira has a comet-like tail caused by the star's loss of material as it hurtles through space. Here's a movie about how this surprising discovery was made, essentially by accident:

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).

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