WB Chapter 1 - Stars I - Classification

The Morgan-Keenan spectral classification


The following illustration represents star classes with the colors very close to those actually perceived by the human eye. The relative sizes are for main sequence or "dwarf" stars.


Class O
Class O stars are very hot and very luminous, being bluish in color; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars. About 1 in 3,000,000 of the main sequence stars in the solar neighborhood are Class O stars. Some of the most massive stars lie within this spectral class. Type-O stars are so hot as to have complicated surroundings which make measurement of their spectra difficult.

O-stars shine with a power over a million times our Sun's output. These stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so massive, class O stars have very hot cores, thus burn through their hydrogen fuel very quickly, and so are the first stars to leave the main sequence. Recent observations by the Spitzer Space Telescope indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the photoevaporation effect.
When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O4 in 1978, and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5. O3 stars are the hottest stars.

Class B
Class B stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a very short time, and thus they do not stray far from the area in which they were formed.
These stars tend to cluster together in what are called OB associations, which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy and contains many of the brighter stars of the constellation Orion. About 1 in 800 of the main sequence stars in the solar neighborhood are Class B stars.

Class A
Class A stars are amongst the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 of the main sequence stars in the solar neighborhood are Class A stars.

Class F
Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 of the main sequence stars in the solar neighborhood are Class F stars

Class G
Class G stars are probably the best known, if only for the reason that our Sun is of this class. About 1 in 13 of the main sequence stars in the solar neighborhood are Class G stars.
Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.

Class K
Class K are orangish stars that are slightly cooler than our Sun. Some K stars are giants and supergiants, while orange dwarfs are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I).
By late K, molecular bands of titanium oxide become present. About 1 in 8 of the main sequence stars in the solar neighborhood are Class K stars. There is a suggestion that K Spectrum stars are very well suited for life.

Class M
Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants, as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to molecules and all neutral metals but hydrogen lines are usually absent. Titanium oxide can be strong in M stars, usually dominating by about M5. Vanadium oxide bands become present by late M.

Extended spectral types

A number of new spectral types have been taken into use from newly discovered types of stars.

Hot blue emission star classes

Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen.

Class W: Wolf-Rayet
Class W or WR represents the superluminous Wolf-Rayet stars, notably unusual since they have mostly helium in their atmospheres instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot stellar winds caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses WN (WNE early-type, WNL late-type) and WC (WCE early-type, WCL late-type, and extend class WO), according to the dominance of nitrogen and carbon emission lines in their spectra (and outer layers).

Cool red and brown dwarf classes
The new spectral types L and T were created to classify infrared spectra of cool stars. This included both red dwarfs and brown dwarfs which are very faint in the visual spectrum. The hypothetical spectral type Y has been reserved for objects cooler than T dwarfs which have spectra that are qualitatively distinct from T dwarfs.

Class L
Class L dwarfs get their designation because they are cooler than M stars and L is the remaining letter alphabetically closest to M. L does not mean lithium dwarf; a large fraction of these stars do not have lithium in their spectra. Some of these objects have masses large enough to support hydrogen fusion, but some are of substellar mass and do not, so collectively these objects should be referred to as L dwarfs, not L stars. They are a very dark red in color and brightest in infrared. Their atmosphere is cool enough to allow metal hydrides and alkali metals to be prominent in their spectra. Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, larger L-type stars can never form in an isolated environment. It may be possible for these L-type supergiants to form through stellar collisions.
They range between 1,300–2,000 K.

Class T: methane dwarfs
Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared. Methane is prominent in their spectra.
Class T and L could be more common than all the other classes combined if recent research is accurate. From studying the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed) then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It is theorized that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long, these smaller stars will accumulate over time.

Class Y
The spectral class Y has been proposed for brown dwarfs that are cooler than T dwarfs and have qualitatively different spectra from them. Although such dwarfs have been modelled, there is no well-defined spectral sequence yet with prototypes, and no certain example of class Y has yet been seen.
Usually below 600 K.
The coolest known brown dwarfs have estimated effective temperatures between 500 and 600 K, and have been assigned the spectral class T9.  The absolute coolest known brown dwarf  which has a surface temperature of 373K. The spectra of these objects display absorption around 1.55 micrometers. Delorme et al has suggested that this feature is due to absorption from ammonia and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature.

Carbon related late giant star classes
Carbon related stars are stars whose spectra indicate production of carbon by helium triple-alpha fusion. With increased carbon abundance, and some parallel s-process heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes G, K and M. The giants among those stars are presumed to produce this carbon themselves, but not too few of this class of stars are believed to be double stars whose odd atmosphere once was transferred from a former carbon star companion that is now a white dwarf.

Class C: carbon stars
Originally classified as R and N stars, these are also known as 'carbon stars'. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6. Another subset of cool carbon stars are the J-type stars, which are characterized by the strong presence of molecules of 13CN in addition to those of 12CN. A few dwarf (that is, main sequence) carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants.

