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Encyclopedia > Carbon star

A carbon star is a late type giant star similar to the red giants (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere, and a strikingly red appearance to human observers. According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... Red Dwarf is a British science fiction comedy franchise, the primary form of which comprises eight series of a post-watershed television sitcom that ran on BBC2 between 1988 and 1999, and which has achieved a global cult following. ... STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Carbon monoxide, with the chemical formula CO, is a colorless, odorless, and tasteless gas. ...


The spectral characteristics of these stars are quite distinctive, and they were first recognized by their spectra by Angelo Secchi in the 1860s — a pioneering time in astronomical spectroscopy. In "normal" stars (such as the Sun), the atmosphere is richer in oxygen than carbon. In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ... Pietro Angelo Secchi (1818–1878) was an Italian astronomer. ... Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ... The Sun (Latin: Sol) is the star at the center of the Solar System. ...

Contents

Astrophysical mechanisms

Current research use to subdivide the carbon stars and explain different classes by different astrophysical mechanisms. McClure [1] distinguishes between classical carbon stars, and other non-classical ones that are less massive.


In the classical carbon stars, the abundance of carbon is thought to be a product of helium fusion, specifically the triple-alpha process within a star, which giants reach near the end of their lives in the so called Asymptotic Giant Branch (AGB). These fusion products have been brought to the stellar surface by episodes of convection after the carbon and other products were made. Normally this kind of AGB carbon star fuses hydrogen in a hydrogen burning shell, but in episodes separated by 104-105, the star transform to burning helium in a shell, while the hydrogen fusion temporarily ceases. In this phase, the star's luminosity rises, and material from the inner of the star (notably carbon) moves up. Since the luminosity rises, the star expands so that the helium fusion ceases, and the hydrogen shell burning restarts. During these shell helium flashes, the mass loss from the star is significant, and after many shell helium flashes, an AGB star is transformed into a hot white dwarf and its atmosphere becomes material for a planetary nebula. Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium. ... Overview of the Triple-alpha process. ... A period of Stellar evolution undertaken by all low to intermediate mass stars (0. ... Convection in the most general terms refers to the internal movement of currents within fluids (i. ... This article or section does not adequately cite its references or sources. ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ...


The non-classical kinds of carbon stars are believed to be binary stars, where one star is observed to be a giant star (or occasionally a red dwarf) and the other a white dwarf. The star presently observed to be a giant star accreted carbon-rich material when it was still a main sequence star from its companion (that is, the star that is now the white dwarf) when the latter was still a classical carbon star. That phase of stellar evolution is relatively brief, and most such stars ultimately end up as white dwarfs. We are now seeing these systems a comparatively long time after the mass transfer event, so the extra carbon observed in the present red giant was not produced within that star.[2] This scenario is also accepted as the origin of the barium stars, which are also characterized as having strong spectral features of carbon molecules and of barium (an s-process element). Sometimes the stars whose excess carbon came from this mass transfer are called "extrinsic" carbon stars to distinguish them from the "intrinsic" AGB stars which produce the carbon internally. Many of these extrinsic carbon stars are not luminous or cool enough to have made their own carbon, which was a puzzle until their binary nature was discovered. Artists impression of a binary system consisting of a black hole, with an accretion disc around it, and a main sequence star. ... Red Dwarf is a British science fiction comedy franchise, the primary form of which comprises eight series of a post-watershed television sitcom that ran on BBC2 between 1988 and 1999, and which has achieved a global cult following. ... This article or section does not adequately cite its references or sources. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... In astronomy, stellar evolution is the sequence of radical changes that a star undergoes during its lifetime (the time in which it emits light and heat). ... Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems. ... Barium stars are G to K class giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455. ... It has been suggested that this article or section be merged with s-process. ...


Other less convincing mechanisms, such as CNO cycle unbalancing and Core Helium Flash have also been proposed as mechanisms for carbon enrichment in the atmospheres of smaller carbon stars. This article does not cite its references or sources. ... A Helium flash is the sudden beginning of helium burning in the core of intermediate mass stars, or on the surface of an accreting white dwarf star. ...


