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Epoch J2000.0 Equinox J2000.0
|Right ascension||12h 45m 07.83s|
|Declination||+45° 26' 24.92"|
|Apparent magnitude (V)||+4.8 to +6.3|
|Proper motion (μ)||RA: -2.20 mas/yr
Dec.: 13.05 mas/yr
|Parallax (π)||4.590 mas|
|Distance||711 ± 113 ly
(218 ± 35 pc)
|Spectral type||C54J, C-N5, C-J4.5|
La Superba is a semiregular variable star, peaking at about +4.8 mag and diminishing to around +6.3 over a 160 day cycle. Known in short form as Y CVn, it is one of the reddest stars in the sky, and it is among the brightest of the giant red "carbon stars". It is the brightest J-star in the sky, a very rare category of carbon stars that contain large amounts of carbon-13 (carbon atoms with 7 neutrons instead of the usual 6). 19th century astronomer Angelo Secchi, impressed with its beauty, gave the star its common name.
La Superba's temperature is believed to be about 2800 K, making it one of the coolest true stars known. Y CVn is almost never visible to the naked eye since most of its output is outside the visible spectrum. Yet, when infrared radiation is considered, Y CVn has a luminosity 4400 times that of the Sun, and its radius is approximately 2 AU. If it were placed at the position of the Sun, the star's surface would extend beyond the orbit of Mars.
To explain its remarkable coloration, it is necessary to understand that mid-sized stars, once they have finished fusing hydrogen to helium in their core, begin to fuse helium to carbon. During this so-called red giant stage, the outer layers expand and cool, causing the star's radiation output to move towards the red end of the electromagnetic spectrum. Near the end of the star's life cycle, fusion products are moved outwards from the core by convection, thus creating a carbon abundance in the outer atmosphere where carbon monoxide and other compounds are formed. These molecules tend to absorb radiation at shorter wavelengths, resulting in a remarkable spectrum with even less blue and violet compared to ordinary red giants, giving the star its distinguished red color.
La Superba is most likely in the final stages of fusing its remaining secondary fuel (helium) into carbon and shedding its mass at the rate of about a million times that of the Sun's solar wind. It is also surrounded by a 2.5 light year-wide shell of previously ejected material, implying that at one point it must have been losing mass as much as 50 times faster than it is now. La Superba thus appears almost ready to eject its outer layers to form a planetary nebula, leaving behind its core in the form of a vanishing white dwarf.