l CAR (l ["el"] Carinae). No, not "1" ("one") Carinae, but lower case Roman letter "l" Carinae, a star that fell well off the end of the Greek letter scheme, following which Lacaille (too far south for Bayer) attached lower case then upper case Roman letters for lesser stars. l (again "el") Car, however, is "lesser" only within the confines of one of the sky's great constellations, Carina (the Keel of Argo), one that contains a spectacular section of the Milky Way and great numbers of bright and important stars, including Eta Carinae, one of the most luminous stars of the whole Galaxy. Not quite fourth magnitude (3.38, maximum, as it is variable), this class G (G3) supergiant actually makes part of the constellation's mythical "pattern," quite the achievement for one that cannot even make the Greek list. But that is only prelude. Among the most interesting and important stars of the sky are the pulsating "Cepheid variables" named after the far-northern hemisphere prototype Delta Cephei, which is well-matched by Eta Aquilae and Mekbuda (Zeta Geminorum). Such stars, all class F, G, and K bright giants and supergiants, are in a critical "zone" of temperature and luminosity that makes them unstable and to pulsate. They exhibit a strict relationship between their variation (pulsation) periods and their true visual luminosities, and thereby make excellent indicators for distances of galaxies. We need only find the Cepheids within another galaxy, and measure their periods and apparent magnitudes. Comparison of the true visual luminosities (found from periods) and apparent magnitudes gives the galaxy's distance. Cepheid distances are the first step to obtaining not just the distances of other galaxies but of establishing the nature of the whole expanding Universe. Little recognized is that even at its great distance of 1850 light years, l Car is visually the brightest of them, varying between 3.38 and 4.10, outshining Delta Cep by two tenths of a magnitude and the other two by 0.3. If Carina had been in the northern hemisphere, the collection of these variables might well have been called the "Carinids." Yet more interesting is that the star's period is extra long, a remarkable 35.52 days (as opposed to 5.4, 7.2, and 10.2 days for Delta Cep, Eta Aql, and Zeta Gem), showing it to be superior in true luminosity as well. l Car is so large that its angular diameter has been measured right through its variation cycle. As it pulses, its radius changes between 160 and 194 times that of the Sun (at its greatest, 90 percent the size of Earth's orbit), being largest rather well before maximum light and smallest before minimum. (Cepheids are brightest at maximum expansion velocity, while dimmest at maximum contraction speed.) While the spectral class varies between roughly F8 and K0, at a typical class of G3, with a temperature roughly 5200 Kelvin (the latter needed to find the amount of infrared or ultraviolet radiation), the star's maximum luminosity is 13,500 times that of the Sun, which gives a radius of 140 solar, not far from the actual measured values, and acceptable within the limits of measurement error. The star's distance is measured from a special technique that uses angular diameter and pulsation velocity. While direct parallax gives a smaller value of 1500 light years, the errors inherent in the measurement allow a value as high as 1920 light years; 1850 light years is best. Using the Cepheid period- luminosity relation, we find a distance of 2100 light years, not far from that determined from the size and pulsation velocity. From the luminosity and temperature, the star must have a mass between 9 and 10 times that of the Sun, putting it perilously close to the limit at which stars explode. It could be cooling at its surface with a dead helium core (at the larger mass), or executing a "blue loop" in which it is either cooling or heating with a core fusing helium into carbon (at the smaller mass). Whatever the details, l Car is clearly takes its place among the more magnificent stars of the sky.
Written by Jim Kaler. Return to STARS.