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.