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In
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, stellar classification is the classification of
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s based on their spectral characteristics.
Electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

Electromagnetic radiation
from the star is analyzed by splitting it with a
prism A prism An optical prism is a transparent optics, optical element with flat, polished surfaces that refraction, refract light. At least one surface must be angled—elements with two parallel surfaces are not prisms. The traditional geometrical ...

prism
or
diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffraction, diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form ...

diffraction grating
into a
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

spectrum
exhibiting the
rainbow A rainbow is a meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting Weather forecasting is the application of sc ...
of colors interspersed with
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spect ...
s. Each line indicates a particular
chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo du ...
or
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient wor ...
, although in some cases there are true abundance differences. The ''spectral class'' of a star is a short code primarily summarizing the
ionization Ionization or ionisation is the process by which an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
state, giving an objective measure of the photosphere's temperature. Most stars are currently classified under the Morgan–Keenan (MK) system using the letters ''O'', ''B'', ''A'', ''F'', ''G'', ''K'', and ''M'', a sequence from the hottest (''O'' type) to the coolest (''M'' type). Each letter class is then subdivided using a numeric digit with ''0'' being hottest and ''9'' being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class ''D'' for
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

white dwarf
s and classes ''S'' and ''C'' for
carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose Stellar atmosphere, atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, ...
s. In the MK system, a luminosity class is added to the spectral class using
Roman numerals Roman numerals are a that originated in and remained the usual way of writing numbers throughout Europe well into the . Numbers in this system are represented by combinations of letters from the . Modern style uses seven symbols, each with a ...
. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class ''0'' or ''Ia+'' is used for ''
hypergiant A hypergiant ( luminosity class 0 or Ia+) is a very rare type of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, ...
s'', class ''I'' for ''
supergiant Supergiants are among the most massive and most luminous star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest ...
s'', class ''II'' for bright ''
giants Giant or Giants may refer to: Mythology and religion *Giant **Giants (Greek mythology) **Giants (Norse mythology) **Giants (Welsh folklore) **Giants (esotericism) **Nephilim, a Hebrew term loosely translated as giants in some Bibles Arts, ente ...
'', class ''III'' for regular ''giants'', class ''IV'' for ''
subgiant A subgiant is a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other sta ...
s'', class ''V'' for ''
main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar Color index, color versus Absolute magnitude, brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams aft ...
stars'', class ''sd'' (or ''VI'') for ''
subdwarf A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the stellar classification#Yerkes spectral classification, Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 Absolute magnitude ...
s'', and class ''D'' (or ''VII'') for ''
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

white dwarf
s''. The full spectral class for the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.


Conventional colour description

The conventional colour description takes into account only the peak of the stellar spectrum. In actuality, however, stars radiate in all parts of the spectrum. Because all spectral colours combined appear white, the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest. This characteristic of 'lightness' indicates that the simplified assignment of colours within the spectrum can be misleading. Excluding colour-contrast effects in dim light, in typical viewing conditions there are no green, indigo, or violet stars.
Red dwarf A red dwarf is the smallest and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs c ...
s are a deep shade of orange, and
brown dwarf A brown dwarf is a type of substellar object that has a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 Jupiter mass, times that of Jupiter (). Unlike main sequence stars, brown dwarfs do not a ...

brown dwarf
s do not literally appear brown, but hypothetically would appear dim grey to a nearby observer.


Modern classification

The modern classification system is known as the ''Morgan–Keenan'' (MK) classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems, such as the
UBV system The photometric system called UBV (from Ultraviolet, Blue, Visual), also called the Harold Johnson (astronomer), Johnson system (or Johnson-Morgan system), is a wide central band of the electromagnetic spectrum, and usually employed for Stellar cla ...
, are based on color indices—the measured differences in three or more color magnitudes. Those numbers are given labels such as "U−V" or "B−V", which represent the colors passed by two standard filters (e.g. ''U''ltraviolet, ''B''lue and ''V''isual).


