|
The classification of stars according to their spectra; each major spectral classification is given a letter, with additional numbers providing further subdivisions.
The light produced by every single star is unique. The temperature of the star, its luminosity, and its chemical composition, will all have effects on the light that it produces, and these effects can be decoded by examining the star's spectrum. We see the Sun's spectrum (albeit in a very coarse form) whenever we look at a rainbow, or use a prism to split sunlight into its constituent colours. Starlight, too, is composed of these seven colours, but the relative intensities of light along the spectrum will vary considerably from star to star.
This fact, as well as providing a wealth of information about stars themselves, provides a basis for a systematic stellar classification. At its most basic, this system consists of sorting stars according to their surface temperature. stars are divided into a number of basic types according to this temperature (and other corresponding factors), and each type is given an identifying letter. Within each type, stars can be further categorised by grading them from 0 to 9 (so for example, an M2 star is hotter than an M7).
The peculiar ordering of the identifying letters is a matter of historical accident, and the letters themselves have no particular significance. Most of the common types have associated colours, which are themselves a consequence of the temperature of the star (hotter stars are bluer, while cooler stars tend to be redder). In decreasing order of surface temperature, the main spectral types are shown below.
| W |
W refers to Wolf-Rayet stars, a rare type of intensely hot star, with surface temperatures up to 50,000 K. There is only one example in the sky that is visible to the naked eye, in the Suhail al Muhlif system in the constellation Vela.
|
| O |
O-type stars are also relatively uncommon, but far more numerous than those of type W. These are bright blue stars which also have very high surface temperatures, in the range 25,000 K to 50,000 K. Examples are Alnitak (O9.5), Naos (O5), Hatysa (O9) and Meissa (O8).
|
| B |
The B-type is the first of the really populous classes. Stars of this type are blue in colour and burn hotly, with surface temperatures lying between 11,000 K and 25,000 K. Prominent examples of blue B-type stars are Rigel (B8), Achernar (B3), Hadar (B1) and Spica (also B1).
|
| A |
A-type stars are those whose surface temperatures lie in the approximate range 7,500 K to 11,000 K. They are white in colour, and some of the brightest and most famous stars in the sky belong to this classification, including Sirius (A0), Vega (A0), Altair (A7) and Deneb (A2).
|
| F |
F-type stars lie between the A-type white stars and G-type 'true' yellow stars, and have a distinctly yellowish light. Their surfaces have a temperature between 6,000 K and 7,500 K. Sometimes called calcium stars, examples of this type include Procyon (F5), Mirfak in Perseus (F5) and Wezen (F8).
|
| G |
The cooler a star, the more complex its chemistry tends to be. G-type stars, with temperatures ranging between 5,000 K and 6,000 K, have spectra that betray the existence of 'metals' (in this context, 'metal' refers to any element heavier than helium). Examples of yellow G-type stars are Alpha Centauri (or Rigil Kentaurus, G2), Capella (G5), Kraz (G5) and Muphrid (G0). Earth's Sun is a G2 star, and also belongs to this type.
|
| K |
K-type stars are occasionally referred to as Arcturian stars, after the brightest of their number. Their surface temperatures are between 3,500 K and 5,000 K, low enough for simple molecules to form. K-type stars are orange in colour, and among the brightest in the sky are Arcturus (K2), Aldebaran (K5), Pollux (K0) and Atria (K2).
|
| M |
The coolest of the common star types, red stars are classified as M-type. They have very cool surface temperatures below 3,500 K, allowing more complex molecules to form. Among the brightest red stars in the sky are Betelgeuse (M2), Antares (M1), Gacrux (M4) and Mirach (M0). The Sun's nearest neighbour in space, Proxima Centauri, is also a red star, classified as M5.
|
Two other rare classes are the C- and S-type stars. These are cool stars that overlap the K- and M-type classifications in terms of temperature, but are placed in separate categories due to unusual chemistry within the star. Very few of these stars are visible without optical aid, though the C-type U Hydrae, and the S-type Chi Cygni are unusual exceptions.
