Stellar Classification

Abstract

Notes about the spectral classification of stars.

Keywords: stellar classification, spectral type, stellar type, star type, Harvard type

Introduction

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.

History

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.

Spectral Types

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

• Type 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.
• Type 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.
• Type 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.
• Type 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).
• Type 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.
• Type 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).
• Type 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.

Type:	O	B	A	F	G	K	M
Mass (M/M¤)	15 – 90	2 – 17	1.4 – 2.1	1 – 1.4	0.84 – 1.15	0.45 – 0.8	0.075 – 0.5
Luminosity (log L/L¤)	~5.5	~3	~1.25	~0.5	~0	~ -0.5	~ -2
Colour	blue-white	blue-white	white	yellow-white	yellow	orange	red
Temperature (K)	>20,000	10 – 20,000	7 – 10,000	6 – 7,000	5 – 6,000	3,500 – 5,000	2,000 – 3,500
Spectral Lines	ionised He	neutral He, H	neutral H (Balmer lines)	ionised Ca (Ca II), neutral H, metals	Ca II, neutral metals (Fe I)	neutral metals (Ca, Fe), molecular bands	TiO bands, neutral Ca
Examples	z Pup i Ori A	b Ori (Rigel) b Cen	a CMa (Sirius) a Lyr (Vega)	a Car (Canopus) a CMi (Procyon)	Sun a Aur (Capella) a Cen A	a Boö (Arcturus) a Tau (Aldebaran)	a Sco (Antares) a Ori (Betelgeuse)
Table 1:  Typical physical properties of main sequencestars by spectral type (various sources).

Subtypes

“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:

• Type S stars are similar to M stars in most respects, but zirconium oxide replaces titanium oxide. (Barium and yttrium are also seen.) The best known examples are T Camelopardalis and U Cassiopeiae.
• Type C stars, known as carbon stars, overlap with late G, K and M stars in most respects but are distinguished by intense molecular absorption bands, most prominently in the blue wavelengths, which indicate compositional differences, typically an unusually high concentration of carbon. The blue absorption lends these stars an unusually red appearance.
  => 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.
• Type WR stars, 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.
• Type 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, and a number of subclasses are recognised, as follows.

Class	General Features	Examples
Ia-0	Extreme, luminous supergiants	r Cas RW Cep
Ia	Luminous supergiants	a Ori b Ori
Ib	Less luminous supergiants	a Car a Sco
II	Bright giants	q Lyr a UMi
III	Normal giants	a Tau b Car
IV	Sub-giants	a CMi A h Boö
V	Main sequence (dwarfs)	a Cen A Sun
VI, sd	Sub-dwarfs	Prox Cen
D	White dwarfs	a CMa B 40 Eri B
Table 2: The luminosity classes (after Carroll & Ostlie 1996, p. 248).

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:

e	indicates the presence of emission lines (hydrogen emission for O stars)
[e]	indicates the presence of “forbidden” emission lines
f	indicates the presence of N III emission lines in the spectrum at 4634, 4640, and 4641 Å of O stars
m	indicates the presence of metallic lines
n	indicates the presence of nebulous (diffuse) lines
p	indicates the presence of peculiar lines
q	indicates the presence of both red- and blue-shifted lines; interpreted to indicate an expanding shell of gas or dust around the star
v	spectrum is variable
wk	weak lines
Table 3: Suffixes for stellar classes (various sources).

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.

References

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.