Harvard spectral classification

The ''Harvard system'' is a one-dimensional classification scheme by astronomer
Annie Jump Cannon Annie Jump Cannon (; December 11, 1863 – April 13, 1941) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astro ...
, who re-ordered and simplified the prior alphabetical system by Draper (see next paragraph). Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions. Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 
K
K
, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest. The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by
Arabic numerals Arabic numerals are the ten numerical digit A numerical digit (often shortened to just digit) is a single symbol used alone (such as "2") or in combinations (such as "25"), to represent numbers in a Positional notation, positional numeral sy ...

Arabic numerals
(0–9), where 0 denotes the hottest stars of a given class. For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star
Mu Normae μ Normae, Latinised as Mu Normae, is a blue supergiant star of spectral type O9.7 Iab, located in the constellation of Norma (constellation), Norma. It shines as bright as 339,000 suns and is weighs 40 solar masses. It varies in visual ...
is classified as O9.7. The
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
is classified as G2. Conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. The fact that the Harvard classification of a star indicated its surface or photospheric
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
(or more precisely, its
effective temperatureThe effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physik ...
) was not fully understood until after its development, though by the time the first
Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The ...
was formulated (by 1914), this was generally suspected to be true. In the 1920s, the Indian physicist
Meghnad Saha Meghnad Saha FRS (6 October 1893 – 16 February 1956) was an Indian astrophysicist who developed the Saha ionization equation, used to describe chemical and physical conditions in stars. His work allowed astronomers to accurately relate the ...
derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere, then to stellar spectra. Harvard astronomer Cecilia Payne then demonstrated that the ''O-B-A-F-G-K-M'' spectral sequence is actually a sequence in temperature. Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals.


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 William Wilson Morgan (January 3, 1906 – June 21, 1994) was an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of Ameri ...
, Philip C. Keenan, and
Edith Kellman Edith Kellman (April 4, 1911, Walworth, Wisconsin – May 11, 2007, Walworth, Wisconsin) was a noted American astronomer who is known for her work on the Yerkes system of stellar classification, also called the MKK system. Early life and education ...
from
Yerkes Observatory Yerkes Observatory ( ) is an Observatory#Astronomical observatories, astronomical observatory located in Williams Bay, Wisconsin, U.S.A. It was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in ...

Yerkes Observatory
. This two-dimensional (
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
and
luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

luminosity
) classification scheme is based on
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spect ...
s sensitive to stellar temperature and
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration In mechanics ...
, which is related to luminosity (whilst the ''Harvard classification'' is based on just surface temperature). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the ''Morgan–Keenan classification'', or ''MK'', and this system remains in use. Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a
giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...
is much lower than for a
dwarf star A dwarf star is a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other s ...
because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as ''luminosity effects'' and a luminosity class can be assigned purely from examination of the spectrum. A number of different ''luminosity classes'' are distinguished, as listed in the table below. Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: *A slash (/) means that a star is either one class or the other. *A dash (-) means that the star is in between the two classes. For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant. Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than the main sequence). Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs. Occasionally, letters ''a'' and ''b'' are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given a luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant. A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the ''Vz'' designation. An example star is HD 93129 B.


Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example,
59 Cygni 59 Cygni is a multiple star system in the northern constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythologic ...
is listed as spectral type B1.5Vnne, indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.


History

The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved.


Secchi classes

During the 1860s and 1870s, pioneering stellar spectroscopist
Angelo Secchi Angelo Secchi (; 28 June 1818 – 26 February 1878) was an Italian Catholic priest, astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They ...

Angelo Secchi
created the ''Secchi classes'' in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra, shown in the table below. In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article. The
Roman numerals Roman numerals are a that originated in and remained the usual way of writing numbers throughout Europe well into the . Numbers in this system are represented by combinations of letters from the . Modern style uses seven symbols, each with a ...
used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and the proposed neutron star classes.


