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Stars are categorized according to their Spectral Class, which Father A. Secchi (Rome, Italy) started.* These spectral classes were originally O, B, A, F, G, K, M, and Q (the mnemonic being "Oh, Be A Fine Girl, Kiss Me Quick !"). This has changed in recent years, thanks to Henry Draper, to O, B, A, F, G, K, M, R, N, and S (this mnemonic being "Oh Be A Fine Girl, Kiss Me Right Now, Sweet !"). This system categorizes stars according to their temperature and brightness, the hottest and brightest being of the type O class, and the dimmest and coolest being the S-type stars.

Spectroscopically the Type O stars show heavy Hydrogen emission lines, with Helium being slightly less prevalent and few or no emission lines of the other elements. Type B stars show heavy Helium emission line spectra, with Hydrogen close behind, quantitatively - again there is a dearth of "heavier" elements in their spectra. In type-A stars, we first notice the spectral lines of metals like Calcium, Sodium, Nickel, and Iron. These emission lines grow in strength through classes F, G, and K, and becoming "landmarks" of classes M, R, N, and S.¹⁰¹

In addition, you should note that the spectral classes - besides being arranged by order of color and temperature - are arranged roughly along the same lines of the spectrum of visible wavelength light - from blue to red. Type O stars, then, are blue-white, type B stars are blue, Class A stars (the exception to the rule) are white, our sun (called Sol) is a type G star which is obviously yellow, type K stars are bright orange, and type M stars are red in color.

To be scientifically correct, the spectral class of stars is now dependent on relative intensities of certain emission lines. Some of these lines are Helium (in hotter stars), Hydrogen, ionized Calcium (K lines), and the 4226 Angstrom emission line of neutral Calcium in type O and B stars. In the cooler stars Titanium Oxide lines are indicative of class M stars, Zirconium Oxide lines are found in the emission lines of type S stars, and Carbon lines are used to classify types R and N stars. Again, there are exceptions to the rules, these typically being Wolf-Rayet stars and white dwarfs in which Carbon is more abundant than Oxygen.²

Luminosity

The Harvard system specifies only the surface temperature and some spectral features of the star. A more precise classification would also include the luminosity of the star. The standard scheme used for this is called the Yerkes classification (or MKK, based on the initials of the authors William W. Morgan, Philip C. Keenan, and Edith Kellman). This system ascertains the shape and nature of certain spectral lines to measure the surface gravity of stars. The gravitational acceleration on the surface of a giant star is much lower than for a dwarf star, since g = G M / R² and the radius of a giant star is much larger than a dwarf. Given the lower gravity, gas pressures and densities are much lower in giant stars than in dwarfs. These differences manifest themselves in different spectral lines which can be measured.

The Yerkes system uses six luminosity classes:

Asymptotic Giant Branch Stars (R, N, and S)

After exhausting the hydrogen supply in its core, nuclear fusion of hydrogen to helium will continue in a shell surrounding the core. The core will essentially be a hot degenerate helium star (or helium white dwarf) encased in a hydrogen burning shell. Grossly simplifying the process, helium produced in the shell around the inert core will add to the core's mass until degenerate pressure heats the core sufficiently to start helium fusion within the core. Helium fusion will then continue in the core until once again, the core fuel supply is exhausted and the star has an inert hot carbon-oxygen white dwarf core surrounded by an inner shell of helium fusion and an outer shell of hydrogen fusion. This double-shell burning phase is a name based on how stellar evolution proceeds when charted on a Hertzprung-Russell diagram.

Stars in the asymptotic giant branch are short-lived. The degenerate core of the star is more massive that it was in the single-shell burning phase, and due to the peculiar nature of degenerate (collapsed) matter, the more massive core is physically smaller. The gravity experienced by overlying layers is hence stronger, requiring higher luminosities to maintain the balance between pressure and gravity. Thus the star expends energy at a very high rate and may well become a red supergiant.

Stars in this phase of stellar evolution are difficult to model with any accuracy. One problem is the instability intrinsic to the burning of the thin, unstable helium shell. Slight positive perturbations in the nuclear energy generate extra pressure and the region is enlarged slightly.Many reaction processes are very temperature sensitive, such as the triple-alpha process which will most likely be dominant in the helium shell. Because the layer is thin, the change in height is slight and hence the change in pressure on the hotter region is changed very little. The higher temperature will likely increase the rate of nuclear reactions. Thus local reaction rates will pick up, generating more heat before it can be diffused. Thus large runaway reaction spots can start from small local condition changes.

This runaway reaction is only checked after considerable expansion, in addition to the creation of a convective cycle to carry away the excess energy. Yet once the runaway is checked and the layer resettles, the same underlying physical problem remains - there is no real stable helium shell burning mode. The star will experience spasms of energy generation with convective cells which may carry material all the way up to the hydrogen burning shell, followed by longer periods of resettling within the radii of the thin shell.

If the convective cells created during this helium fusion runaway reaches all the way to the hydrogen fusion layer, this could provide a mechanism for material deep within the star to be transported up to its surface. This would explain several stellar types which seem analogous with K and M stars with respect to temperature, but show other spectral features as if their outer atmospheres had been enriched with heavier elements - these types would be the R, N, and S types stars.

Be Stars

Be stars are B-type stars which periodically show the spectral lines of the Balmer series, although the emission lines of helium and iron may also be apparent. Specifically, Be stars correspond to class III and V non-supergiant stars. Of these, approximately 10% of the non-supergiant B-type stars present the characteristics of the Be stars, which typically belong to the spectral classes B0 to B7 - 5% are of spectral type O8-O9.5 ; and >1% belong to the spectral type A0 or A1.