A star's full spectral classification usually also includes a 'luminosity class', a Roman numeral from I to VII indicating the star's intrinsic luminosity, which correlates with its mass. The luminosity class is simply appended to the spectral class. So, for example, the Sun's full spectral classification, including its luminosity class, is G2V (where 'V' indicates that it is a 'dwarf' star). The seven standard luminosity classes are listed below.
| 0 |
A classification of the most luminous of all stars, usually known as hypergiants (with many sources prefer an alternative extended supergiant classification of 'Ia+' for hypergiant stars). Hypergiants are comparatively rare compared to the other types, but examples include BP Crucis (B10), Rho Cassiopeiae (G20) or R Puppis (also G20). A subclass of highly variable hypergiant stars (such as Eta Carinae) pass at times through a hypergiant phase, but at other times are much less luminous, and these are typically given their own classification of 'LBV' or 'luminous blue variable'.
|
| I |
Supergiants: extremely massive and luminous stars, usually nearing the end of their lifespan. They are subclassified as Ia or Ib, with Ia representing the most luminous stars of all, other than the rare hypergiants. Examples include Rigel (B8Ia), Betelgeuse (M2Ib) and Antares (M1Ib).
|
| II |
Bright Giants: a relatively uncommon group of giant stars that are particularly luminous, and can be a thousand times more so than the Sun. Examples include Adhara (B2II), Sargas (F1II) and Kraz (G5II).
|
| III |
Standard Giants: the giant stars in this category are typically at least a hundred times more luminous than Earth's Sun, and considerably more massive. Examples of this populous group include Arcturus (K2III), Hadar (B1III) and Aldebaran (K5III).
|
| IV |
Subgiants: though still far more massive and luminous than the Sun, subgiants fall short of the true giants. Examples include Acrux (B0.5IV), Shaula (B1.5IV) and Miaplacidus (A2IV).
|
| V |
Dwarfs: a very numerous class of main sequence stars, whose structure is generally comparable with that of the Sun. Examples include Sirius (A0V), Alpha Centauri (G2V) and Vega (A0V).
|
| VI & VII |
These classes designate subdwarfs and white dwarfs, respectively. They are not now in common use, but are included here for completeness.
|
A few other types or subtypes are also sometimes seen. A 'D' prefix is used to specify a white dwarf (that is, a stellar core remnant, not simply a white star on the main sequence) and is preferred to the older 'VII' luminosity class for such stars. This type is further broken down according to the elements detected within the star (so, for example, 'DA' relates to an outer layer rich in hydrogen, or 'DB' to helium). Examples include Sirius B, the white dwarf companion of Sirius, with a class of DA1.9, or van Maanen's Star, classed as DZ7.5 ('DZ' showing a atmosphere rich in heavier elements).
The 'M-type' classification applies to the coolest stars, but it can also sometimes be used for brown dwarfs. These are objects lacking the mass for nuclear fusion in their cores, and hence are not true stars, but they are nonetheless more massive than any planet, and are able to generate some light and heat of their own. The hottest brown dwarfs overlap with the M-type classification, but three other classifications are also used for objects of this kind. In descending order of temperature, these are classified as L-type, T-type and Y-type. The coolest of these, Y-type brown dwarfs, are sometimes called 'sub-brown dwarfs', and have a temperature of just a few hundred Kelvin, comparable with the surface of the Earth.
A variety of additional symbols are also used to describe spectra that are unusual or uncertain, shown in the table below.
| + |
Binary or multiple systems may have distinct spectra recorded for individual component stars, and in these cases, those spectra are shown separated by a 'plus' sign. For example, the full spectral classification for the star Capella (Alpha Aurigae) is G5IIIe+G0III, representing the distinct classes for the two yellow giants that form the heart of a complex multiple system.
|
| - |
A 'dash' symbol indicates that a star's spectral type lies between two discrete classifications. An example is the bright star Procyon, with a full classification of 'F5IV–V'; that is, the star is in the process of evolving away from its main-sequence or dwarf stage (luminosity class 'V') into a subgiant (luminosity class 'IV').