Draper system

In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the
Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Harvard University is a Private university, private Ivy League research ...
, using the objective-prism method. A first result of this work was the ''Draper Catalogue of Stellar Spectra'', published in 1890.
Williamina Fleming (15 May 1857 – 21 May 1911) was a Scottish astronomer active in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily l ...
classified most of the spectra in this catalogue and was credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars. With the help of the
Harvard computers The Harvard Computers was a team of women working as skilled worker A skilled worker is any Labour (economics), worker who has special skill (labor), skill, training, knowledge which they can then apply to their employment, work. A skilled worker ...
, especially
Williamina Fleming (15 May 1857 – 21 May 1911) was a Scottish astronomer active in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily l ...
, the first iteration of the Henry Draper catalogue was devised to replace the Roman-numeral scheme established by Angelo Secchi. The catalogue used a scheme in which the previously used Secchi classes (I to V) were subdivided into more specific classes, given letters from A to P. Also, the letter Q was used for stars not fitting into any other class.pp. 106–108, Hearnshaw 1986. Fleming worked with Pickering to differentiate 17 different classes based on the intensity of hydrogen spectral lines, which causes variation in the wavelengths emanated from stars and results in variation in color appearance. The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines. The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down the alphabet. This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme.


The old Harvard system (1897)

In 1897, another astronomer at Harvard,
Antonia Maury Antonia Caetana de Paiva Pereira Maury (March 21, 1866 – January 8, 1952) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth ...

Antonia Maury
, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of twenty-two types numbered from I–XXII. : Because the 22 Roman numeral groupings didn't account for additional variations in spectra, three additional divisions were made to further specify differences: Lowercase letters were added to differentiate relative line appearance in spectra; the lines were defined as

: Antonia Maury published her own stellar classification catalogue in 1897 called "Spectra of Bright Stars Photographed with the 11 inch Draper Telescope as Part of the Henry Draper Memorial", which included 4,800 photographs and Maury's analyses of 681 bright northern stars. This was the first instance in which a woman was credited for an observatory publication.


The current Harvard system (1912)

In 1901,
Annie Jump Cannon Annie Jump Cannon (; December 11, 1863 – April 13, 1941) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astro ...
returned to the lettered types, but dropped all letters except O, B, A, F, G, K, M, and N used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one fifth of the way from F to G, and so on. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectra. A common
mnemonic A mnemonic () device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the ...

mnemonic
for remembering the order of the spectral type letters, from hottest to coolest, is "Oh, Be A Fine Guy/Girl: Kiss Me!".


Mount Wilson classes

A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities. This notation system is still sometimes seen on modern spectra.


Spectral types

The stellar classification system is
taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
, based on
type specimens In biology, a type is a particular wiktionary:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anc ...
, similar to classification of species in
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
: The categories are defined by one or more standard stars for each category and sub-category, with an associated description of the distinguishing features.


"Early" and "late" nomenclature

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, K2 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 a late nineteenth century model of
stellar evolution and giant In folklore Folklore is the expressive body of culture shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. These include oral traditions such as Narrative, tales, p ...

stellar evolution
, which supposed that stars were powered by gravitational contraction via the
Kelvin–Helmholtz mechanism The Kelvin–Helmholtz mechanism is an astronomy, astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the internal pressure to drop, and the star or planet shrinks as a result. This compression, in turn ...
, which is now known to not apply to
main-sequence star In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...
s. 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 The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
that were much smaller than what is observed in the
geologic record The geologic record in stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary ...
, and was rendered obsolete by the discovery that stars are powered by
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products ...

nuclear fusion
. The terms "early" and "late" were carried over, beyond the demise of the model they were based on.


Class O

O-type stars are very hot and extremely luminous, with most of their radiated output in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars.These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class. 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. O-type spectra formerly were defined by the ratio of the strength of the  II λ4541 relative to that of He I λ4471, where λ is the radiation
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He I line weakening towards earlier types. Type O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this, the modern definition uses the ratio of the
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
line N IV λ4058 to N III λλ4634-40-42. O-type stars have dominant lines of absorption and sometimes emission for  II lines, prominent ionized ( IV,  III,  III, and  III) and neutral
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
lines, strengthening from O5 to O9, and prominent hydrogen
Balmer lines The Balmer series, or Balmer lines in atomic physics, is one of a set of hydrogen spectral series, six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirica ...
, although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to the extreme velocity of their
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
, which may reach 2,000 km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the
main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar Color index, color versus Absolute magnitude, brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams afte ...
. 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 O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced. Spectral standards: * O7V –
S Monocerotis S Monocerotis, also known as 15 Monocerotis, is a massive multiple star, multiple and variable star system located in the constellation Monoceros (constellation), Monoceros. It is the brightest star in the Christmas Tree cluster, Christ ...
* O9V –
10 Lacertae 10 Lacertae (10 Lac) is a star in the constellation Lacerta (constellation), Lacerta. With an apparent magnitude of 4.9, it is located around distant in the small Lacerta OB1 Stellar association, association. It is a hot O-type main-sequ ...


