Astronomy & Cosmology

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Stars - Stellar Classes

Class C Carbon Stars

La Superba, a red supergiant named for its glorious red coloration through the telescope, is a large cool J type carbon star.  It weighs three stellar masses, but is around 215 times the size of the Sun.  If it were in place of the Sun, it would reach the orbit of Mars.  At 2,800K, it is exceptionally cool.  It is about 711 light-years away.  It is also a semi-regular variable where the visual magnitude varies between magnitude 4.8 and 6.3 over a period of 158 days. As described above, its red color is due to large amounts of carbon, for example carbon monoxide and cyanogen, in its outer layers that absorbs light of shorter wavelengths.  La Superba does not have long to live.  It is close to the end of the helium burning phase, and is loosing huge amounts of mass at a rate about a million times faster than the Sun due to its strong stellar wind.  Eventually, the core will collapse, and form a white dwarf. At present, it is surrounded by a shell formed from material it has ejected that is about 2½ light-years in diameter. 
Generally, these are red giant or supergiant stars nearing the end of their lives, and all have an excess of carbon over oxygen.  They have been burning helium in their core to form carbon which has risen to the surface via convection.  This tends to happen during periods when the star temporarily stops hydrogen burning in its outer shell and starts helium burning.  This is called a shell helium flash, and occurs every 10,000 to 100,000 years.  It causes the stars luminosity to rise, so the star expands which halts the helium burning in the shell, and the star reverts to hydrogen burning.  The excess carbon causes the stars to have a "sooty" atmosphere, and mass ejection from the star forms a cool shell around it.  This filters much of any blue light that is present contributing to the extremely red appearance of these stars.  There are several sub-types:
C-R Roughly equivalent to stars of classes late G to early K, but with an excess of carbon over oxygen. 
C-N Roughly equivalent to stars of classes late K to M, again with an excess of carbon over oxygen.  Example: R Leporis; see below.
C-J
Cool carbon stars with a notable quantity of the carbon isotope 13C and cyanogen (CN).  In fact the 12C/13C ratio is less than 15,
where it is much greater in normal carbon stars. 
C-H Essentially the same as C-R, but consisting of older, less metal rich Population II stars.
C-Hd
These carbon stars are notably deficient in hydrogen and CH.  They are similar to late type G supergiants.
Y Canum Venaticorum (La Superba)
R Leporis is another smoky red carbon star, produced by a similar mechanism to La Superba.  Again, this star has a cloud of carbon surrounding it.  The star is also a Long Period Mira Variable with a period of 427.07 days, and a longer period of Around 40 years overlaid on this.  The intense red color is less obvious when the star is at its brightest, and most obvious when it is at its dimmest.  In the image to the right, you can see the red coloration.  It also shows the apparent brightness of the star in red, green and blue light, with the blue light image being very noticeably dim. 

It is believed to be about 800 light-years away, and could be between 480 and 535 times the diameter of the sun.  If it were in place of the Sun, it would reach half way to the orbit of Jupiter taking in much of the asteroid belt!  It probably only weighs between 2½ and 5 solar masses, and started life as a hot type B star, though it has cooled to well below 3,000K.  It is destined to pass away as a white dwarf in a few million years time. 
R Leporis (Hind's Crimson Star)
Image Credit:
Adam Block/ NOAO/AURA/NSF
WILLIAM & DEBORAH HILLYARD
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