Star types are interesting after SAFIRE observed element transmutation.
SAFIRE can disturb the sequence of star types when some are based on specific elements for the type.
excerpts are from Wikipedia:
My remarks are among the excerpts.
remark: first, the noted elements by atomic number: H-1,He-2, C-6,N-7,O-8,Ne-10,Na-11,Mg-12,Al-13,Si-14,S-16,Ar-18,Ca-20,Ti-22,V-23,Cr-24,Mn-25,Fe-26,Ni-28.
remark: second, the words early vs late confusion:
Stars are often referred to as early or late types. "Early" is a synonym for hotter, while "late" is a synonym for cooler.
Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, and K3.
"Late" is used in the same way, with an unqualified use of the term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9.
In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number.
This obscure terminology is a hold-over from an early 20th century model of stellar evolution, which supposed that stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism, which is now known to not apply to main sequence stars. If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed in the geologic record, and was rendered obsolete by the discovery that stars are powered by nuclear fusion. The terms "early" and "late" were carried over, beyond the demise of the model they were based on.
remark to above: 'The terms were carried beyond the demise of their model.' This is deliberate confusion for their meaning!
remark: third, list of star types by temperature
the 'Modern Classification System' is primarily by decreasing temperature.
type.. temp.. fraction of population
O ≥ 30,000 K. . 0.00003%
B 10,000–30,000 K . . 0.13%
A 7,500–10,000 K . . 0.6%
F 6,000–7,500 K . . 3%
G 5,200–6,000 K. . 7.6%
K 3,700–5,200 K . . 12.1%
M 2,400–3,700 K . . 76.45%
remark: fourth, star types and definition. Types are in the order from O to M and beyond
O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.
remark: 'complicated surroundings' means 'cannot explain'
O-type 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.
remark: 'sometimes' means 'inconsistent'
remark: He II emission line is probably an alpha particle capturing an electron becoming a new helium ion.
remark: here 'later must mean the intensity of lines get stronger from O5 to O9 the quantities of elements change while the O temperature remains the same.
B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at the B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time.
The transition from class O to class B was originally defined to be the point at which the He II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today, the B-class is instead defined by the intensity of the He I violet spectrum, with the maximum intensity corresponding to class B2. For supergiants, lines of silicon are used instead; the Si IV λ4089 and Si III λ4552 lines are indicative of early B. At mid B, the intensity of the latter relative to that of Si II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg II λ4481 relative to that of He I λ4471.
These stars tend to be found in their originating OB associations, which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of the Milky Way and contains many of the brighter stars of the constellation Orion.
remark: the inconsistency of certain elements leads to 'early or 'late' types.
A-type stars are among 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.
remark: Quantity of calcium is noted as increasing at this class A temperature which is lower temp than class B.
F-type stars have strengthening spectral lines H and K 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.
remark: Quantity of certain metals are increasing at this class F temperature which is lower temp than class A.
G-type stars, including the Sun, have prominent spectral lines of Ca II, which are most pronounced 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 CH molecules.
remark: Quantity of Calcium is greatest at G2 temperature. Both ionized and neutral metals appear at class G temperature. A 'spike in CH molecule' quantities, but this G is cooler than F.
[K-type stars] have extremely weak hydrogen lines, if those are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. Mainstream theories (those rooted in lower harmful radioactivity and star longevity) would thus suggest such stars have the optimal chances of heavily-evolved life developing on orbiting planets (if such life is directly analogous to earth's) due to a broad habitable zone yet much lower harmful periods of emission compared to those with the broadest such zones.
remark: 'lower harmful radioactivity' probably means less ultraviolet coming from K-type being cooler than G-type.
Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VV Cephei, Antares, and Betelgeuse, which are also class M. Furthermore, the larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.
The spectrum of a class M star contains lines from oxide molecules (in the visible spectrum, especially TiO) and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum by about M5. Vanadium II oxide bands become present by late M.
remark: the temperature of M-type includes red dwarfs to 'the largest ever super giants' for quite the diversity. M-type is cooler than K-type and has heavier metals like titanium and vanadium.
[class W: Wolf-Rayet] were once included as type O stars; the Wolf-Rayet stars of class W or WR are notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon, and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).
remark: W-type were once included with the hottest O-type. Now there are many subclasses. I recall Robitaille describing WR stars as misunderstood by cosmologists.
remark: A number of special classes follow...
remark: Slash stars for a combination of O and WR types.
The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visible spectrum.
Brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types.
remark: there are even stars barely visible but still have a infrared spectrum.
remark: perhaps any object in the universe whose temperature is above absolute zero could reveal its temperature near the infrared but this confuses a very cold star or very cold matter. These final classes of stars do not seem to be a star.
A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
remark: the word 'luminous' is apparently flexible in the last star types.
The star types associate surface temperature with specific elements on the surface. As the temperature decreases through the types the new elements being observed are increasing in atomic number.
There are no direct connections between just a temperature range and the elements expected.
remark: There are also stellar associations.
Young associations will contain 10–100 massive stars of spectral class O and B, and are known as OB associations. These are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in the Milky Way were formed in OB associations.
Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.
The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun.
These loose clusters are relevant to SAFIRE. In their common local environment, the stars appear to get the same mix of metals.
An explanation is required for formation of individual stars, so they form either a) as unique stars or b) together as a similar type and their individual evolution results in the final types.
I expect cosmologists expect (a) but SAFIRE enables (b).
The M-type has the most stars while ranging from red dwarf to red super giant; this cool type also has the heavier metals.
The Gaia mission will create a precise three-dimensional map of astronomical objects throughout the Milky Way and map their motions, which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements will provide the detailed physical properties of all stars observed, characterizing their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to analyze a wide range of important questions related to the origin, structure, and evolutionary history of our galaxy.
The data are out there but further analysis for this post is out of scope.
Since the Sun is assumed to take millions of years for one orbit. If that 'history' uses dark matter then it is debatable.
Here in one topic (for my reference or others) are the definitions of the current star types.