Class B

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. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from
runaway star In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
s. The transition from class O to class B was originally defined to be the point at which the  II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, 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 Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

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 association In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky Wa ...
s, which are associated with giant
molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
s. The Orion OB1 association occupies a large portion of a
spiral arm Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
and contains many of the brighter stars of the . About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are
B-type main-sequence star A B-type main-sequence star (B V) is a main-sequence (hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the period ...
s. Massive yet non-
supergiant Supergiants are among the most massive and most luminous star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest ...
entities known as "Be stars" are main-sequence stars that notably have, or had at some time, one or more
Balmer lines The Balmer series, or Balmer lines in atomic physics, is one of a set of hydrogen spectral series, six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirica ...
in emission, with the
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
-related
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
series Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used i ...
projected out by the stars being of particular interest. Be stars are generally thought to feature unusually strong
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
s, high surface temperatures, and significant attrition of
stellar massStellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (). Hence, the bright star Sirius has around . A star's mass will vary over its l ...
as the objects
rotate A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line extending from the center an ...
at a curiously rapid rate. Objects known as "B(e)" or "B stars possess distinctive neutral or low ionisation
emission lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spect ...
that are considered to have '
forbidden mechanismIn spectroscopy Spectroscopy is the study of the interaction between matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
s', undergoing processes not normally allowed under current understandings of
quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with ...
. Spectral standards: * B0V – Upsilon Orionis * B0Ia –
Alnilam Alnilam , designation ε Orionis, (Latinisation of names, Latinized to Epsilon Orionis, abbreviated Epsilon Ori, ε Ori) and 46 Orionis (46 Ori), is a large blue supergiant star some 2,000 light-years distant in the constellation of Or ...

Alnilam
* B2Ia – Chi2 Orionis * B2Ib – 9 Cephei * B3V – * B3V – * B3Ia – Omicron2 Canis Majoris * B5Ia –
Eta Canis Majoris Eta Canis Majoris (η Canis Majoris, abbreviated Eta CMa, η CMa), also named Aludra , is a star in the constellation of Canis Major. Since 1943, the stellar spectrum, spectrum of this star has served as one of the stable anchor points by ...
* B8Ia –
Rigel Rigel, designated β Orionis (Latinized Latinisation or Latinization can refer to: * Latinisation of names, the practice of rendering a non-Latin name in a Latin style * Latinisation in the Soviet Union, the campaign in the USSR dur ...

Rigel


Class A

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 ( II,  II,  II) at a maximum at A5. The presence of  II lines is notably strengthening by this point. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars. Spectral standards: * A0Van – Gamma Ursae Majoris * A0Va –
Vega Vega is the brightest star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun ...

Vega
* A0Ib –
Eta Leonis Eta Leonis (η Leo, η Leonis) is a fourth-magnitude star in the constellation Leo (constellation), Leo, about away. Properties Eta Leonis is a white supergiant with the stellar classification A0Ib. Since 1943, the stellar spectrum, ...
* A0Ia – HD 21389 * A1V –
Sirius A Sirius () is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word (, 'glowing' or 'scorching'). The star is designated α Canis Majoris, Latinisation of name ...

Sirius A
* A2Ia –
Deneb Deneb () is a first-magnitude star in the constellation of Cygnus (constellation), Cygnus, the swan. Deneb is one of the vertices of the Asterism (astronomy), asterism known as the Summer Triangle and the "head" of the Northern Cross (as ...

Deneb
* A3Va –
Fomalhaut Fomalhaut , designation Alpha Piscis Austrini (α Piscis Austrini, abbreviated Alpha PsA, α PsA) is the brightest star in the constellation A constellation is an area on the celestial sphere in which a group of ...

