Stellar classification

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Classification of stars from O-M Dwarf Stars.png
Classification of stars from O-M

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or 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, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

Astronomy natural science that deals with the study of celestial objects

Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, which is the study of the Universe as a whole.

Star An astronomical object consisting of a luminous spheroid of plasma held together by its own gravity

A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023) stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

Electromagnetic radiation form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space

In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.


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 dwarfs and classes S and C for carbon stars.

White dwarf Type of stellar remnant composed mostly of electron-degenerate matter

A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf wherein mass is converted to energy. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922.

A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.

In the MK system, a luminosity class is added to the spectral class using Roman numerals. 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 hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd (or VI) for sub-dwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K.

Roman numerals Numbers in the Roman numeral system

The numeric system represented by Roman numerals originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers in this system are represented by combinations of letters from the Latin alphabet. Roman numerals, as used today, employ seven symbols, each with a fixed integer value, as follows:

Sun Star at the centre of the Solar System

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

Conventional color description

Just-saturated RGB-camera discs TernaryColorTmap.PNG
Just-saturated RGB-camera discs

The conventional color 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 colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of 'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading. Excluding color-contrast illusions in dim light, there are no green, indigo, or violet stars. Red dwarfs are a deep shade of orange, and brown dwarfs do not literally appear brown, but hypothetically would appear dim grey to a nearby observer.

Red dwarf An informal category of small, cool stars on the main sequence

A red dwarf is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.

Brown dwarf Type of substellar object larger than a gas giant

A brown dwarf is a type of substellar object occupying the mass range between the heaviest gas giant planets and the lightest stars, having a mass between approximately 13 to 75–80 times that of Jupiter (MJ), or approximately 2.5×1028 kg to about 1.5×1029 kg. Below this range are the sub-brown dwarfs (sometimes referred to as rogue planets), and above it are the lightest red dwarfs (M9 V). Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.

Modern classification

The Morgan-Keenan spectral classification Morgan-Keenan spectral classification.png
The Morgan–Keenan spectral 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, are based on color indexes—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. Ultraviolet, Blue and Visual).

In astronomy, a photometric system is a set of well-defined passbands, with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.

The UBV photometric system, also called the Johnson system, is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star then two subtractions are performed in a specific order to classify it in the system.

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The smaller the color index, the more blue the object is. Conversely, the larger the color index, the more red the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller magnitudes than dimmer ones. For comparison, the yellowish Sun has a B−V index of 0.656 ± 0.005, whereas the bluish Rigel has a B−V of −0.03. Traditionally, the color index uses Vega as a zero point.

Harvard spectral classification

The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. 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, 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.

Annie Jump Cannon American astronomer

Annie Jump Cannon was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification. With Edward C. Pickering, she is credited with the creation of the Harvard Classification Scheme, which was the first serious attempt to organize and classify stars based on their temperatures and spectral types. She was nearly deaf throughout her career. She was a suffragist and a member of the National Women's Party.

The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units (SI).

Class Effective temperature [1] [2] Vega-relative chromaticity [3] [4] [nb 1] Chromaticity (D65) [5] [6] [3] [nb 2] Main-sequence mass [1] [7]
(solar masses)
Main-sequence radius [1] [7]
(solar radii)
Main-sequence luminosity [1] [7]
Fraction of all
main-sequence stars [8]
O  30,000 Kblueblue 16  M  6.6  R  30,000  L Weak~0.00003%
B 10,000–30,000 Kblue whitedeep blue white2.1–16  M 1.8–6.6  R 25–30,000  L Medium0.13%
A 7,500–10,000 Kwhiteblue white1.4–2.1  M 1.4–1.8  R 5–25  L Strong0.6%
F 6,000–7,500 Kyellow whitewhite1.04–1.4  M 1.15–1.4  R 1.5–5  L Medium3%
G 5,200–6,000 Kyellowyellowish white0.8–1.04  M 0.96–1.15  R 0.6–1.5  L Weak7.6%
K 3,700–5,200 Klight orangepale yellow orange0.45–0.8  M 0.7–0.96  R 0.08–0.6  L Very weak12.1%
M 2,400–3,700 Korange redlight orange red0.08–0.45  M  0.7  R  0.08  L Very weak76.45%
The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature. H-R diagram.svg
The Hertzsprung–Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by 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 is classified as O9.7. [9] The Sun is classified as G2. [10]

