Stellar Classification

History

Stars are classified by their spectra. The modern system of classification originated from a series of projects begun around 1886, at the Harvard College Observatory, directed by Edward Pickering.  The work was funded from the estate of the American astronomer Henry Draper, and performed by a corps of women (and the commonly held belief is that women were chosen because they would work for less pay than men). Three in particular made significant intellectual contribution to the projects: Williamina Fleming, Antonia Maury – who happened also to be Draper’s niece – and Annie Jump Cannon. The result was the Henry Draper Catalogue, published in nine volumes beginning in 1918.

Stars were categorised into a number of classes, distinguished by the strength of the spectral lines of different elements, beginning with hydrogen, and were originally assigned letters which ran in order.  Thus, type A spectra show the strongest hydrogen lines. It was Cannon who took a more holistic view and rearranged the formerly alphabetic sequence into the now famous OBAFGKM sequence which reveals a smooth transition from type to type for all elements, and a continuous temperature gradient.

Development of the final two-parameter system, which has remained in current use, were devised by W.W. Morgan and P.C. Keenan in about 1950.

Morgan-Keenan (MK) Spectral Types

Listed from hottest to coolest, the MK spectral types together with their main properties are:

• O stars are massive, very luminous and blue-white, indicating extremely high temperatures (~20,000 K and up). Type O spectra show lines of ionised helium, nitrogen and oxygen. Good examples are Zeta Puppis (Naos) and Iota Orionis A – the star at the ‘tip’ of Orion’s sword.
• B stars are also massive, luminous, blue-white and hot (up to ~ 20,000 K). Their spectra display strong helium lines. Examples are Beta Orionis (Rigel), Beta Centauri and Beta Crucis.
• A stars are luminous, white, with temperatures around 10,000 K. Helium lines are absent from type A spectra, but hydrogen is strongest in this type. Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega) are examples.
• F stars are yellow-white, indicating temperatures around 7,000 K. Their spectra exhibit weaker hydrogen and strong calcium.  Two well-known examples are Alpha Carinae (Canopus) and Alpha Canis Minoris (Procyon).
• G stars, of which the sun is one, are yellow with temperatures around 5,000 to 6,000 K. Hydrogen lines are weaker again; many metals are present. In addition to the sun, another example is Alpha Centauri A.
• K stars are orange, around 4,000 to 4,700 K, and display faint hydrogen lines, strong metallic lines, and some hydrocarbon molecular bands in their spectra. Examples are Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran).
• M stars include the red dwarfs and red giants. They are only very weakly luminous, red, with temperatures around 2,500 to 3,000 K. Their spectra are characterised by many strong metallic lines and also wide titanium oxide bands. Alpha Scorpii (Antares) and Alpha Orionis (Betelgeuse) are the only two first-magnitude examples.

Rare Spectral Types

In addition to the commonly occurring types listed above, several others have been recognised over the years:

• S stars are similar to M stars in most respects, but zirconium oxide replaces titanium oxide. The best known examples are T Camelopardalis and U Cassiopeiae.
• C stars, known as carbon stars, overlap with late G, K and M stars in most respects but are distinguished by compositional differences, typically an unusually high concentration of carbon.
  => R is the carbon analogue of the K stars. These stars are very rare.  Two examples are S Camelopardalis and RU Virginis.
  => N is the carbon analogue of the M stars. One of the best examples is R Leporis, "Hind’s Crimson Star." Others are Y Canum Venaticorum ("La Superba") and V Hydrae.
• WR, comprising several subtypes (all beginning with a W) are the Wolf-Rayet stars.  These are now thought to be the naked cores of massive stars from which hot stellar winds have stripped off the overlying hydrogen layers. They are very luminous (10⁵ to 10⁶ L¤), extremely hot (up to ~50,000 K), with spectra characterised by strong emission bands of ionised elements, and dominated by helium rather than hydrogen. An example is Gamma² Velorum.
• D are the white dwarfs, the cooling naked cores of the most highly evolved stars such as Wolf-Rayet objects. None is bright enough to observe with the naked eye. The most famous example is Sirius B, but a better object for amateur viewing is undoubtedly Omicron² [= 40] Eridani B, the only genuinely easy white dwarf for amateur instruments.

Luminosity Classes

A luminosity class, designated by a Roman numeral, is appended to the Harvard spectral type. In effect, these correlate with the stage of the star’s evolution, and its size:  All else being equal, larger stars are brighter. Six classes, seven including the white dwarfs, are recognised.

Further Refinements

The classification of the spectral types of stars has become more complicated over the years as astronomers have discovered interesting features that they wanted to include. An early refinement was to add a numerical modifier to subdivide the main spectral classes. Thus, class B is sub-divided into B0 (hottest) through B9 (coolest), for example. The scale is theoretically open-ended though, in practise, no star hotter than O3 has been confidently reported.

Various suffixes are sometimes added to create further refinements. Those most commonly encountered are:

References

Carroll, Bradley W.; Ostlie, Dale A. (1996): Introduction to Modern Astrophysics. Addison Wesley.