The Classification of Stellar Spectra

The Classification of Stars

A Short History of Stellar Typing

William Wollaston discovered in 1802 that the solar spectrum (the rainbow) produced by a prism possessed a series of dark lines superimposed opon it, which he attributed to natural boundaries between colors. In 1814, Joseph Fraunhofer made careful measurements of the solar spectrum and found not fewer than 600 dark lines. Capitalizing on the 1859 work of Gustav Kirchhoff and Robert Bunsen (of burner fame), Sir William Huggins, in 1864, matched some of these so-called "Fraunhofer lines" in spectra from the Sun and other stars with the colors emitted by known gaseous substances energized to emit their line-spectra. With this simple experiment, Huggins conclusively showed that stars are made of the same materials found on Earth, rather than mysterious or exotic substances.

Despite these important discoveries, the systematic classification of stars by their spectral features wasn't undertaken until early in the 20th century. The current spectral classification scheme was developed by Henry Draper at Harvard Observatory in 1872. After his death, the majority of the classification work was done by Annie Jump Cannon from 1918 to 1924 (with important contributions from Henrietta Swan Leavitt, Antonia Maury -- who, in 1943, was given the Annie J. Cannon Award in Astronomy, and Williamina Fleming ) under the direction of Harvard College Observatory director Edward Pickering. In her 40 year carrier, Annie Cannon classified and cataloged nearly 500,000 stars. Her original scheme used capital letters running alphabetically but in subsequent revisions, as stellar evolution and typing has become better understood, the basic classification scheme has been reorganized and reduced to eight letters - O, B, A, F, G, K, M, and C.

The modern stellar classification scheme is based on spectral absorption or emission lines, which are sensitive mostly to the star's surface temperatures, rather than differences in gravity, chemical composition, or luminosity. The important spectral lines are 1) the so-called hydrogen Balmer (visible light) lines, 2) lines of neutral and singly ionized helium, 3) iron lines, 4) the doublet of ionized calcium, 5) the molecular absorption band due to the CH molecule, 6) the neutral calcium line, 7) assorted metal lines, and 8) the absorption bands of titanium oxide.

Standard Stellar Types (O, B, A, F, G, K, M)

While it might appear that the differences in stellar spectra indicate differences in stellar chemical compositions, they usually reflect only differences in surface temperatures. Except during dredge-up events, little mixing occurs between the stellar core and stellar atmosphere.

Ordered from highest to lowest temperature, the eight main stellar types are O, B, A, F, G, K, M, and C (plus the asymptotic giant branch classifications R, N, and S). Figure 1 shows, qualitatively, the relationship of stellar surface temperature and the spectral characteristic which predominates the stellar spectrum. The spectral characteristics of these types are summarized in Table 1.

Figure 1. Relationship of predominate spectral lines visible and stellar class and stellar surface temperature. (Adapted from Abell, Morrison, Wolf Realm of the Universe, 4th Ed. Saunders, 1988.)

Type	Color	Approximate Surface Temperature (K)	Main Characteristics	Examples
Table 1. Characteristics of the 8 Basic Star Classifications


O	Blue	> 25,000	Singly-ionized helium lines, doubly- ionized nitrogen, triply-ionized silicon. Hydrogen lines apparent but very weak. Strong ultraviolet continuum.	Alnitak (z-Ori), 10 Lacertae S Monocerotis
B	Blue	11,000-25,000	Neutral helium lines, singly/doubly- ionized silicon, singly-ionized oxygen and magnesium. Hydrogen lines apparent weakly (but stronger than in O-type stars)	Rigel (b-Ori), Spica (a-Vir)
A	Blue	7,500-11,000	Hydrogen lines at maximum strength for A0 stars. Lines of many singly-ionized metals (e.g., Mg, Ca, Fe, Ti) apparent. Lines of some neutral metals weakly present.	Sirius (a-CMa), Vega (a-Lyr), Alioth (e-UMa)
F	Blue to White	6,000-7,500	Hydrogen lines strong, but weaker than in A-type starts. Singly-ionized metals (e.g., Ca, Fe, Cr). Neutral metallic lines become noticeable (e.g., Fe, Cr).	Canopus (a-Car), Procyon (a-CMi), Polaris (a-UMi)
G	White to Yellow	5,000-6,000	Solar-type spectra. Hydrogen lines weaker than in G-type stars. Absorption lines of neutral metallic atoms and ions (e.g., singly-ionized calcium) predominate. Strong CH-radical band.	Sun, Capella (a-Aur), a-Centauri
K	Orange to Red	3,500-5,000	Neutral-metal lines dominate. CH-radical band remains but weakening. Weak in blue end of continuum.	Arcturus (a-Boo), Aldebaran (a-Tau), Dubhe (a-UMa)
M	Red	< 3,500	Strong lines of neutral metals predominate. Molecular bands of titanium oxide noticeable, perhaps dominating..	Betelgeuse (a-Ori), Antares (a-Sco), Gacrux (g-Cru)
C	Red	2000-5400K	CARBON STARS. Strong bands of molecular carbon, CN, CH, or other carbon compounds; no TiO.
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Subtypes