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 [5] 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 (or more precisely, its effective temperature) was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true. [11] In the 1920s, the Indian physicist Meghnad Saha 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. [12]

Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually a sequence in temperature. [13] 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

Spectra for dwarfs (luminosity class V) for standard spectral types taken from Pickles (1998). Several notable spectral lines are indicated. Hydrogen lines are strongest for types A and B, and the overall spectrum peaks at shorter wavelengths for hotter stars. Dwarf star spectra (luminosity class V) from Pickles 1998.png
Spectra for dwarfs (luminosity class V) for standard spectral types taken from Pickles (1998). Several notable spectral lines are indicated. Hydrogen lines are strongest for types A and B, and the overall spectrum peaks at shorter wavelengths for hotter stars.

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. [15] 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 (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, [16] 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 is much lower than for a dwarf star 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. [17]

Yerkes luminosity classes
Luminosity classDescriptionExamples
0 or Ia+ hypergiants or extremely luminous supergiants Cygnus OB2#12 B3-4Ia+ [18]
Ialuminous supergiants Eta Canis Majoris B5Ia [19]
Iabintermediate-size luminous supergiants Gamma Cygni F8Iab [20]
Ibless luminous supergiants Zeta Persei B1Ib [21]
II bright giants Beta Leporis G0II [22]
IIInormal giants Arcturus K0III [23]
IV subgiants Gamma Cassiopeiae B0.5IVpe [24]
V main-sequence stars (dwarfs) Achernar B6Vep [21]
sd (prefix) or VI subdwarfs HD 149382 sdB5 or B5VI [25]
D (prefix) or VII white dwarfs [nb 3] van Maanen 2 DZ8 [26]

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:

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 more luminous than typical may be given a luminosity class of IIIb. [27]

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. [28]

Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. [29]

CodeSpectral peculiarities for stars
 :uncertain spectral value [17]
...Undescribed spectral peculiarities exist
!Special peculiarity
compComposite spectrum [30]
eEmission lines present [30]
[e] "Forbidden" emission lines present
er"Reversed" center of emission lines weaker than edges
eqEmission lines with P Cygni profile
fN III and He II emission [17]
f* N  IV λ4058Å is stronger than the N  III λ4634Å, λ4640Å, & λ4642Å lines [31]
f+Si IV λ4089Å & λ4116Å are emitted, in addition to the N III line [31]
(f)N III emission, absence or weak absorption of He II
(f+) [32]
((f))Displays strong He II absorption accompanied by weak N III emissions [33]
((f*)) [32]
hWR stars with hydrogen emission lines. [34]
haWR stars with hydrogen seen in both absorption and emission. [34]
He wkWeak Helium lines
kSpectra with interstellar absorption features
mEnhanced metal features [30]
nBroad ("nebulous") absorption due to spinning [30]
nnVery broad absorption features [17]
nebA nebula's spectrum mixed in [30]
pUnspecified peculiarity, peculiar star. [nb 4] [30]
pqPeculiar spectrum, similar to the spectra of novae
q P Cygni profiles
sNarrow ("sharp") absorption lines [30]
ssVery narrow lines
sh Shell star features [30]
varVariable spectral feature [30] (sometimes abbreviated to "v")
wlWeak lines [30] (also "w" & "wk")
Abnormally strong spectral lines of the specified element(s) [30]

For example, 59 Cygni is listed as spectral type B1.5Vnne, [35] indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.

Guide for Secchi spectral types ("152 Schjellerup" is Y Canum Venaticorum) Star spectra by Secchi.jpg
Guide for Secchi spectral types ("152 Schjellerup" is Y Canum Venaticorum)


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 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. [36] [37] [38]

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article. [39] [40] [41]

Class numberSecchi class description
Secchi class IWhite and blue stars with broad heavy hydrogen lines, such as Vega and Altair. This includes the modern class A and early class F.
Secchi class I
(Orion subtype)
A subtype of Secchi class I with narrow lines in place of wide bands, such as Rigel and Bellatrix. In modern terms, this corresponds to early B-type stars
Secchi class IIYellow stars hydrogen less strong, but evident metallic lines, such as the Sun, Arcturus, and Capella. This includes the modern classes G and K as well as late class F.
Secchi class IIIOrange to red stars with complex band spectra, such as Betelgeuse and Antares.
This corresponds to the modern class M.
Secchi class IVIn 1868, he discovered carbon stars, which he put into a distinct group: [42]
Red stars with significant carbon bands and lines, corresponding to modern classes C and S.
Secchi class VIn 1877, he added a fifth class: [43]
Emission-line stars, such as Gamma Cassiopeiae and Sheliak, which are in modern class Be. In 1891, Edward Charles Pickering proposed that class V should correspond to the modern class O (which then included Wolf-Rayet stars) and stars within planetary nebulae. [44]

The Roman numerals used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes.

