The happy hunting grounds of the “Hunting Dogs”, Canes Venatici are well placed. This glorious area beneath the handle of Ursa Major. A favourite area and kicking off point for the Coma Virgo galaxy clusters.
It holds many galaxies including the magnificent M51( 27 million light years away) M94 (16 million lys away)and M63. A few years ago these were good targets from the edge of town. Light pollution has worsened . On rare nights “The Whale”, “Cocoon” and “Hockey Stick” still come out. The most stunning being NGC 4449 which resolves to stars. However for small scopes and bad skies there are some stars worth locating.
Canes V is marked out as a line between Cor Caroli (“Heart of Charles”)at 115 lys away and fainter Chara at 27 lys away. These are useful pointers for galaxies. Cor Caroli is a wonderful binary for small scopes. It’s easy to find by eye and half way between it and Arcturus is the globular bright M3. A most useful pointer.
In addition there is a lovely star in CNv which I use as a tester for observing conditions, the fabulous “La Superba”. Once again, small aperture will serve you well, scooping up maximum colour. See what you think. If you can still get “Hind’s Crimson” star, you’ll see the much finer colour of this “Vampire Star”. Be aware that both these carbon giants are variable , catch them at their brightest !
Back to the superb one.Here’s its location and a few notes from Jim Kaler.
Y Canum Venaticorum – La Superba
Y Canum Venaticorum, called “La Superba” by the 19th-century Italian astronomer Father Angelo Secchi, is one of the deeply red-toned “carbon stars.” Y CVn is a semi-regular (SRb) variable star; its magnitude range is from 4.8 to 6.4, over a period that averages roughly 157 days. Other periods, including one of 2000 days, are suspected. “Y” is one of the reddest stars in the sky, and is classified variously as a C7 supergiant, or as a CN5 supergiant. Its beautiful poppy-red tone is easy to see in 50 mm binoculars.
Carbon stars are highly evolved cool red giants with atmospheres rich in carbon molecules. Most red giants and supergiants are richer in oxygen than carbon; carbon stars reverse the ratio. The unusually deep red color of these stars is the consequence of the efficiency of these carbon molecules in absorbing the star’s blue light.
Carbon stars were originally classed as warmer “R” and cooler “N,” and are now combined into class “C.” As giants, they are dying, and are in a mass range where the carbon byproducts of helium nuclear fusion are lofted to the surface before escaping into space. Huge absorptions by carbon monoxide, cyanogen or CN, carbon-2, and carbon-3 are present, giving the star its remarkable spectrum. The beauty of the spectrum is what caused Father Secchi to gave the star its name. It was described by Agnes Clerke in 1905 as having “extraordinary vivacity of its prismatic rays, separated into dazzling zones of red, yellow, and green, by broad spaces of profound obscurity.”
With a surface temperature of 2200 K, La Superba is one of the coolest of naked eye stars, though one authority puts it at 2800. At 710 light years away, the star’s luminosity is 4400 times the Sun’s, after a large correction for infrared radiation. This gives it a radius of about 2 AU – 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 is not well defined, but was probably initially at least three times the Sun’s.
Typical of its breed, Y CVn is losing mass, at a rate of about one ten millionth of a solar mass per year – a million times that of our Sun’s own solar wind – with a flow velocity of about 10 km/sec. Y CVn is surrounded by a huge detached shell of matter with a diameter of around 2.5 light years. The shell subtends an astounding 11′, or 0.2 degrees, as seen from Earth. It implies that the mass loss rate was 50 times higher in the past. La Superba seems poised to eject its outer envelope, becoming a planetary nebula with its dead white dwarf core at the center.
La Superba is also the sky’s brightest “J star.” These are a very rare set of carbon stars which have a huge elevation of the heavy isotope carbon-13. Though carbon-13 (with 7 neutrons in its nucleus rather than 6) is readily made in the nuclear reactions that help generate stellar energy. But no one quite understands what causes it to be so abundant in the J stars.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]
clear skies ! Nick.