|
| / |
A 'slash' sign is used where there are ambiguities within the spectrum, with different characteristics pointing toward different types. For example, a class 'K0III/IV' (as for the star Timir or HD 148427 in Ophiuchus) shows that there are some indications that the star is a giant, while other features point toward it being a subgiant.
|
| : |
Where uncertainties exist within the classification, this is marked by adding a colon symbol to the spectral type. A prominent example is Rigel in Orion, a blue supergiant classified as 'B8Iae', sometimes given as 'B8Iae:' with a colon suffix because of difficulties in assessing the precise luminosity of the star.
|
| ... |
An ellipsis indicates that there are unusual features in the spectrum that are not specifically described. The use of this notation is rare, and usually spectral pecularities are specified using one of the code letters listed below, or indicated by 'p' or 'pec' for 'peculiar'. An example is the extreme hypergiant Eta Carinae, sometimes listed as simply 'pec', or sometimes as 'LBV' for 'Luminous Blue Variable'.
|
| ! |
An exclamation mark formally describes a spectrum showing a special pecularity, such as an unusually strong magnetic field or levels of metallic elements. In practice the use of this mark is exceptionally rare.
|
| (elements) |
Where spectral lines for a particular element are unusually strong or weak in the spectrum, the symbol for that element is appended to the spectral classification code. This symbol is sometimes shown with a '+' or '-" to indicate strong or weak lines, and sometimes a value (which repesents the strength relative to the spectrum of the Sun). A prominent example is Arcturus, with a full spectral type of K1IIIbCN-1, where 'CN-1' shows weak lines for 'cyano radicals' (which are simple molecules of C, carbon, and N, nitrogen).
|
In addition to these symbols, a full spectral classification can also include any of a wide range of alphabetical codes, usually added as a suffix to the main classification. The main suffix codes are listed in the table below.
| a |
Indicates that the star is unusually luminous for its class, a suffix typically reserved for supergiants (which often have an additional code added to further specify the type) but occasionally seen appended to other classes. For exceptionally luminous stars, 'a+' will sometimes be added to the classification (not to be confused with '+' above for binary or multiple systems). Rigel in Orion is an example of a luminous supergiant (full type B8Iae:), while Vega in Lyra is another example of an 'a' suffix (Vega is a main sequence star, not a supergiant, but its spectrum of A0Va shows that it is particularly luminous for a star of its type).
|
| ab |
Used of supergiants to indicate that the star is of intermediate luminosity between the highly luminous 'Ia' type and the less luminous 'Ib' type. Betelgeuse in Orion is an example of this type, with a full type of 'M1-M2Ia-Iab' reflecting its variability (at times it has the luminosity of a 'Ia' supergiant, while at other times it falls into the 'Iab' class).
|
| b |
Indicates that the star belongs to the less luminous group of its class, and is used particularly to supergiants that have lost their outer hydrogen shell, so that helium fusion takes place in their outer layer. An example of such a supergiant is Mirfak or Alpha Persei, with a classification of 'F5Ib'.
|
| β |
The Greek letter β (beta) is one of a series of specific codes used to specify the surface gravity of a brown dwarf. The full sequence is α (alpha) for normal, β (beta) for intermediate (or lower than normal) and γ (gamma) for notably low surface gravity. Note that while α (alpha) is technically part of this sequence, its meaning of 'normal' is not usually informative, and it is almost never seen in practice.
|
| blue |
Used only in classifications of brown dwarfs, this suffix indicates an unusual shift towards a bluer spectrum, but without the reduced metallicity that would typically account for this effect. Note that 'bluer' here is a relative term, referring to variations in the near-infrared part of the spectrum. A 'bluer' object in these terms would still appear reddish in visible colouration.
|
| comp |
Short for 'composite', indicating a spectrum that is formed from two blended underlying spectra, indicating that the star in question is actually a close binary.