Within each of the original seven classes, Cannon assigned subclasses numbered 0 to 9. A star midway through the range between F0 and G0 would be an F5 type star. Smaller subtype numbers are hotter stars in the class. The Sun is a G2 type star with a surface temperature of 5800 K. Figure 2 shows spectral characteristics of several different star classes with subtypes.

Figure 2. Stellar spectra with absorption line spectra characteristic of elements in the stellar atmospheres.
(From R. Bell, University of Maryland, and M. Briley, University of Wisconsin, Oshkosh.)

Luminosity Classes

To further complete the classification of a star, more than surface temperature and spectral features must be considered. A more complete classification also includes the luminosity of the star and its location on the Hertzsprung-Russell diagram. The so-called Yerkes classification (or MKK, from the initials of the authors W.W. Morgan, P.C. Keenan, and E. Kellman) uses the shape and nature of selected spectral lines to identify the size and evolutionary history of the star. The Yerkes scheme uses six luminosity classes labeled in Roman numerals from I-V (plus two rarely used classes, VI and VII).

Table 2. The Eight Luminosity Classes

Luminosity
Class

Characteristic

Examples

Most luminous supergiants

Rigel (b-Ori) B8Ia,

Less luminous supergiants

Betelgeuse (a-Ori) M2Ib,
Antares (a-Sco) M1Ib
Alnitak (z-Ori) O9.5Ib

Luminous giants

Albireo (b₁-Cyg) K3II
Tarazed (g-Aql) K3II

III

Normal giants

Arcturus (a-Boo) K2III,
Aldebaran (a-Tau) K5III,
Gacrux (g-Cru) M4III

Subgiants

Procyon (a-CMi) F5IV,

Main sequence stars

Sol (our sun) G2V,
Dubhe (a-UMa) F7V,
Spica (a-Vir) B1V

Subdwarfs

Rarely used

VII

White dwarfs

Rarely used

Other Classification Nomenclature

A star's spectrum reveals virtually every measurable characteristic of the star. Often, the spectrum reveals information about the star which cannot be simply classified as shown above. Thus, stellar peculiarities are indicated in the form of lowercase letter added to the end of a spectral type.


Table 3. Selected Spectral Peculiarity Codes
Code	Characteristic	Example
c	sharp lines
comp	Composite spectrum; two spectral types are blended, indicating that the star is an unresolved binary.	Dubhe (a-UMa) F7V comp Capella (a-Aur) M1 comp
d	main-sequence dwarf star
e	Emission lines are present (usually hydrogen).	Cor Caroli (a₂-CVn) A0spe
em	Emission lines of metals are present (or me)	56-Cygni
ep	Peculiar emission lines are present (or pe)	Cor Caroli (a₂-CVn) A0spe
f	Emission line of helium and neon in O-type stars
g	Giant
k	Interstellar lines present
m	Abnormally strong "metals" (elements other than hydrogen and helium) for a star of a given spectral type; usually applied to A stars.	Castor (a-Gem) A2Vm Sirius (a-CMa) A0m
n	Broad ("nebulous") absorption lines due to fast rotation.	g-Camelopardalis A2IVn
nn	Very broad lines due to very fast rotation.	9-g-Trianguli A1Vnn
neb	A nebula's spectrum is mixed with the star's.
p	Unspecified peculiarity, except when used with type A, where it denotes abnormally strong lines of "metals".	Arcturus (a-Boo) K2IIIp Cor Caroli (a₂-CVn) A0spe
s	Very narrow ("sharp") lines.	Cor Caroli (a₂-CVn) A0spe
sh	Shell star (B to F main sequence star with emission lines from a shell of gas).
var	Varying spectral type.	Kochab (b-UMi) K4IIIvar
wd	White dwarf
wk	Weak lines (suggesting an ancient, "metal"-poor star)