Fomalhaut


Class F

F-type stars have strengthening spectral lines ''H'' and ''K'' of  II. Neutral metals ( I,  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 (3.03%) of the main-sequence stars in the solar neighborhood are F-type stars. Spectral standards: * F0IIIa –
Zeta Leonis , caption=Location of ζ Leonis (circled) Zeta Leonis (ζ Leonis, abbreviated Zeta Leo, ζ Leo), also named Adhafera , is a third-magnitude star in the constellation of Leo (constellation), Leo, the lion. It forms the second star (after ...
* F0Ib –
Alpha Leporis Alpha Leporis (α Leporis, abbreviated Alpha Lep, α Lep), formally named Arneb , is the brightest star in the constellation of Lepus constellation, Lepus. Nomenclature ''Alpha Leporis'' is the star's Bayer designation. The traditional ...
* F2V – 78 Ursae Majoris


Class G

G-type stars, including the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, have prominent spectral lines ''H'' and ''K'' of  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 the G band of molecules. Class G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood. Class G contains 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 unstable
yellow supergiant A yellow supergiant (YSG) is a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. M ...
class. Spectral standards: * G0V –
Beta Canum Venaticorum Beta Canum Venaticorum (β Canum Venaticorum, abbreviated Beta CVn, β CVn), also named Chara , is a G-type main-sequence star in the northern constellation of Canes Venatici. At an apparent visual magnitude of 4.26, it is the second-bright ...
* G0IV – Eta Boötis * G0Ib –
Beta Aquarii Beta Aquarii (β Aquarii, abbreviated Beta Aqr, β Aqr) is a star in the constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing ...
* G2V –
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
* G5V – Kappa1 Ceti * G5IV –
Mu Herculis Mu Herculis (μ Herculis) is a nearby star system about 27.1  light years from Earth in the constellation Hercules (constellation), Hercules. Its main star, Mu Herculis A is fairly similar to the Sun although more highly stellar e ...

Mu Herculis
* G5Ib – 9 Pegasi * G8V – 61 Ursae Majoris * G8IV – Beta Aquilae * G8IIIa – Kappa Geminorum * G8IIIab – Epsilon Virginis * G8Ib – Epsilon Geminorum


Class K

K-type stars are orangish stars that are slightly cooler than the Sun. They make up about 12% of the main-sequence stars in the solar neighborhood. There are also giant K-type stars, which range from
hypergiant A hypergiant ( luminosity class 0 or Ia+) is a very rare type of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, ...
s like RW Cephei, to
giants Giant or Giants may refer to: Mythology and religion *Giant **Giants (Greek mythology) **Giants (Norse mythology) **Giants (Welsh folklore) **Giants (esotericism) **Nephilim, a Hebrew term loosely translated as giants in some Bibles Arts, ente ...
and
supergiant Supergiants are among the most massive and most luminous star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest ...
s, such as Arcturus, whereas K-type main-sequence star, orange dwarfs, like Alpha Centauri B, are main-sequence stars. They have extremely weak hydrogen lines, if those are present at all, and mostly neutral metals (manganese, Mn I,  I,  I). By late K, molecular bands of titanium monoxide, 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. Spectral standards: * K0V – Sigma Draconis * K0III – Pollux (star), Pollux * K0III – Epsilon Cygni * K2V – Epsilon Eridani * K2III – Kappa Ophiuchi * K3III – Rho Boötis * K5V – 61 Cygni, 61 Cygni A * K5III – Gamma Draconis


Class M

Class M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars.This rises to 78.6% if we include all stars. (See the above note.) However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest-known M class main-sequence star is Lacaille 8760, class M0V, with stellar magnitude, magnitude 6.7 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found. 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 dwarf A brown dwarf is a type of substellar object that has a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 Jupiter mass, times that of Jupiter (). Unlike main sequence stars, brown dwarfs do not a ...

brown dwarf
s 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
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
s (in the visible spectrum, especially Titanium(II) oxide, 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. Spectral standards: * M0IIIa – Beta Andromedae * M2III – Chi Pegasi * M1-M2Ia-Iab – Betelgeuse * M2Ia – Mu Cephei ("William Herschel, Herschel’s garnet")