|
| e |
Specifies that the spectrum contains emission lines, groupings of brighter lines that demonstrate the existence of an element that is being heated to the extent that emits photons. Each element has its own distinct 'fingerprint' of emission lines, with hydrogen being the most commonly found, but by no means the only example. Emission lines can appear in many different types of stellar spectra, but a common example is a 'Be' star (that is, B-type blue star showing emission lines in its spectrum). In this case the emission is due to hot material expelled by a star rotating rapidly on its axis. The classical example, and indeed the first such star to be identified, is Tiansi or Gamma Cassiopeiae (with a full spectral type of B0IVnpe(shell)). Emission lines are usually indicated by a lowercase 'e', except in the spectra of white dwarfs, where an uppercase 'E' is conventional.
|
| [e] |
The use of square brackets around an 'e' marker indicates a particular type of emission line within the spectrum, specifically a 'forbidden' line. Such lines indicate a transition in energy state of electrons that would be 'forbidden' under typical conditions, but can be seen in extremely low-density states. Where such lines appear, therefore, they indicate a particularly low density in the outer layers of a star.
|
| (e) |
Where 'e' appears within parentheses, this indicates that there are suggestions of emission lines in the spectrum, but that their detection is not conclusive or continuous. Either the emission lines are exceptionally weak, or they are transient (that is, they may appear at some times, but not at others.)
|
| em |
One of a number of extended codes for particular types of emission lines (see 'e' above). A star whose spectral code includes an 'em' suffix shows emission lines for metals (and in this context, 'metals' refers to any element heavier than helium). This effect is particuarly common in Wolf-Rayet stars (or W-type stars), unusually hot stars that show particularly strong emission lines for heavier elements, most commonly nitrogen or carbon.
|
| ep |
This code specifies that the spectrum shows emission lines that are 'peculiar', showing any of various unusual or unexpected features in the emission spectrum. An example of this is the bright blue main sequence star Achernar in Eridanus, sometimes classified as 'B3Vep' (or sometimes as 'B3Vpe', where 'pe' indicates peculiar absorption as well as emission lines; see 'pe' below).
|
| eq |
A highly specific type of emission line spectrum, in which emission and absorption lines are simultaneously present. This type of spectrum is more fully known as a 'P Cygni profile', after the prototype star P Cygni, a variable supergiant classed as B1Iapeq. This unusual combination of spectral lines is caused by an envelope of gas expanding rapidly outward from the star due to the pressure of its stellar wind.
|
| er |
A rare suffix indicating reversed emssion lines in the spectrum. In this case, 'reversed' describes emission lines that are fainter in the centre and stronger on their edges, a phenomenon that occurs when photons emitted by an element are absorbed by other atoms of the same element, reducing the strength of the emission lines for that element.
|
| f |
A family of suffixes used in the spectra of O-type stars to mark unusually strong or weak lines for specific ionised elements (primarily helium, nitrogen and silicon). The most prominent example is the star Naos or Zeta Puppis, with a spectral type of 'O5Iaf' showing that it is an extremely hot O-type supergiant with emission lines for ionised helium and nitrogen. Variations on the 'f' suffix, marked with asterisks, question marks, brackets and so on, show different combinations of emission lines for different elements at specific wavelengths.
|
| γ |
A Greek γ (gamma) follows the same series listed under 'β' (beta) above, describing the surface gravity of a brown dwarf. The γ symbol represents the lowest of the three possible designations, indicating a brown dwarf with particularly low surface gravity.
|
| H |
A capital 'H' is used exclusively for the spectra of stars in the white dwarf class, and indicates that the star has been detected to possess a specific configuration of magnetic field. It is common for white dwarfs to show magnetic fields, and for those field to affect the orientation of the photons emitted by the star. In some cases, however, no such polarisation is detected, and white dwarf spectra showing this omission register the fact with an 'H' indicator.
|
| h |
A lowercase 'h' applies specifically to Wolf-Rayet stars, unusually hot stars that typically show only low levels of hydrogen and abundances of other elements. The 'h' suffix shows that, untypically for its class, a Wolf-Rayet star demonstrates distinct emission lines for hydrogen. Such evolved Wolf-Rayet stars (sometimes abbreviated as 'WNh' stars, using this 'h' code) are unusual in that they continue to burn hydrogen in their cores. A variant on the 'h' suffix is 'ha', used when the star shows both emission and absorption lines for hydrogen.
