Carbon Stars

Carbon Stars are late stage stars whose upper atmosphere contains more carbon atoms than oxygen atoms which gives them their distinctive ruby-red color. Due to the relative insensitivity of the human eye to red light these stars can be a challenge to view since they don’t present much of a contrast against the inky blackness of the sky – often taking on the appearance of a smoldering red ember on black velvet. 

Long exposure photos of carbon stars tend to diminish the rich deep red color of these stars due to saturation effects on the camera sensor. I find that a truer representation of the color of these stars is best captured in a star trail photo where the exposure time on any given sensor pixel is kept relatively short and is a better representation of what is visually observed through a telescope.   Below is a selection of star trail photos showing the striking color variations to be found in this class of stars.

The color index in the chart at left is a measure of a stars color as well as its temperature and is found by measuring the magnitude of a star through two different standard color filters -most commonly through a Blue filter (B) and a Yellow-Green filter (V). The color index is the difference of these magnitudes (B-V). The smaller the number the bluer and hotter the star, and conversely a higher index value indicates a redder star with a lower temperature. Our yellow Sun has a color index of 0.656 while a hot blue star such as Rigel has a color index of -0.03. The reddish carbon stars have color indices well above +2.0 indicating a cooler and redder star.

The Pictures below are ordered alphabetically by the Letter Prefix of the Star.

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LX Cygni is a very unique star – it apparently evolved from an S Class star to a C Class (Carbon Star) over the years from 1975 to 2008.  During this time span its period of variability rapidly lengthened from 460 days to 588 days which researchers believe coincided with a “Dredging up of Carbon” from its interior to its surface, thus drastically changing its spectral signature.
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Click Here for a Paper that discusses the unusual Light Curve of RS Cygni.
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Though it has the appearance of being a double star there is no physical relation between T Draconis and UY Draconis. Their apparent proximity however provides a beautiful contrast in colors. The brightness of T Draconis varies by 5 magnitudes (7.2 – 13.5) over a period of 422 days.
T Lyrae was my first attempt to view a carbon star and I had a hard time seeing it. The star trail photo is a better representation of the low-contrast presented by these objects compared to the typical “brilliant white” stars that are much easier to discern. Once you have trained your eye and know what to expect they become easier to spot.
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TX Piscium is among the closest and brightest carbon stars in our solar neighborhood and stands out like an orange beacon in the sparsely populated constellation of Pisces. In terms of astronomical time-scales It may have transitioned from an oxygen-rich atmosphere to carbon-rich atmosphere fairly recently . Here is a LINK to James Kaler’s “Stars” website for more info.
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