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

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 wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
s, 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). WR spectra range is listed below: * WN – spectrum dominated by N III-V and He I-II lines ** WNE (WN2 to WN5 with some WN6) – hotter or "early" ** WNL (WN7 to WN9 with some WN6) – cooler or "late" ** Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars ** h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption * WN/C – WN stars plus strong C IV lines, intermediate between WN and WC stars * WC – spectrum with strong C II-IV lines ** WCE (WC4 to WC6) – hotter or "early" ** WCL (WC7 to WC9) – cooler or "late" * WO (WO1 to WO4) – strong O VI lines, extremely rare, extension of the WCE class into incredibly hot temperatures (up to 200 kK or more) Although the central stars of most planetary nebulae (CSPNe) show O-type spectra, around 10% are hydrogen-deficient and show WR spectra. These are low-mass stars and to distinguish them from the massive Wolf–Rayet stars, their spectra are enclosed in square brackets: e.g. [WC]. Most of these show [WC] spectra, some [WO], and very rarely [WN].


The "Slash" stars

The ''slash'' stars are O-type stars with WN-like lines in their spectra. The name "slash" comes from their printed spectral type having a slash in it (e.g. "Of/WNL"). There is a secondary group found with this spectra, a cooler, "intermediate" group designated "Ofpe/WN9". These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference from other Wolf–Rayet stars. Recent discoveries of even rarer stars have extended the range of slash stars as far as O2-3.5If*/WN5-7, which are even hotter than the original "slash" stars.


The magnetic O stars

They are O stars with strong magnetic fields. Designation is Of?p.


Cool red and brown dwarf classes

The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and
brown dwarf A brown dwarf is a type of substellar object that has a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 Jupiter mass, times that of Jupiter (). Unlike main sequence stars, brown dwarfs do not a ...

brown dwarf
s that are very faint in the visible spectrum. Brown dwarfs, stars that do not undergo hydrogen fusion, cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes, faster the less massive they are; the highest-mass brown dwarfs cannot have cooled to Y or even T dwarfs within the age of the universe. Because this leads to an unresolvable overlap between spectral types
effective temperatureThe effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physik ...
and
luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

luminosity
for some masses and ages of different L-T-Y types, no distinct effective temperature, temperature or
luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

luminosity
values can be given.


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. Some of these objects have masses large enough to support hydrogen fusion and are therefore stars, but most are of substellar object, substellar mass and are therefore brown dwarfs. 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 surface gravity in giant stars, Titanium monoxide, TiO- and Vanadium monoxide, VO-bearing condensates never form. Thus, L-type stars larger than dwarfs can never form in an isolated environment. However, it may be possible for these L-type supergiants to form through stellar collisions, an example of which is V838 Monocerotis while in the height of its luminous red nova eruption.


Class T: methane dwarfs

Class T dwarfs are cool
brown dwarf A brown dwarf is a type of substellar object that has a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 Jupiter mass, times that of Jupiter (). Unlike main sequence stars, brown dwarfs do not a ...

brown dwarf
s with surface temperatures between approximately . Their emission peaks in the infrared. Methane is prominent in their spectra. Study of the number of protoplanetary disk, proplyds (protoplanetary disks, clumps of gas in nebulae from which stars and planetary systems are formed) indicates that the number of stars in the galaxy should be several order of magnitude, orders of magnitude higher than what was previously conjectured. It is theorized that these proplyds are in a race with each other. The first one to form will become a protostar, 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 dwarfs of the L and T classes, which are quite invisible to us.


Class Y

Brown dwarfs of spectral class Y are cooler than those of spectral class T and have qualitatively different spectra from them. A total of 17 objects have been placed in class Y as of August 2013. Although such dwarfs have been modelled and detected within forty light-years by the Wide-field Infrared Survey Explorer (WISE) there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2. The spectra of these prospective Y objects display absorption around 1.55 micrometers. Delorme et al. have suggested that this feature is due to absorption from ammonia, and that this should be taken as the indicative feature for the T-Y transition. In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class. However, this feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature. The latest brown dwarf proposed for the Y spectral type, WISE 1828+2650, is a > Y2 dwarf with an effective temperature originally estimated around 300 , the temperature of the human body.European Southern Observatory
"A Very Cool Pair of Brown Dwarfs"
23 March 2011
Parallax measurements have, however, since shown that its luminosity is inconsistent with it being colder than ~400 K. The coolest Y dwarf currently known is WISE 0855−0714 with an approximate temperature of 250 K. The mass range for Y dwarfs is 9–25 Jupiter masses, but young objects might reach below one Jupiter mass, which means that Y class objects straddle the 13 Jupiter mass deuterium-fusion limit that marks the current International Astronomical Union, IAU division between brown dwarfs and planets.