|
| k |
This suffix indicates that the spectrum incorporates interstellar absorption features. That is, the light from the star passes through intervening material in the interstellar medium before reaching observers on Earth, and that intervening material absorbs specific wavelengths of the spectrum. These features of the spectrum have characteristics that allow them to be distinguished from that of the star itself, providing information about the nature of the regions of space between the source star and the Sun. An example is Pincoya or HD 164604 in Sagittarius, an orange dwarf with the spectral type 'K3.5Vk'. (Note that this lowercase suffix 'k' is not to be confused with the commonplace capital 'K' - both appear in this example - used to designate an orange star.)
|
| m |
This 'm' suffix stands for 'metals', and in terms of spectral analysis this refers to any elements heavier than hydrogen or helium. This suffix is used where the indicated levels of these metals is significantly greater than would be expected for a star of a given type. This 'm' code is applied most commonly to white (A-type) stars, and indeed the shorthand 'Am star' (or metallic-line star) is used of white stars showing metal-rich spectra. A prominent example is Sirius A, the brightest star in Earth's sky, with a full spectral type of 'A1Vm', where (in this case) the 'm' relates to unusually high levels of iron in the atmopshere of the star. |
| n |
A spectral class marked with an 'n' indicates that the absorption lines of the spectrum are 'nebulous'. This use of the term 'nebulous' in this sense has no connection with nebulae (for which see 'neb' below), but simply denotes that the lines are broader and less distinct than in most spectra, a phenomenon typically due to a star rotating unusually rapidly on its axis.
|
| neb |
When the light of star passes through an intervening nebula, the spectral features of that nebula can become combined with that of the source star. Spectra like this, which integrate elements of the star's chemistry with that of the nebula, are suffixed with 'neb'.
|
| nn |
An extension of 'n' above, indicating especially diffuse absorption lines in the spectrum, which in turn indicate that a star is rotating extremely rapidly on its axis. The brightest example of an 'nn' star in the sky is the white main sequence Gamma Trianguli (spectral type A1Vnn), with an estimated rotational velocity of 254 km/s (compared to 2 km/s for the Sun). Such rapid rotation causes the star's equator to project outward, so that 'nn' stars tend to have a 'flattened' spheroidal shape.
|
| p |
A 'p' suffix shows that the star's spectrum demonstrates a 'peculiarity' that is sufficiently unusual that it is not otherwise coded. The brightest example of such a peculiar star in the sky is Achernar in Eridanus (with a spectral type of B3Vep, showing that it is a star of the 'Be' type, but with unusual properties). The 'p' code is usually non-specific, except in a handful of circumstances, especially if it follows another suffix code (in which case it is taken to relate to the condition described by that preceding code). A more specific use applies to white A-type stars, where 'p' typically relates to unusually abundant metals within the star, and those metals are then commonly listed after the 'p' code.
|
| P |
A capital 'P' suffix relates to the magnetic fields of white dwarfs, and specifically indicates the presence of polarisation in those fields. An example of such a classification is the white dwarf V1201 Orionis (also identified as Gliese 1087 or LHS 212), with a spectrum classified as 'DA8P' showing that it has a magnetic field with (in this case) circular polarisation.
|
| pe |
An extended version of the 'p' code for 'peculiar' features, 'pe' specifies that the peculiarity in question relates to the absorption lines in the spectrum. An example is the primary component of the Omicron Andromedae system, a shell star named Alfarasalkamil, which shows rapid and unpredictable variations in its spectrum, and is classified - at least on some systems - as 'B6IIIpe'. The 'pe' code for peculiar absorption lines is related to 'ep', described above, which describes pecularities in the emission lines of the spectrum.
|
| pec |
An equivalent to the more common 'p' above, also meaning 'peculiar'. This extended form tends to be used primarily to indicate peculiarities in the spectra of brown dwarfs. The same suffix, with the same meaning, can also be used of white dwarfs, in which case it is typically capitalised as 'PEC'.
