HIND'S CRIMSON STAR (R Leporis). Though stars are colored, their shades are really rather washed out and subject to discussion. Some see Betelgeuse as reddish, others more as orange or even yellow-orange. There is little doubt, however, about the redness of the select set of deep red "carbon stars." Not only are they among the coolest known (such that red and infrared dominate), but they have also -- through internal nuclear fusion and convection (up and down gas motion) -- changed their external ratios of carbon to oxygen. Oxygen usually well dominates carbon, but in the carbon stars, carbon is in the ascendancy, and the resulting carbon molecules remove what little blue light is left, leaving the stars a vivid red. Among the best of them is R Leporis, discovered in 1845 in the constellation Lepus (the Hare) by J. R. Hind (1823-1895), and in his honor known as "Hind's Crimson Star."
R Lep Shining with a deep red light, Hind's Crimson Star, the carbon star R Leporis, lies at center. A long-period Mira variable, R Lep was captured here near maximum light. Mu Leporis shines toward lower left. Such stars are so cool and loaded with carbon compounds that the blue component of starlight is pretty much removed, leaving them a vivid red. Only barely visible to the naked eye, and then only during an especially bright cycle, the star is really a telescopic or binocular object.
R Leporis also has the distinction of being a wonderful "long period variable" like Mira (hence the set also called "Mira Variables"). Such stars have lost their sense of balance, and pulsate in radius, temperature, and luminosity. R Lep, a class C6 carbon star, varies in a complex way over a period of 432 days by about four magnitudes with a much longer superimposed cycle of some 40 years that takes it between extremes of sixth magnitude (making it just visible to the naked eye) and twelfth, 100 times fainter (most of the variation caused by temperature changes during the pulsation cycle). It was at its brightest between 1968 and 1973, and then during the 1990s faded dramatically, for a time barely making 9th magnitude at maximum. The origin of the long cycle is unknown, but may involve the ejection of a dusty shell, as the star loses mass at the rate of a millionth of so of a solar mass per year, nearly 100 million times that of the solar wind. Carbon stars are rare, and most far away. At a distance of about 1100 light years (the distance from parallax corrected for statistical error), R Leporis is one of the closer. Calculation of its parameters is complicated by the surrounding dust shell. A temperature of only 2245 to 2290 Kelvin and luminosity of between 5200 and 7000 Suns (almost all coming out in the infrared) lead to a whopping radius between 480 and 535 times that of the Sun, or between 2.2 and 2.5 Astronomical Units. Were it our star, it would extend nearly halfway to Jupiter, and well into the asteroid belt. The ratio of carbon to oxygen is estimated at 1.2, over twice the solar value. From the dust shell we see an intense "maser" (microwave laser) produced by the hydrogen cyanide (HCN) molecule. R Lep, like its sister stars, is brightening with a dead carbon and oxygen core (but still fusing helium to carbon and hydrogen to helium in surrounding shells), and is now in the process of shedding its external envelope. Its mass is not directly determinable, but carbon stars tend to be in the range of 2.5 to 5 solar masses, meaning R Lep began life as a hot class B star. Once the envelope is ejected, the hot core will light the surrounding wind in a "planetary nebula" and will then finally expire as a cooling, fairly massive white dwarf akin to Sirius B. Much of the carbon (and many other chemical elements) of the Universe came from such stars that died off long before the Sun was born 4.5 billion years ago, including that from which life is made.
Written by Jim Kaler. Return to STARS.