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R LYR (R Lyrae = 13 Lyrae). The first variable star found in aconstellation (that already did not have aname) received the appellation"R," the second "S," and so on, the Roman letter (or double letter,such "RR") a clear indication that the star varies in brightness. R Lyrae (which is also known by itsFlamsteed number, 13 Lyrae), atmid-fourth magnitude (4.0) even though 350 light years away, isalso the brightest true (intrinsic) variable in the constellationofLyra (the Lyre).Sheliak, Beta Lyrae, is apparentlybrighter, but varies because of eclipses, not because of internalprocesses. Well advanced in its evolution, this class M (M5) redgiant is also a "semi-regular" variable, and in the trade is knownas an "SRb star." Semi-regulars are low-level long periodpulsating variables likeMira, R Lyrchanging between magnitude 3.9 and 5.0 over a 46.0 day period.

R Lyrae varies by several tenths of a magnitude over a 46 dayperiod. Erratic wanderings in maximum and minimum brightnessduring the nearly 14 years of observation are readily apparent. The scale on the bottom is the "Julian Date" of 2440000 plus thenumber that appears, where the Julian Date is the number of dayssince January 1, 4713 BC of the Julian Calendar and is commonlyused for variable phenomena in astronomy. JD 2446500 correspondsto March 11, 1986. The left-hand scaleexpresses the difference between the apparent visual magnitude ofR Lyrae and a nearby comparison star. (From an article in thePublications of the Astronomical Society of the Pacificby J. R. Percy, J. B. Wilson, and G. W. Henry.)

(The term "semi-regular" is something of a misnomer, as these starsare indeed rather regular in their variation, though they canbecome erratic. The SRa class is rather artificial and is largelyincorporated into SRb, and SRc stars are supergiants). As an SRbstar, R Lyr is clearly in its death process, is slowly brighteningwith adead carbon-oxygencore surrounded by fusing shells of helium (into carbon) andhydrogen (into helium), and is losing its outer envelope. Thestar's characteristics are not well defined, the difficultyexaggerated by the variability. Three temperature measures,determined through measure of angular size from interferometry,range from 3175 Kelvin to 3750 Kelvin, a wide gap. The probleminvolves wavelength of observation. Measures in a color in whichtitanium oxide (which defines class M giants) absorbs naturallygive larger diameters than those in a color in which the star'sgases are more transparent; neither value is wrong: they justmeasure different things. Diameter measures range from 150 to 200times that of theSun, the latter about thesize of the Earth's orbit! Luminosity estimates range from 1000solar to 7500 solar. The best evaluation comes from thetemperature and diameter measures, which give luminosities around4000 times that of the Sun. From the average temperature andluminosity, the star's original (and quite uncertain) mass wasabout four times solar. Now reddish, it began life as a hot blue-white class B star. Winds that go along with advanced evolutionwill soon begin to whittle it down to lower mass. For a time,variations in the spectrum led astronomers to believe that R Lyraewas a close double. The variations, however, are not due to orbit(and the resulting Doppler effect), but to the expansion andcontraction associated with the pulsation. The pulsations are alsoquite complex, with additional periods of 53 and 64 days.

Written byJim Kaler. Return toSTARS.

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