Draper system

Classifications in the Draper Catalogue of Stellar Spectra [45] [46]
IA, B, C, DHydrogen lines dominant.
IIE, F, G, H, I, K, L
IVNDid not appear in the catalogue.
VOIncluded Wolf–Rayet spectra with bright lines.
VPPlanetary nebulae.
 QOther spectra.
Classes carried through into the MK system are in bold.

In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue.

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. [45] [46]

Harvard system

In 1897, another worker at Harvard, 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 to XXII. [47] [48]

In 1901, Annie Jump Cannon 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. [49] [50] Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. [51] [52] This is essentially the modern form of the Harvard classification system.

A common mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is "Oh, Be A Fine Guy/Girl, Kiss Me". [53]

Mount Wilson classes

Proper motion of stars of early type in -/+ 200 000 years Ba b do8mag c6 big.gif
Proper motion of stars of early type in -/+ 200 000 years

A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities. [54] [55] [56] This notation system is still sometimes seen on modern spectra. [57]

The movement of stars of late type around the apex (left) and antapex (right) in -/+ 200 000 years Astro 4D stars proper radial g b 8mag big.gif
The movement of stars of late type around the apex (left) and antapex (right) in -/+ 200 000 years

Spectral types

The stellar classification system is taxonomic, based on type specimens, similar to classification of species in 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. [58]

"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, 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. [59] The terms "early" and "late" were carried over, beyond the demise of the model they were based on.

Class O

The spectrum of an O5v star. O5v-spectre.png
The spectrum of an O5v star.

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. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars. [nb 5] [8] 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 He  II λ4541 relative to that of He I λ4471, where λ is the wavelength, measured in ångströms. 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 line N IV λ4058 to N III λλ4634-40-42. [60]

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. 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.

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. [61] The MKK scheme was extended to O9.7 in 1971 [62] and O4 in 1978, [63] and new classification schemes that add types O2, O3 and O3.5 have subsequently been introduced. [64]

Spectral standards: [58]

Class B

B-class stars in the Jewel Box cluster (Credit: ESO VLT) A Snapshot of the Jewel Box cluster with the ESO VLT.jpg
B-class stars in the Jewel Box cluster (Credit: ESO VLT)

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 stars.

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 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 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. [60]

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. About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type main-sequence stars. [nb 5] [8]

Massive yet non-supergiant entities known as "Be stars" are main-sequence stars that notably have, or had at some time, one or more Balmer lines in emission, with the hydrogen-related electromagnetic radiation series projected out by the stars being of particular interest. Be stars are generally thought to feature unusually strong stellar winds, high surface temperatures, and significant attrition of stellar mass as the objects rotate at a curiously rapid rate. [65] Objects known as "B(e)" or "B[e]" stars possess distinctive neutral or low ionisation emission lines that are considered to have 'forbidden mechanisms', undergoing processes not normally allowed under current understandings of quantum mechanics.

Spectral standards: [58]

Class A

Class A Vega (left) compared to the Sun (right). Size Vega.png
Class A Vega (left) compared to the Sun (right).

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. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars. [nb 5] [8] [66]

Spectral standards: [58]

Class F

Canopus, an F-type supergiant and the second brightest star in the night sky Canopus.jpg
Canopus, an F-type supergiant and the second brightest star in the night sky

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. Their color is white. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are F-type stars. [nb 5] [8]

Spectral standards: [58]

Class G

The Sun, a G2 main-sequence star, with dark sunspots Sun920607.jpg
The Sun, a G2 main-sequence star, with dark sunspots

G-type stars, including the Sun [10] have prominent spectral lines H and K 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 the G band of CH molecules. Class G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood. [nb 5] [8]

G is host to the "Yellow Evolutionary Void". [67] 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 yellow supergiant G class, as this is an extremely unstable place for a supergiant to be.

