Spectral Classification of Stars

Also Known as Spectral Types: O B A F G K M

Below is an image of the license plate of an obviously enthusiastic amateur astronomer, Bill Dellinges, who is a member of the Superstition Mountain Astronomical League. I also am a member of this group, which generally is composed of persons who live at the eastern edge of the Phoenix metropolitan area, and more specifically in the communities of Gold Canyon, Apache Junction and eastern Mesa. The photo was supplied by Bill Dellinges and is used with his permission.

Using my homemade star spectrograph, I obtained the spectrum of at least one star of each spectral type. What follows is an example stellar spectrum from each of the seven spectral types, followed by the Luminosity Class and, if needed, the Spectrum Descriptor and any other suffixes. These terms will be described after all seven spectral types are presented. Each image is presented as it appeared full frame on my Canon T4i digital camera that I used on my homemade prism star spectrograph. Because my star spectrograph uses a prism rather than a diffraction grating, my spectral images are distinctly non-linear in their wavelength dimension, and this difference between linear and non-linear instrumental response will be explored near the end of this web page.

Above: the spectrum of an O9 Ib star, Alnitak, in the belt of Orion

Above: the spectrum of a B7 V star, Regulus, the front foot of Leo the lion

Above: the spectrum of an A1 V star, Sirius, the brightest star in Canis Major and, indeed, the brightest star in the entire sky

Above: the spectrum of an F5 IV-V star, Procyon, the brightest star in Canis Minor

Above: the spectrum of a G8 III star, Capella, the brightest star in Auriga

Above: the spectrum of a K5 III star, Aldebran, the reddish eye of Taurus, "the Bull"

Above: the spectrum of an M2 Iab star, Betelgeuse, a reddish star at the right shoulder of Orion and the brightest star in that constellation.

The above image is a rotated and scaled composite of the previous seven stellar spectra. This allows an easier comparison of the relative strengths of individual absorption lines that appear in adjacent spectra.

Explanation of symbols used, based in whole or part on the book A Spectroscopic Atlas of Bright Stars: A Pocket Field Guide by Jack Martin

Spectral Type - major characteristics of optical spectrum. Each spectral type (OBAFGKM) is subdivided into 10 numeric catagories (0123456789), so our sun is a G2 spectral type star and Sirius, the brightest star in the sky, is an A1 spectral type star.

O --- singly ionized helium (HeII) in emission or absorption

B --- neutral helium (HeI) lines in absorption

A --- neutral hydrogen (HI) lines in absorption

F --- once ionized calcium (CaII) lines in absorption

G --- neutral metallic lines and once ionized calcium (CaII) lines in absorption

K --- metallic lines in absorption

M --- some molecular bands of TiO in absorption

Spectral Type - temperature of stellar atmosphere (and color of star):

O --- 28,000 - 50,000 degrees K (color: blue)

B --- 9,900 - 28,000 degrees K (color: bluish white)

A --- 7,400 - 9,900 degrees K (color: white)

F --- 6,000 - 7,400 degrees K (color: yellowish white)

G --- 4,900 - 6,000 degrees K (color: yellow)

K --- 3,600 - 4,900 degrees K (color: orange)

M --- 2,000 - 3,600 degrees K (color: red)

C-R --- 2,800 - 5,100 degrees K (carbon star of G/K) (color: deep red)

C-N --- 2,600 - 3,100 degrees K (carbon star of K/M) (color: deep red)

Luminosity Class

Ia --- Bright supergiants

Ib --- Supergiants

II --- Bright Giants

III --- Giants

IV --- Sub Giants

V --- Main Sequence

VI --- Sub dwarfs

VII --- White dwarfs

Luminosity class suffixes for emission lines:

e --- Emission line star

em --- Emission by metal lines

ep --- Pecular emission

eq --- P Cygni emission

er --- Reversed emission

f --- Helium and Nitrogen emission

Other suffixes:

k --- Interstellar lines

m --- Strong metallic absorption

n --- Nebulous diffuse lines

nn --- Very diffuse lines

p --- Chemically pecular spectrum

s --- Sharp lines

si --- Silicon star

v --- Variation in spectrum

wk --- Weak lines

Linear vs non-linear spectra portrayal

The two spectra below are both of G spectral class stars: the upper one is of the Sun (a G2 star) and the one beneath it is of the bright star Capella (a G8 star). Both images were made with my Canon T4i digital camera. I made the solar spectrum with my homemade solar spectrograph, which uses a commercial mirrored diffraction grating that I obtained from eBay, so its spectrum is linear, or very nearly linear. I made the one beneath with my homemade prism star spectrograph, so its spectrum is distinctly non-linear. The differences between these two spectra are due primarily to the differences in the linearity and non-linearity of the instrumental responses rather than the slight differences in their spectral types.

Relative to the upper linear solar spectrum, the lower non-linear stellar spectrum expands the blue end (left) of the spectrum and contracts the red end (right). Absorption lines present in both spectra are likewise displaced accordingly.

For additional images that illustrate the differences of portraying optical spectra in a linear vs non-linear portrayal, see the following links:

For information about the Superstition Mountain Astronomical League, see the following:

For information about the East Valley Astronomy Club, see the following: http://evaconline.org/

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