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Notes about the spectral classification of stars.
Keywords: stellar classification, spectral type, stellar type, star type, Harvard type
The different wavelengths of light coming from a star – its “spectrum” (plural spectra) – determine its colour. In extreme cases, say bluish Zeta Puppis or red Antares, the colour differences are obvious to the naked eye. Observers of double stars often enjoy studying the contrasting colours of their quarry. Professionally, however, stellar spectra are studied using specialised instruments.
Stars are classified by their spectra. “In general, the higher the star’s temperature the simpler is the absorption spectrum and, though stars consist mainly of hydrogen and helium, the lines of these elements rarely dominate, either in emission or in absorption spectra. The spectra are simple in hot stars because the electrons are stripped from their atoms and the resulting positive ions do not absorb visible light. With cooler stars, elements and simple compounds can form in stellar atmospheres. This is what produces the complexity of the absorption spectra of stars like the Sun and has a much more marked effect on the perceived colour of very cool stars such as Antares and Betelgeuse” (Malin & Frew 1995, p. 33).
When we look into the night sky, we observe literally millions of hot, bright, “early” type stars. This is the subjective reality of our eyes. Although the cool, dim “late” (K and M, see below) stars outnumber all of the earlier spectral types combined, many times over, we see only a few giants: Antares, Aldebaran, Betelgeuse…. Even the brightest G type star in the sky, Alpha Centauri, is not so conspicuously different from its neighbours as to invite comment. So, while there is a spirit of truth in the commonly repeated claim that the Sun is an “average” star, in fact it is one of the 5% of largest, hottest, and brightest of all stars.
Although Angelo Secchi first began to study stellar spectra as early as the 1860s, 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 “types”, 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 two-parameter system, incorporating both spectral type and luminosity, and which remains in current use, was devised by W.W. Morgan and P.C. Keenan in about 1950.
Listed from hottest to coolest, the Harvard spectral types together with their main properties are:
“With greater understanding and better instrumentation, the main classes [originally] defined by Annie Cannon have been further subdivided by the addition of a number…. The temperature difference between classes is very large for the hot O and B stars, so for these types the numerical subdivisions are [often] further subdivided by a decimal. Alnitak, z Ori, is thus an O9.5 star” (Malin & Frew 1995, p. 33).
Rare Spectral Types
The commonly occurring types, listed above, account for around 90% of stars. As in many sciences, the study of the unusual helps to better understand the normal, also, so the unusual stars which do not conform to one of the common categories, are also of great interest. The following additional types have been recognised over the years:
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, and a number of subclasses are recognised, as follows.
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:
Usage – Putting it together
The hot supergiant, Zeta Puppis, is type O4I(n)f. Thus, it belongs to Harvard spectral type O, subtype 4, luminosity class I, and exhibits diffuse spectral lines and also N III emission lines.
At the opposite end of the main sequence lies Alpha Orionis, or Betelgeuse. Betelgeuse is a variable red supergiant; the luminosity varies by a small amount over an irregular period. The spectrum also changes irregularly, approximately pacing the luminosity changes. The spectral type quoted for this star is not always consistent, but M1-2Ia-Iab is a good stab. The Harvard spectral type is therefore M, subtype 1 to 2, with the luminosity class varying from Ia to something intermediate between Ia and Ib.
Carroll, B.W.; Ostlie, D.A. 1996: Introduction to Modern Astrophysics. Addison Wesley.
Malin, D.; Frew, D. 1995: Hartung’s astronomical objects for southern telescopes (2nd ed). Cambridge University Press: 1-428.
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