Spectral standards: [58]

Class K

Arcturus, a K1.5 giant compared to the Sun and Antares Redgiants.svg
Arcturus, a K1.5 giant compared to the Sun and Antares

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. [nb 5] [8] There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri  B, 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. There is a suggestion that K-spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone. [68]

Spectral standards: [58]

Class M

UY Scuti, an M4 supergiant UY Scuti zoomed in, Rutherford Observatory, 07 September 2014.jpeg
UY Scuti, an M4 supergiant

Class M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars. [nb 5] [nb 6] [8] 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 M0V Lacaille 8760, with magnitude 6.6 (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 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.

Spectral standards: [58]

Extended spectral types

A number of new spectral types have been taken into use from newly discovered types of stars. [69]

Hot blue emission star classes

UGC 5797, an emission-line galaxy where massive bright blue stars are formed Galactic refurbishment.jpg
UGC 5797, an emission-line galaxy where massive bright blue stars are formed

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

Class W: Wolf–Rayet

Hubble Space Telescope image of the nebula M1-67 and the Wolf-Rayet star WR 124 in the center M1-67 & WR124.png
Hubble Space Telescope image of the nebula M1-67 and the Wolf–Rayet star WR 124 in the center

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). [34]

WR spectra range is listed below: [71] [72]

  • WN [34] 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 [34]
    • 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 [34]
  • WC [34] 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

Although the central stars of most planetary nebulae (CSPNe) show O type spectra, [73] around 10% are hydrogen-deficient and show WR spectra. [74] 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" [60] ).

There is a secondary group found with this spectra, a cooler, "intermediate" group designated "Ofpe/WN9". [60] 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. [75]

The magnetic O stars

They are O stars with strong magnetic fields. Designation is Of?p [60]

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 dwarfs that are very faint in the visible spectrum. [76]

Brown dwarfs, whose energy comes from gravitational attraction alone, 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 temperature and luminosity for some masses and ages of different L-T-Y types, no distinct temperature or luminosity values can be given. [7]

Class L

Artist's impression of an L-dwarf L-dwarf-nasa-hurt.png
Artist's impression of an L-dwarf

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 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. [77] [78] [79]

Due to low surface gravity in giant stars, TiO- and 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

Artist's impression of a T-dwarf T-dwarf-nasa-hurt.png
Artist's impression of a T-dwarf

Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 550 and 1,300 K (277 and 1,027 °C; 530 and 1,880 °F). Their emission peaks in the infrared. Methane is prominent in their spectra. [77] [78]

Classes T and L could be more common than all the other classes combined if recent research is accurate. Because brown dwarfs persist for so long—a few times the age of the universe—in the absence of catastrophic collisions these smaller bodies can only increase in number.

Study of the number of 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 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

Artist's impression of a Y-dwarf WISE 1828+2650 Brown dwarf.jpg
Artist's impression of a Y-dwarf

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. [80] Although such dwarfs have been modelled [81] and detected within forty light-years by the Wide-field Infrared Survey Explorer (WISE) [69] [82] [83] [84] [85] there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2. [86]

The spectra of these prospective Y objects display absorption around 1.55  micrometers. [87] 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. [87] [88] In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class. [86] However, this feature is difficult to distinguish from absorption by water and methane, [87] and other authors have stated that the assignment of class Y0 is premature. [89]

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  K, the temperature of the human body. [82] [83] [90] 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. [91]

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 IAU division between brown dwarfs and planets. [86]

Late giant carbon-star classes

Carbon-stars are stars whose spectra indicate production of carbon—a byproduct of 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

Image of the carbon star R Sculptoris and its striking spiral structure Curious spiral spotted by ALMA around red giant star R Sculptoris (data visualisation).jpg
Image of the carbon star R Sculptoris and its striking spiral structure

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 13 CN in addition to those of 12 CN. [92] 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 13C.
  • C-H Population II analogues of the C-R stars.
  • C-Hd Hydrogen-deficient carbon stars, similar to late G supergiants with CH and 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 ZrO absorption bands analogous to the TiO bands of class M stars, whereas those most similar to carbon stars have strong sodium D lines and weak C2 bands. [93] 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 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.

White dwarfs come into an entirely different place near the bottom of the Hertzsprung-Russell diagram. Hertzsprung-Russel StarData.png
White dwarfs come into an entirely different place near the bottom of the Hertzsprung-Russell diagram.

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.

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 stars in the asymptotic giant branch.