Peculiar brown dwarfs

Young brown dwarfs have low Surface gravity, surface gravities because they have larger radii and lower masses compared to the field stars of similar spectral type. These sources are marked by a letter beta () for intermediate surface gravity and gamma () for low surface gravity. Indication for low surface gravity are weak CaH, K and Na lines, as well as strong VO line. Alpha () stands for normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta (). The suffix "pec" stands for peculiar. The peculiar suffix is still used for other features that are unusual and summarizes different properties, indicative of low surface gravity, subdwarfs and unresolved binaries. The prefix sd stands for
subdwarf A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the stellar classification#Yerkes spectral classification, Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 Absolute magnitude ...
and only includes cool subdwarfs. This prefix indicates a low metallicity and kinematic properties that are more similar to Galactic halo, halo stars than to Thin disk, disk stars. Subdwarfs appear bluer than disk objects. The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content. The blue suffix describes objects with blue near-infrared colors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as 2MASS J11263991−5003550 and are explained with thin and/or large-grained clouds.


Late giant carbon-star classes

Carbon-stars are stars whose spectra indicate production of carbon – a byproduct of Triple-alpha process, triple-alpha helium 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. Equivalent classes for carbon-rich stars are S and C. The giants among those stars are presumed to produce this carbon themselves, but some stars in this class are double stars, whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf, when the companion was a carbon-star.


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 C–J-type stars, which are characterized by the strong presence of molecules of Carbon-13, 13 in addition to those of Carbon-12, 12. A few main-sequence carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. There are several subclasses: * C-R – Formerly its own class (''R'') representing the carbon star equivalent of late G- to early K-type stars. * C-N – Formerly its own class representing the carbon star equivalent of late K- to M-type stars. * C-J – A subtype of cool C stars with a high content of Carbon-13, 13C. * C-H – Population II analogues of the C-R stars. * C-Hd – Hydrogen-deficient carbon stars, similar to late G supergiants with Methylidyne radical, CH and diatomic carbon, C2 bands added.


Class S

Class S stars form a continuum between class M stars and carbon stars. Those most similar to class M stars have strong Zirconium monoxide, ZrO spectral bands, absorption bands analogous to the Titanium monoxide, TiO bands of class M stars, whereas those most similar to carbon stars have strong sodium D lines and weak Diatomic carbon, C2 bands. Class S stars have excess amounts of zirconium and other elements produced by the s-process, and have more similar carbon and oxygen abundances than class M or carbon stars. Like carbon stars, nearly all known class S stars are asymptotic giant branch, asymptotic-giant-branch stars. The spectral type is formed by the letter S and a number between zero and ten. This number corresponds to the temperature of the star and approximately follows the temperature scale used for class M giants. The most common types are S3 to S5. The non-standard designation S10 has only been used for the star Chi Cygni when at an extreme minimum. The basic classification is usually followed by an abundance indication, following one of several schemes: S2,5; S2/5; S2 Zr4 Ti2; or S2*5. A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO. A number following a slash is a more-recent but less-common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10, where a 0 would be an MS star. Intensities of zirconium and titanium may be indicated explicitly. Also occasionally seen is a number following an asterisk, which represents the strength of the ZrO bands on a scale from 1 to 5.


Classes MS and SC: Intermediate carbon-related classes

In between the M and S classes, border cases are named MS stars. In a similar way, border cases between the S and C-N classes are named SC or CS. The sequence M → MS → S → SC → C-N is hypothesized to be a sequence of increased carbon abundance with age for
carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose Stellar atmosphere, atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, ...
s in the asymptotic giant branch.


