Stellar classification

In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using Wien's displacement law; but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.

Morgan-Keenan spectral classification

This stellar classification is the most commonly used. The common classes are normally listed from hottest to coldest, and are:

Class	Temperature	Star Color	Mass	Radius	Luminosity
O	30,000 - 60,000 K	Ultraviolet-violet	60	15	1,400,000
B	10,000 - 30,000 K	Blue	18	15	20,000
A	7,500 - 10,000 K	White-green	3.2	2.5	80
F	6,000 - 7,500 K	Yellow-white	1.7	1.3	6
G	5,000 - 6,000 K	Yellow	1.1	1.1	1.2
K	3,500 - 5,000 K	Yellow-orange	0.8	0.9	0.4
M	2,000 - 3,500 K	Orange-red	0.3	0.4	0.04

* 1 = Sun. Values are averages.

A popular mnemonic for remembering the order is "Oh Be A Fine Girl, Kiss Me" (there are many variants of this mnemonic). This scheme was developed in the 1900s, by Annie J. Cannon and the Harvard College Observatory. The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature. It should be noted that while these descriptions of stellar colors are traditional in astronomy, they really describe the light after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has essentially the colour temperature of a black body of 5780 K; this is a white with no trace of yellow which is sometimes used as a definition for standard white.

The reason for the odd arrangement of letters is historical. When people first started taking spectra of stars, they noticed that stars had very different hydrogen spectral lines strengths, and so they classified stars based on the strength of the hydrogen balmer series lines from A (strongest) to Q (weakest). Other lines of neutral and ionized species then came into play (H&K lines of calcium, sodium D lines etc). Later it was found that some of the classes were actually duplicates and those classes were removed. It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface temperature of the star. The basic work was done by the "girls" of Harvard College Observatory, primarily Cannon and Antonia Maury, based on the work of Williamina Fleming. These classes are further subdivided by arabic numbers (0-9). A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The sun is classified as G2.

Spectral types

• Class O stars are very hot and very luminous, being strongly violet in colour; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as little as 1/32,000th of the total.(LeDrew) O-stars shine with a power over a million times our Sun's output. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines.

Examples: Zeta Puppis, Epsilon Orionis

• Class B stars are extremely luminous and blue. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds. The Orion OB1 association is an entire spiral arm of our Galaxy (brighter stars make the spiral arms look brighter, there aren't more stars there) and contains all of the constellation of Orion. They constituting about 0.13% of main sequence stars--rare, but much more common than those of class O.(LeDrew)

Examples: Rigel, Spica

• Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white or green. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals. They comprise perhaps 0.63% of all main sequence stars.(LeDrew)

Examples: Vega, Sirius

• Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow. These represent 3.1% of all main sequence stars.(LeDrew)

Examples: Canopus, Procyon

• Class G stars are probably the most well known if only for the reason that our Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. These are about 8% of all main sequence stars.(LeDrew)

Examples: Sun, Capella

• Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others like Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals. These make up some 13% of main sequence stars.(LeDrew)

Examples: Arcturus, Aldebaran

• Class M is by far the most common class if we go by the number of stars. All our red dwarfs go in here and they are plentiful; over 78% of stars are red dwarfs, such as Proxima Centauri.(LeDrew) M is also host to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars. The red colour is deceptive; it is because of the dimness of the star. When an equally hot object, a halogen lamp (3000 K) which is white hot is put at a few kilometers distance, it appears like a red star.

Examples: Betelgeuse, Barnard's star

Spectral types for rare stars

A number of new spectral types have been taken into use for rare types of stars, as they have been discovered:

• W: Up to 70,000 K - Wolf-Rayet stars.
• L: 1,500 - 2,000 K - Stars with masses insufficient to run the regular hydrogen fusion process ( brown dwarfs). Class L stars contain lithium which is rapidly destroyed in hotter stars.
• T: 1,000 K - Cooler brown dwarfs with methane in the spectrum.
• C: Carbon stars.

