A star becomes a Carbon Star when the star has come to the end of its Main Sequence stage, when it will be no longer be converting Hydrogen into Helium anymore because it has run out of hydrogen. The star will grow in size because of the outward pressure overriding the inward pressure. This is true of our star but the good news is that there is still a lot of hydrogen left to convert and it won't occur for at least another billion years so we are safe from the Sun growing.
Most Carbon Star are variable stars in which their variability takes place over a period of time, normally a long period.
Once all the hydrogen is used up, the next step is convert Helium into either Carbon or Oxygen. Carbon is produced when three helium nucleii are "merged" and Oxygen is produced with four helium nucleii. When the star has more Carbon than Oxygen then it is a Carbon Star. If a star has more Oxygen then its just a normal, regular star or giant star.
If a star has an equal amount of Carbon and Oxygen, these stars are referred to as S-Type Stars. Examples of S stars are GP Orionis and BS Cygni. The S in the spectral type must be a capital because a small s suffixed with a d means Sub-Dwarf.
Carbon and Oxygen molecules in the atmosphere will combine leaving only carbon left in the wake, the resulting in an atmosphere of Carbon Monoxide which is one Carbon, One Oxygen molecule. Once the Oxygen has all been used up, the Carbon can go on to create more "advanced" carbon molecules.
Carbon Stars will tend to have a spectral type begining with the letter C, for example, R Leporis is "C7,6e" and RT Ursae Majoris is "C4+,4 C". When you look at a Carbon Star, they will be a shade of red.
Some Carbon Stars used to be marked as being either R and N. This is an older form of classification but is still used. The classification was created by Annie Jump Canon who did the work whilst at Harvard Observatory in the United States. Those stars that showed unusually strong carbon were put into R band and those which had a strong cyanogen bands were N. Cyanogen is a chemical compound that is CN2
These bands would later be further enhanced by Donald Shane which would break the two groups into smaller groups, R0-R9 and N0-N7.
A newer way and a more common way is to see Carbon Stars to be classified in terms of temperature. The stars are classified from C0 (hottest - 4,500 K) to the coolest C9 (coldest - 3000K). There is an additional number to indicate the strength of the carbon band between 1 and 5 with 5 the strongest. The star CW Leonis could be said to be the coldest type carbon star with the strongest carbon band with a spectral of C9,5E.
This is one of the better known Carbon stars in the galaxy. It is a brilliant red due to the carbon in the atmosphere. In addition to it being a carbon star, it is also referred to as a vampire star, a star that is sucking hydrogen from a close star to keep it young and near youthful.
It is known because of its brilliant red colour that makes it stand out like the red star in the picture below. Note, that the below picture does not show R Leporis. It is highly variable and it won't always be visible so you would need to use visual aids in order to see it.
Saiph is a main sequence star that is at the bottom left of the constellation of Orion . Orion is one of the most easy to recognise constellations in the night sky, when you locate Orion, you should look down and spotting Lepus will be easy.
Picture Generated using Stellarium
La Superba is another well know star and it is one that can be seen in the sky without any visual aids. To get the best out of the star, you would need a telescope or binoculars to see it. La Superba
The following is a picture taken by the Hubble Space Telescope that shows a Carbon Star in the Globular cluster NGC 2108 which was discovered by John Herschel, son of the Sir William Herschel who discovered Uranus.
Picture copyright : N.A.S.A. / Hubble
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