White dwarf classifications

The class D (for Electron-degenerate matter, Degenerate) is the modern classification used for white dwarfs—low-mass stars that are no longer undergoing
nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products ...

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 Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
-rich atmosphere or outer layer, indicated by strong Balmer hydrogen
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spect ...
s. * DB – a
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
-rich atmosphere, indicated by neutral helium, spectroscopic notation, He I, spectral lines. * DO – a helium-rich atmosphere, indicated by ionized helium, spectroscopic notation, He II, spectral lines. * DQ – a carbon-rich atmosphere, indicated by atomic or molecular carbon lines. * DZ – a metal (astronomy), 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/''T''eff, where ''T''eff is the effective temperature, 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 that displays more than one of the spectral features above.


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 A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars:


Non-stellar spectral types: Classes P and Q

Finally, the classes P and Q are left over from the system developed by Annie Jump Cannon, Cannon for the ''Henry Draper Catalogue''. They are occasionally used for certain non-stellar objects: Type P objects are stars within planetary nebulae (typically young white dwarfs or hydrogen-poor M giants); type Q objects are novae.


Stellar remnants

Stellar remnants are objects associated with the death of stars. Included in the category are
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

white dwarf
s, and as can be seen from the radically different classification scheme for class D, non-stellar objects are difficult to fit into the MK system. The Hertzsprung–Russell diagram, which the MK system is based on, is observational in nature so these remnants cannot easily be plotted on the diagram, or cannot be placed at all. Old neutron stars are relatively small and cold, and would fall on the far right side of the diagram. Planetary nebulae are dynamic and tend to quickly fade in brightness as the progenitor star transitions to the white dwarf branch. If shown, a planetary nebula would be plotted to the right of the diagram's upper right quadrant. A black hole emits no visible light of its own, and therefore would not appear on the diagram. A classification system for neutron stars using Roman numerals has been proposed: type I for less massive neutron stars with low cooling rates, type II for more massive neutron stars with higher cooling rates, and a proposed type III for more massive neutron stars (possible exotic star candidates) with higher cooling rates. The more massive a neutron star is, the higher neutrino flux it carries. These neutrinos carry away so much heat energy that after only a few years the temperature of an isolated neutron star falls from the order of billions to only around a million Kelvin. This proposed neutron star classification system is not to be confused with the earlier Secchi spectral classes and the Yerkes luminosity classes.


Replaced spectral classes

Several spectral types, all previously used for non-standard stars in the mid-20th century, have been replaced during revisions of the stellar classification system. They may still be found in old editions of star catalogs: R and N have been subsumed into the new C class as C-R and C-N.


Stellar classification, habitability, and the search for life

While humans may eventually be able to colonize any kind of stellar habitat, this section will address the probability of life arising around other stars. Stability, luminosity, and lifespan are all factors in stellar habitability. We only know of one star that hosts life, and that is our own—a G-class star with an abundance of heavy elements and low variability in brightness. It is also unlike many stellar systems in that it only has one star in it (see Habitability of binary star systems). Working from these constraints and the problems of having an empirical sample set of only one, the range of stars that are predicted to be able to support life as we know it is limited by a few factors. Of the main-sequence star types, stars more massive than 1.5 times that of the Sun (spectral types O, B, and A) age too quickly for advanced life to develop (using Earth as a guideline). On the other extreme, dwarfs of less than half the mass of our Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see Habitability of red dwarf systems). While there are many problems facing life on red dwarfs, many astronomers continue to model these systems due to their sheer numbers and longevity. For these reasons NASA's Kepler Mission is searching for habitable planets at nearby main-sequence stars that are less massive than spectral type A but more massive than type M—making the most probable stars to host life dwarf stars of types F, G, and K.


See also

* * * * , survey of stars *


Explanatory notes


References


External links


Libraries of stellar spectra
by D. Montes, UCM
Spectral Types for Hipparcos Catalogue Entries


by Richard O. Gray and Christopher J. Corbally
Spectral models of stars
by P. Coelho *

{{DEFAULTSORT:Stellar Classification Stars by spectral type, Stars by luminosity class, Hertzsprung–Russell classifications, Stellar astronomy, Classification Star types, Concepts in astronomy