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MIRA (Omicron Ceti). Mira, its very name telling us that we shouldtake strong notice, Mira "the amazing one," the word coming fromthe same root as "miracle," Mira the onlyproper-named star in the sky that for atime is too faint to be seen with the naked eye. Few of the starsin its resident constellationCetus, theWhale, are prominent; onlyDenebKaitos (Beta Ceti) is of second magnitude. Mira itself wasrelegated by Johannes Bayer to be the "Omicron" (the 15th letter intheGreek alphabet) star. In 1572, TychoBrahe studied a "new star" inCassiopeiaso bright that for a time it was visible in daylight. Not new atall,Tycho's Star is now known to havebeen an old star that exploded and disappeared forever. Two dozenyears 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, a classM7 red giant 420 light yearsaway, is the brightest and nearest of the red class M "long period variables,"thousands of which are known. The star varies from about thirdmagnitude (though sometimes it can reach second) all the way downto tenth, 40 or so times fainter than the human eye can see alone,and then back again over a 330 day period (the spectral classvarying as well between M5 and M9 coolest whenfaintest). As a result it issometimes a part of itsconstellation,sometimes not. The temperatures and radii of such stars are highlyproblematic. They depend on where the star is in its variationcycle and on the wavelength ("color") at which they are observed. If we observe at a wavelength at which the star's outer gases arevery opaque (the result of molecular absorption of light), we willfind a radius that might be twice as high (and the temperaturelower) than it appears at a "transparent" wavelength. Thecalculation of luminosity is then confused as well. Mira, however,is so close that we can easily measure its angular radius, hencephysical radius, which ranges from about 2 Astronomical Units (500solar radii) at visual wavelengths to double that in the infrared,or from 20 percent bigger than the orbit of Mars to nearly half thesize of the orbit of Jupiter. From a typical temperature (foundfrom the angular size) of around 3000 Kelvin (other estimates goingas low as 2000 Kelvin) and the radius itself,we find a luminosity of around 8500 times that of theSun, close to that deduced from the visualbrightness, distance, and an estimate of the huge amount ofinfrared radiation produced bythe cool "surface." The star is approaching the last stages of itslife. Long ago, the hydrogen fusion that powered its core ran out,and then the by-product of that fusion, helium, fused to carbon andoxygen, and now the helium has also run out. The result of theseinternal changes is a hugely distended, very luminous star. Thelight variations are caused by pulsation, changes in size that alsoaffect the star's temperature and thus the amount of light thatleaks out at visual wavelengths (the infrared variation nowherenear so large). Mira's great size and instability promote a dustywind that blows at a rate of about a tenth of a millionth of asolar mass per year (10 million times that of the solar wind) thatwill soon evaporate away its outer envelope to produce an ephemeralplanetary nebula (such as theRing orSaturnnebulae), the inner nuclear burning portions of the star eventuallycondensing into a burnt-outwhite dwarf, a tiny star thesize of Earth, the rest of the star lost to interstellar space. These long period variables help enrich the interstellar gases, outof which new stars condense, with dust and chemical elements formedin their nuclear cauldrons. Most of the carbon in the Universeseems to have come from them. Mira has a white dwarfcompanion that orbits some 65 AUaway from Mira proper to which all these events have alreadyhappened. (Many billions of years from now, the same will happento our Sun.) The two are close enough that the white dwarf drawsmass from Mira's wind. Massive white dwarfs that are closer totheir mass-donating companions can overload themselves so much thatthey can collapse and explode assupernovae. Such an event isbelieved to have created Tycho's Star, and we are back full circle.As Mira moves through the gases of interstellar space, it leaves ashocked wake more than 15 light years long. Written byJim Kaler 10/23/98; revised02/13/09. Return toSTARS.
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