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R LEO (R Leonis). Among the most dramatic and fascinating stars ofthe sky are the "long-periodvariables," the "LPVs," epitomized by the first one found,Mira (Omicron Ceti, in 1596), so much so thatthese ultra-variables are also called "Miras." To the eye, theytypically vary by several magnitudes (Mira usually from third to10th, though it has reached nearly to first) with periods of theorder of a year (50 to several hundred days). A handful of othersare visible to the naked eye when they are near maximum light,includingChi Cygni (which can hit closeto third magnitude) and our star here, R Leonis (capital Romanletters from R to Z then double letters used toidentify variables), whichresides in westernLeo west ofRegulus. Very cool, Mira variables arealso among the reddest of stars, the general class beaten out onlyby the deep red carbon stars, some of which (likeHind's Crimson Star, R Leporis) are alsoMiras.

Deep red R Leonis, one of the first long-period variables known,lies inLeo to the right of center justbeneath a somewhat brighter white star.Regulus (Alpha Leonis) appears at far left,Subra (Omicron Leo) at lower right. Atminimum light the star would be gone, beneath the limit of theimage presented here.

R Leo, discovered in 1782, was the fourth such variable known, theearlier ones Mira itself (1596), Chi Cyg (1686), and R Hydrae(1704). R Leo typically varies between magnitudes 5 and 10.5 andback over a period of 312 days, though the maxima and minima varyby a magnitude or so over the years as well. LPVs, of whichthousands are known, are among the biggest of all stars, exceededonly by high mass supergiants. R Leo is a deep class M (M8)giant, whose class also varies,between M7 and M9, giving it a lovely color described by Burnham as"rosy scarlet with often a seeming touch of purple."

Three thousand days in the variation of R Leonis are plotted asvisual magnitudes versus the running count ofJulian Day number, which begins January 1, 4713BCE. JD 2452500 is August 13, 2002, 2454750 October 10, 2008. Notonly does the star vary over its 312-day period, but the maxima andminima vary from one cycle to the next and over longer periods oftime as well. Courtesy of the American Association of VariableStar Observers (AAVSO).

These stars are so large that they do not have well-definedsurfaces, making temperatures, sizes, masses, even distancesthrough parallax uncertain. The best modern measure places R Leo240 light years away, making it one of the closer Miras. Taking itas magnitude 5 at maximum light, adopting a temperature of 3000Kelvin to account for a huge amount of infrared radiation, gives aluminosity 4000 times that of theSun, aradius of 1.1 Astronomical Units (that is, it's as big as the orbitof the Earth), and a nominal birth mass of 1.5 solar. But don'ttake any of that too seriously. Direct measure of radius (byinterferometry and lunar occultation) depends strongly on color andwhere we are in the pulsational cycle, the star's radius measureddirectly from as small as 1 AU (agreeing with that from luminosityand temperature) to as large as two AU, bigger than the orbit ofMars. Direct examination also shows the star to be asymmetric witha distinctive "point source" in its southern hemisphere. Longperiod variables are all well advanced in their evolution, and areexpanding as giants for the second time, with dead carbon andoxygen cores. At some point they become quite unstable, andpulsate, expanding and contracting, which causes them to vary. Thestrong visual variation is a result more of temperaturefluctuations produced by the pulsation. In total luminosity (andin the infrared), the variation is nowhere near as great. Miraslose mass at fierce rates, and at their maximum luminosities (whichdepend on mass), they lose their entire hydrogen envelopes and dieas hot white dwarfs. During the transition, the hot cores light upthe fleeing mass to create "planetarynebulae." And R Leo is indeed losing mass, the surroundingmatter radiating powerful silicon monoxide (SiO) and hydroxyl (OH)masers, in which the molecules beam out laser-like microwave radiowaves. Water is observed as well. Fluctuations in the quality ofthe SiO maser implies a magnetic field and the possibility of aplanet, one you most certainly would not to live on.

Written byJim Kaler 7/31/09. Return toSTARS.

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