Carbon star spectra

By definition carbon stars have dominant spectral Swan Bands from the molecule C2. Many other carbon compounds use to be present at high levels, such as CH, CN (cyanogen), C3 and SiC2. Carbon is formed in the core and circulated into its upper layers, dramatically changing the layers' composition. Other elements formed through helium fusion and the s-process are also "dredged up" in this way, including lithium and barium. Cyanogen is a chemical compound (CN)2. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/grey Atomic mass 6. ... General Name, Symbol, Number barium2, Ba, 56 Chemical series alkaline earth metals Group, Period, Block 2, 6, s Appearance silvery white Standard atomic weight 137. ...


When astronomers developed the spectral classification of the carbon stars, they got into considerable hardships when trying to correlate the spectra to the stars' effective temperatures. The trouble was with all the atmospheric carbon hiding the absorption lines normally used as temperature indicators for the stars. In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ...


Secchi

Carbon stars were discovered already in the 1860's when spectral classification pioneer Pater Angelo Secchi erected the Secchi class IV for the carbon stars, who in the late 1890's were reclassified as N class stars. [3] Pietro Angelo Secchi (1818–1878) was an Italian astronomer. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ...


Harvard

Using this new Harvard classification, the N class was later enhanced by a R class for less deeply red stars sharing the characteristic carbon bands of the spectrum. Later correlation of this R to N scheme with conventional spectra, showed that the R-N sequence approximately run in parallel with c:a G7 to M10 with regards to star temperature. [4]

MK-type R0 R3 R5 R8 Na Nb
giant equiv. G7-G8 K1-K2 ~K2-K3 K5-M0 ~M2-M3 M3-M4
Teff 4300 3900 ~3700 3450 --- ---

Morgan-Keenan C system

The later N classes correspond less well to the counterparting M types, because the Harvard classification was only partially based on temperature, but also carbon abundance; so it soon became clear that this kind of carbon star classification was incomplete. Instead a new dual number star class C was erected so to deal with temperature and carbon abundance. Such a spectrum measured for Y CVn, was determined to be C54, where 5 refers to temperature dependent features, and 4 to the strength of the C2 Swan bands in the spectrum. (C54 is very often alternatively written C5,4). [5] This article needs to be cleaned up to conform to a higher standard of quality. ...

MK-type C0 C1 C2 C3 C4 C5 C6 C7
giant equiv. G4-G6 G7-G8 G9-K0 K1-K2 K3-K4 K5-M0 M1-M2 M3-M4
Teff 4500 4300 4100 3900 3650 3450 --- ---

The Revised Morgan-Keenan system

This two-dimensional classification replaced the older R-N classifications during the 1960-1993, but the Morgan-Keenan C system failed to fulfill the creators expectations:

  1. it failed to correlate to temperature measurements based on infrared,
  2. originally being two-dimensional it was soon enhanced by suffixes, CH, CN, j and other features making it impractical for en-masse analyses of foreign galaxies' carbon star populations,
  3. and it gradually occurred that the old R and N stars actually were two distinct types of carbon stars, having real astrophysical significance.

A new revised Morgan-Keenan classification was published in 1993 by Philip Keenan, defining the classes: C-N, C-R and C-H. Later the classes C-J and C-Hd were added. [6] This constitutes the established classification system used today [7]:

class spectrum population MV[8] theory example(s) # known
classical carbon stars
C-R: the old Harvard class R reborn: are still visible at the blue end of the spectrum, strong isotopic bands, no enhanced Ba line medium disc pop I 0 red giants? S Camelopardalis ~25
C-N: the old Harvard class N reborn: heavy diffuse blue absorption, sometimes invisibile in blue, s-process elements enhanced over solar abundance, weak isotopic bands thin disc pop I -2.2 AGB R Leporis ~90
non-classical carbon stars
C-J: very strong isotopic bands of C2 and CN unknown unknown unknown Y Canum Venaticorum ~20
C-H: very strong CH absorption halo pop II -1.8 bright giants, mass transfer V Arietis, TT Canum Venaticorum ~20
C-Hd: hydrogen lines and CH bands weak or absent thin disc pop I -3.5 unknown HD 137613 ~7

