R LYR (R Lyrae = 13 Lyrae). The first variable star found in a constellation (that already did not have a name) 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 its Flamsteed number, 13 Lyrae), at mid-fourth magnitude (4.0) even though 350 light years away, is also the brightest true (intrinsic) variable in the constellation of Lyra (the Lyre). Sheliak, Beta Lyrae, is apparently brighter, but varies because of eclipses, not because of internal processes. Well advanced in its evolution, this class M (M5) red giant is also a "semi-regular" variable, and in the trade is known as an "SRb star." Semi-regulars are low-level long period pulsating variables like Mira, R Lyr changing 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 day period. Erratic wanderings in maximum and minimum brightness during the nearly 14 years of observation are readily apparent. The scale on the bottom is the "Julian Date" of 2440000 plus the number that appears, where the Julian Date is the number of days since January 1, 4713 BC of the Julian Calendar and is commonly used for variable phenomena in astronomy. JD 2446500 corresponds to March 11, 1986. The left-hand scale expresses the difference between the apparent visual magnitude of R Lyrae and a nearby comparison star. (From an article in the Publications of the Astronomical Society of the Pacific by J. R. Percy, J. B. Wilson, and G. W. Henry.)
(The term "semi-regular" is something of a misnomer, as these stars are indeed rather regular in their variation, though they can become erratic. The SRa class is rather artificial and is largely incorporated into SRb, and SRc stars are supergiants). As an SRb star, R Lyr is clearly in its death process, is slowly brightening with a dead carbon-oxygen core surrounded by fusing shells of helium (into carbon) and hydrogen (into helium), and is losing its outer envelope. The star's characteristics are not well defined, the difficulty exaggerated 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 problem involves wavelength of observation. Measures in a color in which titanium oxide (which defines class M giants) absorbs naturally give larger diameters than those in a color in which the star's gases are more transparent; neither value is wrong: they just measure different things. Diameter measures range from 150 to 200 times that of the Sun, the latter about the size of the Earth's orbit! Luminosity estimates range from 1000 solar to 7500 solar. The best evaluation comes from the temperature and diameter measures, which give luminosities around 4000 times that of the Sun. From the average temperature and luminosity, the star's original (and quite uncertain) mass was about four times solar. Now reddish, it began life as a hot blue- white class B star. Winds that go along with advanced evolution will soon begin to whittle it down to lower mass. For a time, variations in the spectrum led astronomers to believe that R Lyrae was a close double. The variations, however, are not due to orbit (and the resulting Doppler effect), but to the expansion and contraction associated with the pulsation. The pulsations are also quite complex, with additional periods of 53 and 64 days.
Written by Jim Kaler. Return to STARS.