• R: Formerly a class on its own representing the carbon star equivalent of Class K stars, e.g. S Camelopardalis.
• N: Formerly a class on its own representing the carbon star equivalent of Class M stars, e.g. R Leporis.

• S: Similar to Class M stars, but with zirconium oxide replacing the regular titanium oxide.
• D: White dwarfs, e.g. Sirius B.

Class W represents the superluminous Wolf-Rayet stars, being notably different since they have mostly helium instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot stellar winds caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses WN and WC according to the dominance of nitrogen or carbon in their spectra (and outer layers).

Class L stars get their designation from the lithium present in their core. Any lithium would be destroyed in ongoing nuclear reactions in regular stars, which indicates that these objects have no ongoing fusion processes. They are a very dark red in colour and brightest in infrared. Their gas is cool enough to allow metal hydrides and alkali metals to be prominent in the spectrum.

Class T stars are very young and low density stars often found in the interstellar clouds they were born in. These are stars barely big enough to be stars and others that are substellar, being of the brown dwarf variety. They are black, emitting little or no visible light but being strongest in infrared. Their surface temperature is a stark contrast to the fifty thousand kelvins or more for Class O stars, being merely up to 1,000 K. Complex molecules can form, evidenced by the strong methane lines in their spectra.

Class T and L could be more common than all the other classes combined, if recent research is accurate. From studying the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed) then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It’s theorised that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other propylids in the vicinity, stripping them of their gas. The victim propylids will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long (no star below 0.8 solar masses has ever died in the history of the galaxy) then these smaller stars will accumulate over time.

Class R and N stars are carbon stars (red giants thought to reach the end of their life) which run 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.

Class S stars have ZrO lines rather than TiO, and are in between the Class M stars and the carbon stars. Class S stars have their carbon and oxygen abundances almost exactly equal, and both elements are locked up almost entirely in CO molecules. For stars cool enough for CO to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" on the normal main sequence, "leftover carbon" on the C sequence, and "leftover nothing" on the S sequence.

In reality the relation between these stars and the traditional main sequence suggest a rather large continuum of carbon abundance and if fully explored would add another dimension to the stellar classification system.

Finally, the classes P and Q are occasionally used for certain non-stellar objects. Type P objects are planetary nebulae and type Q objects are novae.

White dwarf classifications

The class D is sometimes used for white dwarfs, the state most stars end their life in. Class D is further divided into classes DA, DB, DC, DO, DZ, and DQ. Note the letters are not related to the letters used in the classification of true stars, but instead indicate the composition of the white dwarf's outer layer or "atmosphere".

The white dwarf classes are as follows:

• DA: a hydrogen-rich "atmosphere" or outer layer, indicated by strong Balmer hydrogen spectral lines.
• DB: a helium-rich "atmosphere" or outer layer, indicated by neutral helium spectral lines.
• DQ: a carbon-rich "atmosphere" or outer layer, indicated by atomic or molecular carbon lines.
• DZ: a 'metal'-rich "atmosphere" or outer layer, indicated by calcium II lines.
• 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.

All class D stars use the same sequence from 1 to 9, with 1 indicating a temperature above 37,500 K and 9 indicating a temperature below 5,500 K. [1]

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, Phillip C. Keenan and Edith Kellman of Yerkes Observatory.

This classification is based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature.

Since the radius of a giant star is much larger than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf.

These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.

A number of different luminosity classes are distinguished:

• 0 hypergiants (later addition);
• Ia most luminous supergiants;
• Ib less luminous supergiants;
• II bright giants;
• III normal giants;
• IV subgiants;
• V main sequence stars (dwarfs);
• VI subdwarfs (rarely used);
• VII white dwarfs (rarely used)

Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant.

UBV system

The UBV system, also called the Johnson system, is a photometric system for classifying stars according to their magnitude. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.