The Webfooted Astronomer: Secrets of the Harvard Classification Revealed

The Webfooted Astronomer - February 2000

The Secrets of the Harvard Classification Revealed

By Dave Irizarry

WHEN reading astronomy articles relating to stars and stellar phenomena, the reader often encounters a stellar classification system known as the Harvard Classification. This system establishes a notation used to designate the spectral class of a star and hence its temperature. Using this system, it's possible to describe the overall characteristics of a star using a very concise notation. The format used in the Harvard Classification is as follows:

[prefix] [letter code] [numeric code] [suffix]

The prefix depicts spectral line features of a star and can consist of the any of the following codes:

c - The star exhibits very sharp spectral lines. Characteristic of supergiant stars such as Deneb
g - The star exhibits the spectral features of a giant star
d - The star exhibits the spectral features of a dwarf star

The letter code describes the spectral class of the star. Valid codes are as follows:

W - Very hot stars called Wolf-Rayet stars with surface temperatures up to 106K. Spectral features reveal the presence of show ionized helium, carbon, nitrogen and oxygen. These lines arise in an atmospheric envelope around the star expanding at speeds of 2000 km/sec.
WC - Wolf-Rayet stars with an apparent overabundance of carbon and oxygen
WN - Wolf-Rayet stars with an overabundance of nitrogen
O - Average temperature 30,000 K. These stars appear blue hot and show the presence of highly ionized elements such as helium, nitrogen, oxygen.
B - Temperature range is 13,000 K to 20,000 K. These stars exhibit a bluish–white color and show the spectra of neutral helium, ionized silicon, ionized oxygen and ionized magnesium. Strong spectral lines of hydrogen begin to appear a B0 and increase in strength as the spectral class reaches B9. Example: Rigel in Orion.
A - Temperatures average around 10,000K. At A0, spectral lines of helium and other ionized elements are weaker. The Balmer lines of Hydrogen dominate the spectrum. K-line of Calcium begins to appear and strengthen throughout the range of this class. Examples of A type stars are Sirius, Vega, and Altair.
F - Yellow–white stars with a temperature range of 7,000K to 9,000K. Spectral lines due to hydrogen begin to weaken, the K line of calcium continues to get stronger, and characteristic spectral lines (neutral and ionized) of such as Fe, Mn, and Na begin to appear.
G - Yellow stars, such as our sun, with surface temperatures ranging from 5,200 to 6,000K. The K line of calcium is very strong, while spectral lines due to hydrogen become weaker. Lines due to metals also increase in intensity.
K - Orange stars with surface temperatures ranging from 4000K to 5100K. The spectra of these stars exhibit strong lines due to Calcium and neutral metals. Lines due to titanium oxide begin to appear. Examples are Arcturus and Aldebaran.
M - With temperatures averaging 3000K, the spectra of these stars show strong molecular bands due to titanium oxide. Lines due to other metals (such as FeI) are also present.
N, R, S - The temperature of stars in this class range from around 2300K to 2600K. N type stars possess spectra that exhibit bands due to carbon compounds. R and N stars are sometimes combined into one class: C. Stars of class S exhibit complex spectra containing bright hydrogen lines and molecular bands due to zirconium oxide.

The numeric code places the star more precisely within the spectral sequence. The range of this numeric code is from 0 to 9 and indicates how far to the left or right a star is in its spectral sequence. For instance a G9 star is much closer in spectral type to a K0 star than a G2 star is. A portion of the spectral sequence is shown below:

... F9 – G0 – G1 – G2 – G3 ... G8 - G9 – K0 – K1...

Suffixes allow for the additional description of the stellar spectrum. Codes are listed below:

n - These stars exhibit broad, shallow lines indicating high stellar rotation speeds
s - Sharp spectral lines (not as fine as the lines designated by the prefix c)
e - These stars show emission lines in their spectra
v - Stars with variable spectra
ev - Stars with spectral emission lines that exhibit variability
k - Spectra with interstellar absorption features
p - Peculiar spectrum – strong spectral lines due to metals
pq - Peculiar spectrum, similar to the spectra of novae

Sometimes we see a luminosity classifier added to this classification scheme. These roman numerals are called the Morgan-Keenan Luminosity Classes. They are not part of the original Harvard Classification scheme, but were added by W.W. Morgan and P.C. Keenan (Yerkes Observatory). The classes and their meanings are listed below:

Class	Subclass	Name
I	Ia, Iab, Ib	Supergiant
II	IIa, IIab, IIb	Bright Giant
III	IIIa, IIIab, IIIb	Giant
IV	IVa, IVab, IVb	Subgiant
V	Va, Vab, Vb	Dwarf
VI	VI	Subdwarf

So, using the system outlined above, it is possible to describe a star using a concis and articles, so familiarizing yourself with the Harvard Classification System will enrich your study sessions.

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