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MIRA
Mira, its very name telling us that we should take strong notice, Mira "the amazing one," the word coming from the same root as "miracle," Mira the only proper-named star in the sky that for a time is too faint to be seen with the naked eye. Few of the stars in its resident constellation Cetus, the Whale, are prominent; only the Alpha and Beta stars are of the second magnitude. Mira itself was relegated by Johannes Bayer to be the "Omicron" (the 15th letter in the Greek alphabet) star. In 1572, Tycho Brahe studied a "new star" so bright that for a time it was visible in daylight. Not new at all, Tycho's star is now known to have been an old star that exploded and disappeared forever. Two dozen years later, David Fabricius thought he might have found another, though much fainter one, in Cetus. This star, however, returned, and has been doing so for over 400 years. Mira is the brightest of the red class M "long period variables," thousands of which are now known. Our star varies from about third magnitude (though sometimes it can reach second) way down to tenth, 40 or so times fainter than the human eye can see alone, and then back again over a 330 day period. As a result it is sometimes a part of its constellation, sometimes not. Mira, with a temperature just above 2000 degrees Kelvin, is one of the coolest stars in the sky. From its distance of 420 light years, we calculate an average luminosity (that includes invisible infrared radiation) 15,000 times that of the Sun. The star is approaching the last stages of its life. Long ago, the hydrogen fusion that powered its core ran out, and then the by- product of that fusion, helium, fused to carbon and oxygen, and now the helium has also run out. The result of these internal changes is a hugely distended, very luminous star that is double the size of the orbit of Mars. Hubble Space Telescope observations show that the star is so unstable that it is not even round. The light variations are caused by pulsation, changes in size that also affect the star's temperature. Mira's great size and instability promote a wind that will soon blow away its outer envelope, the inner nuclear burning portions condensing into a burnt-out "white dwarf," a tiny star the size of Earth, the rest of the star lost to interstellar space. These long period variables help enrich the interstellar gases, out of which new stars condense, with chemical elements formed in their nuclear cauldrons. Most of the carbon in the Universe seems to have come from them. Mira has a white dwarf companion to which all these events have already happened. Many billions of years from now, the same will happen to our Sun.