Characteristics

• Hydrogen, helium, and iron emission lines vary from example to example, may be confused with the background (completely obliterated), and the rate of change of the variability changes.
• Be stars are thought to be in their "shell" phase - where a narrow and deep absorption line is in the center of a broad emission line.

The process that produces the emission line spectra is related to the presence of a accretion disk that rotates very quickly, with an apparent velocity >300 km/sec. which may be why these stars show the quality of Doppler Broadening (widened emission lines on the spectrographic plate). However, the measured speed is V * (sin) i, the projected rotational velocity, and not V, the disk velocity. If the disk matrix is optically opaque, then narrow absorbtion lines from the disk is projected in front of the underlying star spectra. In this situation, the material is moving at right angles to the line of sight, so no Doppler Broadening is in evidence.

Amateur astronomers are encouraged to study and accumulate photometry information because at the present time there is very little data of this type available on Be-type stars. In addition, so little is known about the morphology of Be-type stars, that any and all data collected may be useful.

More on Be-type Stars

R and N type stars

A number of giant stars appear to be K or M type stars, but also show significant excess spectral features of carbon compounds. They are often referred to as "carbon stars" and many astronomers collectively refer to them as C type stars. The most common spectral features are from C₂, CN, and CH. The abundance of carbon to oxygen in these stars is four to five times higher than in normal stars. The presence of these carbon compounds will tend to absorb the blue portion of the spectrum, giving R and N type giants a distinctive red colour. R stars are those with hotter surfaces which otherwise more closely resemble K type stars. S type stars have cooler surfaces and more closely resemble M stars.

Carbon Stars

A primordial complex of organic chemicals that could be the precursors of life are created soon after the birth of stars, new research suggests.^77b The findings indicate that life may have an easier time forming than was first speculated. A study by the Infrared Space Observatory showed that large organic molecules evolve within a few thousand years from chemicals in the gaseous envelope that surrounds some stars.

This conclusion is based on infrared spectra data from short-lived, carbon-rich stars that are engulfed in clouds of gas and dust. These clouds are rich in some of the most advanced organic molecules ever detected in outer space. Such chemicals would eventually be ejected into interstellar space, which makes it possible that they could end up on planets such as Earth.

Among the chemicals detected was acetylene, a building block for benzene and other aromatic molecules that can form complex hydrocarbons. There may be a process that allows amino acids to be manufactured around stars, but this molecule cannot be detected by the current generation of space telescopes.

S type stars

S type stars have photospheres with enhanced abundances of s-process elements. These are isotopes of elements which have been formed from the capture of a free neutron (changing the isotope of the element) followed by a beta decay (a neutron decays into a proton and an electron, thus changing the element to one with a higher atomic number and an isotope with one less neutron). The s-process is one of the mechanisms by which elements with atomic numbers higher than 56 (Iron) can be made. The s stands for slow. By way of contrast, its partner r-process (for rapid) takes place when there are a sufficient supply of free neutrons for additional neutrons to be acquired in the atomic nucleus before the captured neutron has a chance to beta decay. Instead of (or in addition to) the usual lines of titanium, scandium, and vanadium oxides characteristic of M type giants, S type stars show heavier elements such as zirconium, yttrium, and barium. A significant fraction of all S type stars are variable.

Peculiar Stars

Wolf-Rayet Stars (W)

Wolf-Rayet stars are similar to O type stars, but have broad emission lines of hydrogen and ionized helium, carbon, nitrogen, and oxygen with very few absorption lines. Current theory holds that these stars exist in binary systems where the companion star has stripped away the Wolf-Rayet star's outer layers. Thus the spectra observed is from the exposed stellar interior rather than the normal surface material. The broadness of the lines also indicates that the material observed may be from high velocity gases streaming away from the star, with the range of velocities smearing out the observed lines.

T Tauri Stars (T)

T Tauri stars are very young stars, typically found in bright or dark interstellar clouds from which they have presumably just formed. Typically T Tauri stars are irregular variable stars, with unpredictable changes in their brightness. Their spectra contains bright emission lines and a number of "forbidden lines" (so-called because they are not observable in typical laboratory conditions) which indicate extremely low densities. Spectral lines also show Doppler shifts with respect to the rest velocity of the star, indicating that matter is streaming out from them.

^{The Hertzsprung-Russell Diagram}

This Hertzsprung-Russell diagram compares SPECTRAL CLASS (the color) of stars to their ABSOLUTE LUMINOSITY and their ABSOLUTE VISUAL MAGNITUDE (intrinsic brightness). This diagram may be drawn for any series of stars located in a particular star field, and may also be drawn to represent a composite of all stars for a given criteria. There may be a transcribed H-R diagram, in which the luminosity (in terms of SOLAR luminosity - a comparison given by the equation L_abs = L_obs / L) is compared to the Absolute Luminosity and the Effective Temperature of a series of stars. The "ZAMS" line is the Zero-Age Main Sequence line where stars in their prime (Main Sequence - like our sun) tend to congregate. Please note: This is a coarse drawing of an H-R diagram, and is not absolutely accurate - it is for the purpose of showing the reader what a typical H-R diagram looks like, and what may be represented in such a diagram.

Hubble Constant

* - On December 14th Father Angelo Secchi (1818-1878), official astronomer to Pope Pius IX, conducted the first analysis of starlight with a spectroscope

101 - This work was done originally by E.C. Pickering (Director of Harvard College Observatory), with the assistance of Williamina P.S. Fleming, Antonia C. Maury, and Annie J. Cannon.