2007.02.17
Local:Pátio (Leiria 39.75N 8.82W alt:60m)

Mira, the Wonderfull

Mira (omicron ceti) is the variable stars prototype of long period with variation greater than 2.5 magnitudes.
At 420 light years, a distance that wasn't enough to prevent it's solar disk being measured (0,048 seconds of arc), having a diameter of 3 AU, about the double of the Mars orbit.
Mira had left the main sequence long ago, having now a contracting carbon and oxygen nucleus inclosed in helium and hydrogen shells, the heating causes unstability, forcing that the size, temperature and spectral class to change. When it is hottest at about 3000K, as it is at this moment, radiates a good part of its radiation at the visual spectrum being thus visible to our eyes. When it cools, the radiation starts to be stronger at the infra-red ray thus disappearing from the visible spectrum. In the reality the luminosity variation is of only two magnitudes what really changes is zone of the spectrum where radiates - wonderfull trick of magic!

Mira is one of the most observed stars on the study of variable stars. Curiously the first record at AAVSO was made in 1902 at the island of S.Miguel, Açores by a Portuguese João of Moraes Pereira. The complete history of this star is here. An excellent book on this and 99 stars more is The Hundred Greatest Stars de James B. Kaler.

The reason of this photo is that Mira is at this moment at its maximum brightness, this rebirth is almost annual due a period of about 330 days, becoming for some time the brightest star its gigantic but relatively tenues Cetus constellation (Whale), of which the image only shows the head and part of the body. To have one a better notion of the enormous light variation (more than 1500x) that this star crosses, its minimum 10,0 magnitude is shown by the encircled star a bit above of Mira.

- Image made two 1 minute photos at 400 ISO Nikon D70 and 50mm lens on the top of a Teegul Sky Patrol II.

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