Class S
Class S stars have zirconium oxide lines in addition to (or, rarely, instead of) those of titanium oxide, and are in between the Class M stars and the carbon stars. S stars have excess amounts of zirconium and other elements produced by the s-process, and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both carbon and oxygen being locked up almost entirely in carbon monoxide molecules. For stars cool enough for carbon monoxide to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form titanium oxide) in stars of normal composition, "leftover carbon" (which becomes available to form the diatomic carbon molecules) in carbon stars, and "leftover nothing" in the S stars. The relation between these stars and the ordinary M stars indicates a continuum of carbon abundance. Like carbon stars, nearly all known S stars are giants or supergiants.

Classes MS and SC: intermediary carbon related classes
In between the M class and the S class, border cases are named MS stars. In a similar way border cases between the S class and the C-N class are named SC or CS. The sequence M → MS → S → SC → C-N is believed to be a sequence of increased carbon abundance with age for carbon stars in the asymptotic giant branch.

White dwarf classifications
The class D (for Degenerate) is the modern classification used for white dwarfs, low-mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down. Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere.
The white dwarf types are as follows:

• DA: a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.
• DB: a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines.
• DO: a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines.
• DQ: a carbon-rich atmosphere, indicated by atomic or molecular carbon lines.
• DZ: a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM).
• DC: no strong spectral lines indicating one of the above categories.
• DX: spectral lines are insufficiently clear to classify into one of the above categories.

The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/Teff, where Teff is the effective surface temperature, measured in kelvins. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.
Two or more of the type letters may be used to indicate a white dwarf which displays more than one of the spectral features above. Also, the letter V is used to indicate a variable white dwarf.

Extended white dwarf spectral types:

• DAB: a hydrogen- and helium-rich white dwarf displaying neutral helium lines.
• DAO: a hydrogen- and helium-rich white dwarf displaying ionized helium lines.
• DAZ: a hydrogen-rich metallic white dwarf.
• DBZ: a helium-rich metallic white dwarf.

Variable star designations:

• DAV or ZZ Ceti: a hydrogen-rich pulsating white dwarf.
• DBV or V777 Her: a helium-rich pulsating white dwarf.
• GW Vir, sometimes divided into DOV and PNNV: a hot helium-rich pulsating white dwarf (or pre-white dwarf.) 

Non-stellar spectral types: Class P & Q
Finally, the classes P and Q are occasionally used for certain non-stellar objects. Type P objects are planetary nebulae and type Q objects are novae.


Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.

Code	Spectral peculiarities for stars
:	Blending and/or uncertain spectral value
...	Undescribed spectral peculiarities exist
!	Special peculiarity
comp	Composite spectrum
e	Emission lines present
[e]	"Forbidden" emission lines present
er	"Reversed" center of emission lines weaker than edges
ep	Emission lines with peculiarity
eq	Emission lines with P Cygni profile
ev	Spectral emission that exhibits variability
f	N III and He II emission
f*	NIV is stronger than the NIII lines
f+	SiIV are emission in addition to the NIII line
(f)	Weak emission lines of He
((f))	Displays strong HeII absorption accompanied by weak NIII emissions
h	WR stars with emission lines due to hydrogen
ha	WR stars with hydrogen emissions seen on both absorption and emission
He wk	Weak He lines
k	Spectra with interstellar absorption features
m	Enhanced metal features
n	Broad ("nebulous") absorption due to spinning
nn	Very broad absorption features due to spinning very fast
neb	A nebula's spectrum mixed in
p	Unspecified peculiarity, peculiar star.
pq	Peculiar spectrum, similar to the spectra of novae
q	Red & blue shifts line present
s	Narrowly "sharp" absorption lines
ss	Very narrow lines
sh	Shell star features
v	Variable spectral feature (also "var")
w	Weak lines (also "wl" & "wk")
d Del	Type A and F giants with weak calcium H and K lines
d Sct	Type A and F stars
Code	If spectrum shows enhanced metal features
Ba	Abnormally strong Barium
Ca	Abnormally strong Calcium
Cr	Abnormally strong Chromium
Eu	Abnormally strong Europium
He	Abnormally strong Helium
Hg	Abnormally strong Mercury
Mn	Abnormally strong Manganese
Si	Abnormally strong Silicon
Sr	Abnormally strong Strontium
Tc	Abnormally strong Technetium
Code	Spectral peculiarities for white dwarfs
:	Uncertain assigned classification
P	Magnetic white dwarf with detectable polarization
E	Emission lines present
H	Magnetic white dwarf without detectable polarization
V	Variable
PEC	Spectral peculiarities exist

Yerkes spectral classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity.

Since the radius of a giant star is much greater than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.

A number of different luminosity classes are distinguished:

• 0 hypergiants
• I supergiants
• Ia (luminous supergiants)
• Ib (less luminous supergiants)
• II bright giants
• III normal giants
• IV subgiants
• V main sequence stars (dwarfs)
• VI subdwarfs. Subdwarfs are generally represented with a prescript of sd or esd (extreme subdwarf) in front of the spectra.
• VII (uncommon) white dwarfs. White dwarfs are represented with a prescript wD or WD.

Something to note would be that all this information will not be used to generate, without a great background in astronomy would make it hard to even try. So I will stick with O0Ia-M9VII. I may add in some exras just to keep it intersting but for now that is what it will be.