A note on the spectral atlas and spectral classification

This spectral atlas contains a sample of the standard spectral type stars, peculiar stars, variable stars, and some special stars. The atlas is divided into pages for each spectral type, with each page containing a short description of the stellar type, characteristic spectral features, and a brief physical explantaion. For the standard spectral types, the spectra are sorted into luminosity classes, and displayed in increasing sub-type (decreasing temperature) for each luminosity class.

OBAFGKM and more. Each spectral type is divided into 10 subclasses, A0, A1, A2, ...A9 etc. The spectral types and sub-classes represent a temperature sequence, from hotter (O stars) to cooler (M stars), and from hotter (subclass 0) to cooler (subclass 9). The temperature defines the star's "color" and surface brightness.

Spectral Type Surface Temperature Distinguishing Features
O > 25,000K H; HeI; HeII
B 10,000-25,000K H; HeI; HeII absent
A 7,500-10,000K H; CaII; HeI and HeII absent
F 6,000-7,500K H; metals (CaII, Fe, etc)
G 5,000-6,000K H; metals; some molecular species
K 3,500-5,000K metals; some molecular species
M < 3,500K metals; molecular species (TiO!)
C < 3,500K metals; molecular species (C2!)

Stars are also classified by luminosity class. Luminosity classes are determined from spectral features and photometric measurements, coupled with information regarding the distance to the star and theamount of extinction of the starlight from interstellar material. The luminosity class designation describes the size (gravitational acceleration in photosphere) of a star from the atmospheric pressure. For larger stars of a given spectral type, the surface gravity decreases relative to what it was on the main sequence, and this decreases the equivalent widths of the absorption lines.

Luminosity Class Description Comments
0 Hypergiants extreme
Ia Supergiants! large and luminous
Ib Supergiants! less luminous than Ia
II Bright Giants  
III Giants  
IV Sub-Giants  
V Dwarfs Main Sequence
sd Sub-Dwarfs  
D White Dwarfs  

Peculiar features. Many stars have peculiar features, their spectral types are coded with additional designations.

Peculiarity Code Description
comp composite spectrum
e emission lines present
f NIII and HeII emission (O stars)
m enhanced metal features
n broad absorption features
nn very broad absorption features
neb nebular features present
p other peculiarity
s very narrow absorption lines
sh shell star
var variable spectral features
wl weak features
: uncertainty

All of the spectra were obtained with the Mt. Hopkins Whipple Observatory 60" Telescope and FAST spectrograph. Spectrograph set-up of 300gpmm grating and 3" slit, yielding 6Å resolution across the entire optical spectrum of ~3650Å to ~7650Å. Permission to present the spectra kindly allowed by various PI's.

Simplified Stellar Evolution For Single Stars (post proto-star collapse):

Mass: 0.8-11 Solar Masses
Main sequence star (B, A, F, or G) > Red giant with He core >
Red supergiant with carbon-oxygen core > Planetary nebula with central star >
White Dwarf

Mass: 11-50 Solar Masses
Main sequence star (O or B) > Red or blue supergiant with He core >
Red supergiant with iron core > Type II Supernova > Neutron Star

Mass: > 50 Solar Masses
Main sequence O star > Wolf-Rayet Star >
Type Ib Supernova > Black Hole