Mira Honors Its Name
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Mira is the popular name of a star designated Omicron Ceti, located 350 light-years out in the constellation of Cetus, "the Whale". Mira is Latin for "amazing", "wonderful" or "astonishing", and, indeed, the star does amaze with its changing brightness going up to 3 magnitudes and all the way down to fainter than 10 (invisibility to the unaided eye) in a period of 332 days.

This long period variable star is a spectral class M red giant with a surface temperature of 2000°K. Cool class M stars are at the end of their natural life cycle where hydrogen fusion to helium is about to cease. Hence with an unpowered core, Mira is in an unstable state where its shape is no longer round but elongated as the Hubble Space Telescope confirmed. Periodic changes in the thickness of Mira's atmosphere are believed to cause the variation in brightness. Known as the first of its kind since 1596, Mira is the modern representative of the category of Mira-type cool pulsating red giant stars, a classification by the nature of variability.

Mira is similar in mass to our star, the Sun, however, this is the only physical similarity. Mira's atmosphere spans over a distance that is greater than the diameter of Mars' orbit or over 700 times that of our sun. First resolved by the Hubble Space Telescope in 1995, Mira has a white dwarf companion star in 100AU average distance which is surrounded by a disk of matter supplied by Mira's stellar winds and swirling onto the compact companion. The so-called accretion disk is the source of x-rays that has been monitored by the Chandra x-ray Observatory.

In January 2007, astronomers announced discovery of a protoplanetary disk around Mira's companion, a unexpected reason to assume that planets may form from material provided by dying stars. Since dying stars should be rich of carbon, a by-product of helium, planets, if they will form, are likely to support life; an amazing theory, indeed.



On August 15, 2007 NASA informed that its Galaxy Evolution Explorer has imaged an enormous trail that can only be seen in ultraviolet light. Unseen for over 400 years of observations, the comet-like tail stretches over 13 light years (more than 3 times the distance to the nearest star to our sun). If Mira's tail could be seen with bare eyes, it would span four full moon discs or 2 degrees worth of sky.

Mira's fast space motion of 130 km/sec plus the outflow of material over 30,000 years (approximate time required to travel 13 light-years at that speed) explains the existence of the yet surprising and unparalleled trail of gas and dust. Travelling faster than a supersonic bullet, Mira is pushing ahead of it a bow shock of gas. Thus heated hydrogen gas that loses energy by emitting ultraviolet light causes the tail to glow like fluorescence further explaining why this phenomena can be observed in ultraviolet wavelengths only, hence missed by other telescopes. In only ten years, Mira ejects material worth the mass of Earth that may be recycled into new stars, planets, and eventually, life.
Further reading in the GALEX Press Release

Mira is an aged star near the edge of becoming a planetary nebula. Billions of years ago, Mira was like our sun today and now demonstrates us the destiny of our home star. Mira will be back to naked eye visibility in November and to its full glory early next year.