White dwarf classifications

Sirius A and B (a white dwarf of type DA2) resolved by Hubble Sirius A and B Hubble photo.jpg
Sirius A and B (a white dwarf of type DA2) resolved by Hubble

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: [94] [95]

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. [94] [96]

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. [94]

Extended white dwarf spectral types: [94]

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

CodeSpectral peculiarities for stars
PMagnetic white dwarf with detectable polarization
EEmission lines present
HMagnetic white dwarf without detectable polarization
PECSpectral peculiarities exist

Non-stellar spectral types: Classes P and Q

Finally, the classes P and Q, left over from the Draper system by Cannon, are occasionally used for certain non-stellar objects. Type P objects are stars within planetary nebulae and type Q objects are novae.

Stellar remnants

Stellar remnants are objects associated with the death of stars. Included in the category are white dwarfs, 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. [97]

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

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 Planetary habitability, under the binary systems section).

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). [98] While there are many problems facing life on red dwarfs, due to their sheer numbers and longevity many astronomers continue to model these systems.

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. [99]

See also


  1. This is the relative color of the star if Vega, generally considered a bluish star, is used as a standard for "white".
  2. Chromaticity can vary significantly within a class; for example, the Sun (a G2 star) is white, while a G9 star is yellow.
  3. Technically, white dwarfs are no longer “live” stars, but rather the “dead” remains of extinguished stars. Their classification uses a different set of spectral types from element-burning “live” stars.
  4. When used with A-type stars, this instead refers to abnormally strong metallic spectral lines
  5. 1 2 3 4 5 6 7 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.
  6. This rises to 78.6% if we include all stars. (See the above note.)

Related Research Articles

Supergiant star large star

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,450 K to over 20,000 K.

Red supergiant star supergiant stars

Red supergiants are stars with a supergiant luminosity class of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

Wolf–Rayet star Stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. Their surface temperatures range from 30,000 K to around 200,000 K, hotter than almost all other stars.

Blue giant stars of early spectral type

In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II. In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.

Blue supergiant star type of star

Blue supergiant stars are hot luminous stars, referred to scientifically as OB supergiants. They have luminosity class I and spectral class B9 or earlier.

Giant star type of star with a radius 10-100 times, and luminosity 10-1000x that of the Sun

A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.

Subdwarf star of luminosity class VI under the Yerkes spectral classification system

A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On a Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.

Subgiant type of star larger than main-sequence but smaller than a giant

A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.

RV Tauri variable type of variable star

RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.

Yellow hypergiant

A yellow hypergiant is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest with just 15 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison to O- and B-type stars, and sometimes as warm hypergiants in comparison to red supergiants.

O-type main-sequence star stellar classification

An O-type main-sequence star is a main-sequence star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun.

Yellow supergiant star

A yellow supergiant star is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.

Hypergiant Rare star with tremendous luminosity and high rates of mass loss by stellar winds

A hypergiant (luminosity class 0 or Ia+) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants may be classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.

SU Andromedae star

SU Andromedae is a carbon star in the constellation of Andromeda. It is a variable star classified as a slow irregular pulsating supergiant, and varies from an apparent visual magnitude of 8.5 at minimum brightness to a magnitude of 8.0 at maximum brightness with no clear period.

HD 5980 triple star system in the constellation Tucana

HD 5980 is a multiple star system in NGC 346 in the Small Magellanic Cloud (SMC) and is one of the brightest stars in the SMC.

Melnick 42 star

Melnick 42 is a massive blue supergiant star in the Tarantula Nebula in the Large Magellanic Cloud located in the constellation Dorado. Although it is only 21 times the size of the sun, its high temperature of 47,300 K makes it one of the most luminous stars of the Tarantula Nebula at 3,600,000 L. It is less than two parsecs from the centre of the R136 cluster, although that is well outside the central core.

WR 25 star

WR 25 is a binary star system in the turbulent star forming region Carina Nebula, about 7,500 light-years from Earth. It contains a Wolf-Rayet star and a hot luminous companion, and is a member of the Trumpler 16 cluster.

WR 24 star

WR 24 is a Wolf-Rayet star in the constellation Carina. It is one of the most luminous stars known. At the edge of naked eye visibility it is also one of the brightest Wolf Rayet stars in the sky.

O-type star type of star according to Stellar classification (O, B, A, F, G, K, and M)

An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvins (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.

3 Geminorum highly luminous blue supergiant star in the constellation Gemini

3 Geminorum is a blue supergiant star in the constellation Gemini. It is a small amplitude pulsating variable and a close double star, with a mean combined apparent visual magnitude of 5.75.


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