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Asymptotic Giant Branch Stars (R, N, and S)

The asymptotic giant branch (AGB) phase corresponds to the short stage in a star's evolution during which intermediate-mass stars attain their highest luminosities (become the largest they ever will) but simultaneously experience extreme mass loss. It is during this time in which they rapidly progress towards the planetary-nebula phase and the final cooling to white dwarfs. AGB stars are most revealing for critical issues in stellar structure, nucleosynthesis, and evolution. The outer atmospheres of AGB stars are the major factories of cosmic dust (e.g., silicates).

After reducing the hydrogen supply in its core to a point where hydrogen fusion to helium can can no longer be sustained, fusion of hydrogen will continue in a shell surrounding the core. The core will essentially be a helium white dwarf enveloped by a hydrogen-burning (fusion) shell. Helium produced in the shell surrounding the core will combine with the degenerate core until gravitational compression heats the core sufficiently to initiate helium fusion (the helium flash). Fusion of helium into carbon (via the triple-alpha process) will continue in the core (with a hydrogen-burning shell around the helium-burning core) until the core helium supply is depleted and what remains is an inert carbon-oxygen white dwarf core surrounded by an inner shell of helium fusion (triple-alpha) which is itself surrounded by an outer shell of hydrogen fusion. This double-shell burning phase is known as the asymptotic giant branch (AGB) stage, a name derived from the evolving star's location on the Hertzprung-Russell diagram. A star in the AGB phase is likely become a relatively short-lived red supergiant.

Stars in the AGB phase of stellar evolution have proven difficult to model. A significant problem under intense scrutiny is one of heavy element dredge-up from the carbon-rich core. One current theory is that convection cells in the fusion layers form, which reach from the core to the hydrogen fusion layer and provides the necessary mechanism for material deep within the star to be dredged up to the surface. This may explain stellar types which have cooler surface temperatures (<3200 K) than M stars but have spectral features as if their outer atmospheres had been enriched with heavier elements. These are the R, N, and S types.

R and N type stars

A number of giant stars appear to be K or M type stars, but also show significant strong spectral features of carbon compounds. They are often referred to as C-class stars or "carbon stars". The most common spectral features are molecular absorption bands from C₂, CN, and CH. High core temperatures are necessary to produce carbon but low surface temperatures are necessary for the carbon compounds to survive. These carbon compounds characteristically absorb strongly in the blue region of the visible light spectrum giving R- and N-type giants a distinctive red color. R stars are those with hotter surfaces which otherwise resemble K-class stars. S-type stars have cooler surfaces and more closely resemble M-class stars. The abundance of carbon, nitrogen, and oxygen in these stars is four to five times higher than in normal stars.

S type stars

The photospheres of S-type stars appear to have enriched abundances of s-process elements. The so-called s-process (s = slow) produces isotopes of elements which have been formed via neutron capture (thus changing the isotope of the element) followed by b-decay (increasing the atomic number of the element by one and decreasing the neutron number by one). In the s-process, the kinetics of neutron capture are slower than b-decay. The s-process is one stellar nucleosynthetic mechanism by which elements larger than iron (atomic number 26) may be produced. In contrast, the r-process (r = rapid) occurs when there is a sufficient number of free neutrons such that the rate of neutron capture by the nucleus is faster than b-decay. The r-process is more pertinent to nucleosynthesis during supernovae rather than in AGB stars.

For the s-process to operate -- the predominant process in AGB stars -- a source of neutrons is required. Sources of neutrons include ²²Ne (via ²²Ne(a,n)²⁵Mg) and during the production of ¹⁶O in the CNO-cycle (¹³C(a,n)¹⁶O). However, these two mechanisms will not provide a sufficient number of neutrons to sustain the s-process but other process during dredge-up can provide the additional neutrons.

In addition to the usual lines of titanium, scandium, and vanadium oxides characteristic of M-type giants, S-type stars show heavier elements such as zirconium, yttrium, and barium. Virtually all S-type stars are variable.