General Name, Symbol, Number barium2, Ba, 56 Chemical series alkaline earth metals Group, Period, Block 2, 6, s Appearance silvery white Standard atomic weight 137. ... A period of Stellar evolution undertaken by all low to intermediate mass stars (0. ... R Leporis, sometimes called Hinds Crimson Star, is a well-known variable star, in the constellation Lepus, near the border with Eridanus. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Other qualities

Most classical carbon stars are variable stars: miras, irregular or semiregular variables due to the chaoticity of their modes of fusion. Most stars are of almost constant luminosity. ... Mira variables, named after the star Mira (IPA [ˈmai. ... An irregular variable is a type of variable star in which variations in brightness show no regular periodicity. ... Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. ...


Observing carbon stars

Due to the insensitivity of night vision to red and a slow adaption of the red sensitive eye rods to the light of the stars, amateur astronomers making magnitude estimates of red variable stars, especially carbon stars, have to know how to deal with the Purkinje effect in order to not overstate the luminosity of the observed star. Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. ... The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... Most stars are of nearly constant luminosity. ... The Purkinje effect (sometimes called the Purkinje shift, or dark adaptation) is the tendency for the peak sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels. ...


Interstellar carbon sowers

Owing to its low surface gravity, as much as half (or more) of the total mass of a carbon star may be lost by way of powerful stellar winds. The star's remnants, carbon-rich "dust" similar to graphite, therefore become part of the interstellar dust. This dust is believed to be a significant factor in providing the raw materials for the creation of subsequent generations of stars and their planetary systems. The material surrounding a carbon star may blanket it to the extent that the dust absorbs all visible light. Gravity is a force of attraction that acts between bodies that have mass. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ...


References

  1. ^ The carbon and related stars
  2. ^ R. McClure, Journal of the Royals Astronomical Society of Canada, vol 79, pp. 277-293, Dec. 1985
  3. ^ Classification of Stellar Spectra: Some History
  4. ^ Carbon Stars (Peripatus.gen)
  5. ^ The Classification of the Red Carbon Stars. Keenan, Philip C.; Morgan, W. W.
  6. ^ Revised MK spectral classification of the red carbon stars - Keenan, Philip C.
  7. ^ Spectral Atlas of Carbon Stars (Barnbaum+ 1996)
  8. ^ Absolute Visual Magnitude

In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. ...

See also

Hinds Crimson Star, also known as R Leporis, is a well-known variable star, in the constellation Lepus, near the border with Eridanus. ... IRC +10216 or CW Leonis is the brightest and best studied carbon star, but also a very peculiar one with the central star being embedded in a thick dust envelope. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Marc Aaronson (August 24, 1950 – April 30, 1987) was an American astronomer. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as auroras and cosmic background radiation). ...

External links


  Results from FactBites:
 
carbon star (311 words)
Carbon stars, also known as C stars, have carbon/oxygen ratios that are typically four to five times higher than those of normal red giants and show little trace of the light metal oxide bands that are the usual red giant hallmark.
Carbon stars lose a significant fraction of their total mass in the form of a stellar wind which ultimately enriches the interstellar medium – the source of material for future generations of stars.
Carbon stars were previously classified as stars of spectral type R (hotter, with surface temperatures of 4,000 to 5,000 K) and N (up to 10 times more luminous but cooler, with a temperature of about 3,000 K).
Spartanburg SC | GoUpstate.com | Spartanburg Herald-Journal (1128 words)
The non-classical kinds of carbon stars are believed to be binary stars, where one star is observed to be a giant star (or occasionally a red dwarf) and the other a white dwarf.
The star presently observed to be a giant star accreted carbon-rich material when it was still a main sequence star from its companion (that is, the star that is now the white dwarf) when the latter was still a classical carbon star.
Carbon stars were discovered already in the 1860's when spectral classification pioneer Pater Angelo Secchi erected the Secchi class IV for the carbon stars, who in the late 1890's were reclassified as N class stars.
  More results at FactBites »

 
 

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