LA SUPERBA (Y Canum Venaticorum). Small packages can hold
remarkable treasures. The constellation Canes Venatici (the Hunting Dogs) is best known for its
luminary, Cor Caroli (Alpha-2 Canum
Venaticorum), a class A magnetic star with a magnetic field 1500
times the strength of the Sun's. Such stars are not all that rare.
Much rarer is La Superba, or Y Canum Venaticorum. Its
Roman letter
name gives it away as a variable star. Of a kind called "semi-
regular" (technically SRb), it peaks at mid-fifth (about 4.8) and
varies to nearly seventh (6.3 or so) over a 160 day period (which
makes it seem more regular than its class implies). Other periods,
one of 2000 days, are suspected. Most "red" stars are not really
so red. Not so "Y," which is one of the reddest stars in the sky,
as it is among the brightest of the "
carbon stars," the star classified variously as a C7
supergiant, sometimes as a
CN5 supergiant. (In evolutionary terms
the star is most likely really a
red giant with a carbon-oxygen
core, as its mass is nowhere near that of a true supergiant -- it
is just highly developed and overly bright.) Carbon stars were
originally classed as warmer "R" and cooler "N," and are now
combined into class "C." Most red giant and supergiants are richer
in oxygen than carbon: carbon stars reverse the ratio. As giants,
they are dying, and are in a mass range in which the by-products of
nuclear fusion, here carbon (from the nuclear "burning" of helium),
are lofted to the surface before escaping into space. Huge
absorptions by carbon molecules (carbon monoxide, cyanogen or CN,
carbon-2, and carbon-3) are present, giving the star a remarkable
spectrum, a combination of them cutting out blue and violet light
and making the star quite red. The beauty of the spectrum caused
the great 19th century classifier, Father Angelo Secchi, to gave
the star its name "La Superba". It was described by Agnes Clerke
(1905) in terms of the "extraordinary vivacity of its prismatic
rays, separated into dazzling zones of red, yellow, and green, by
broad spaces of profound obscurity," the "spaces" the dark carbon
absorptions. La Superba, 710 light years away, is one of the
coolest of naked eye stars, its temperature but 2200 Kelvin (though
one authority puts it at 2800). After a large correction for
infrared radiation,
the star is seen to shine with a luminosity
4400 times that of the Sun, giving a radius
of about 2 astronomical units, notably larger than the orbit of
Mars. La Superba is most likely in the process of becoming a
luminous giant for the "second time," brightening with a dead
carbon-oxygen core, its mass not well defined, but initially
probably around (or greater than) three times that of the Sun.
Typical of the breed, it is losing mass, La Superba at a rate of
about a tenth of a millionth of a solar mass per year (a million
times that of the solar wind), with a flow velocity of about 10
kilometers per second. The star is surrounded by a huge detached
shell of matter of its own making with a diameter of around 2.5
light years (from Earth an astounding 11 minutes of arc, 0.2
degree!), implying that the mass loss rate was 50 times higher in
the past. La Superba seems poised to eject its outer envelope in
the process of becoming a white dwarf. But that is not all. La
Superba is the sky's brightest "J star." A very rare set of carbon
stars has a huge elevation of the heavy isotope of carbon, carbon-
13 (7 neutrons in the nucleus rather than 6), and are classed
carbon-J. Though carbon-13 is readily made in the nuclear
reactions that help generate stellar energy, no one quite knows how
J-stars are actually made. Go look for this red denizen of the
northern skies, its red color obvious in binoculars, and gaze in
wonder at one of the sky's rarer sights. Thanks to Bob Arr and
Mark Seidel, who suggested this star.
Update: The second Hipparcos reduction gives a distance of 1045
light years, plus or minus 115. With an uncertain revised
temperature of 3000 Kelvin, needed to assess the mount of infrared
radiation, the luminosity comes out to be 87,000 times that of the
Sun and the radius 5.1 Astronomical Units, close to the size of
Jupiter's orbit. These figures are all uncertain and are probably
upper limits. If we use the faintest magnitude, the luminosity drops
to 22,000 Suns and the radius to 2.6 AU, The mass is indeterminate.
Whatever the details, it's big and luminous and nearing the end of
its life.
Written by Jim Kaler 4/11/03; updated 5/23/14
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