|
| pq |
A specific variant of the 'peculiar' subclass, 'pq' indicates a spectrum that shows signs of nova-like activity, or similarities to the spectra of novae. This suffix is also sometimes used for cataclysmic variables, binary systems in which the transfer of mass onto a white dwarf component can cause periodic nova effects.
|
| q |
A suffix alternative and equivalent to eq, describing a star with an expanding envelope of gas of the kind described by spectra that follow the 'P Cygni profile' (see eq above for further details).
|
| red |
A code applied specifically to the spectra of brown dwarfs, for which a redder infrared spectrum would usually indicate that the object is relatively young in stellar terms. The 'red' prefix is used for brown dwarfs that show this reddening of the spectrum, but for reasons unrelated to their age (typically because of an unusually high dust content in their atmospheres).
|
| s |
An abbreviation for 'sharp', this suffix indicates that absorption lines in a spectrum are unsually narrow. There can be various reasons of this but, typically, sharper absorption lines in a spectrum relate to a more diffuse stellar atmosphere (because narrower lines are due to light interacting with comparatively fewer atoms as it is emitted from the star). Thus 's' spectra are most commonly associated with giant or supergiant stars, which have more diffuse outer shells than main sequence stars.
|
| sd |
Used as a prefix rather than a suffix, 'sd' indicates that a star belongs to the subdwarf class, with low mass, luminosity and metallicity. A prominent example is Kapteyn's Star (VZ Pictoris), classified as either 'sdM1' or 'M1VIp'. The luminosity class 'VI' also represents subdwarfs, so the 'sd' prefix or 'VI' class specifier can be used interchangeably.
|
shell
sh |
Stars with spectra marked as 'shell' or 'sh' are shell stars: hot, rapidly rotating stars of spectral types O, B, A or (more rarely) F. These stars possess a circumstellar disc thrown out from the star due its rapid rotation, and the light from that disc combines with that of the star itself to produce a characteristic spectral pattern. A common subgroup of this type is composed of B-type blue stars with strong emission lines in their spectra (denoted by 'e', as described above), and from their 'B' colouration and 'e' spectral lines they take their name of 'Be stars'.
|
| Si |
This is a use of the 'Si' symbol for silicon. As described under '(elements)' above, a wide range of chemical symbols can be used in this way, but a 'silicon star' represents one of a particular class of chemically peculiar objects. These are usually hot stars of the white A-type or blue B-type, showing abundant levels of silicon in their surface layers, and possessing particularly intense magnetic fields.
|
| ss |
Extending 's' above, 'ss' is used to indicate exceptionally sharp or narrow absorption lines, indicative of either a star with very low density (that is, an exceptionally diffuse giant or supergiant). Sharp lines like this might also indicate that the star is rotating unusually slowly (thus producing the opposite effect of the nebulous lines indicated by the 'n' suffix.)
|
var
v
V |
The suffix 'var' is commonly abbreviated to simply 'v', and describes a star that shows variability within its spectral type. Note that this usage is distinct from the general sense of stellar variability, which can describe a whole range of different effects and driving forces. Rather, as part of a spectral code, 'v' describes a star whose spectral type itself is subject to variability (for example, a star's temperature can change over time, which might affect its spectral type). The variation 'V' (that is, capital 'v') has the same meaning, but is applied exclusively to white dwarf stars.
|
wk
wl
w |
This group of suffixes relate to weak absorption lines a stellar spectrum, especially in relation to those that indicate metals (that is, heavier elements). Weak lines typically describe ancient stars that formed before heavier elements were abundant in the Galaxy. Where absorption lines are particularly weak for a specific element, that element can be specified in the code: for example, 'Hewk' specifies weak absorption lines for (and therefore a relative scarcity of) the element helium ('He').
|
| z |
Finally, the rare 'z' suffix describes hot young stars, usually of the O-type class. Specifically, this 'z' code describes a spectrum showing strong a helium line at 486.6 nanometres, which in physical terms describes an extremely dense star that will, in turn, have a strong gravitational field. These are features of young massive stars, and - because they are still early in their lifecycles - such stars are often found still embedded within the star-forming regions from which they recently emerged.
|
|
|
