EXAMPLES FOR SPECTRAL CLASSES 
 ---------------------------
 John Pazmino
 NYSkies Astronomy Inc
 www.nyskies.org
 nyskies@nyskies.org
 2009 July 5 
Introduction
 ----------
    Following my article 'Stellar spectrometry' one of the frequent 
inquiries related to specimina of stars for each spectral and 
luminosity class. 'Which star can I point to as a good example of a B7 
IV, G4 III, F5 II, star?'. 
    The inquiry is a good one, because at starviewing sessions 
spectrometry is a common topic. After a talk on the subject it would 
be helpful to show stars illustrating the various classes. 
    I went back to the new book 'Stellar spectrum classification', 
mentioned in my previous piece. It discusses in deep detail the 
selection and maintenance of standard stars to assess the spectral 
class of a given target. 
Extending the standard 
 -------------------- 
    The book notes that these standards are bright stars, within bare 
eye range but they are increasing falling out of favor exacta mente 
because they are bright. They would blind or burn out spectrometers 
built for faint stars. 
    We do spectrometry on very dim stars, thanks to the supersize 
telescopes and more sensitive spectrometers. Spectrometry is now done 
on stars in other galaxies! Efforts are underway to build standards in 
the fainter magnitude ranges, like within certain clusters, for this 
extended reach of stellar spectrometry. 
    Never the less, the primary standard, against which new standards 
must be qualified, are still valid, specially for home spectrometry 
where the targets are mostly in the bare-eye range. 
Classification
 ------------
    The skill and art of spectral classification is entirely founded 
on the internal evidence in the very spectrum. It is the detailed 
texture of the spectral lines that determines the class. Class is not 
based on theory or other star properties. These other parameters are 
attributes of the class, yes, but not the determinant.
    A class G2 star may have a temperature of 5,900K by other studies 
of its radiation. By some evolution of theory or new finding, the 
temperature is now assigned to be 6,100K. The star does not now have 
to be reclassified into G1 or G0, steps hotter than G2. 
    As long as the actual spectrum remained the same (the star is a 
stable one), it is still a G2 star. The attribute of G2 is now altered 
to a 6,100K temperature.
    On the other hand, if the star does get hotter, as a variable 
star, and the spectral structure is altered as a result, then the  
class is reevaluated. We say the star varies from G0 to G2 in 
spectrum, along with other parameters. 
    This method, the Morgan-Keenan system, is sometimes lost in 
explanations of spectral class in some texts. The home astronomer can 
be misleaded to believe that a diversity of tests is done on the star 
to arrive at a best-fit spectral class, 
Luminosity class
 --------------
    The criteria for luminosity are developed for each spectral class. 
There is no overall scheme that can be applied across all the classes. 
Criteria for one class may not be appropriate in an other.
    That's why on a HR diagram the luminosity curves are irregular 
lines. Besides that, the luminosity classes bunch up toward the left 
side of the chart so they are less well segregated. In fact, at the 
far left, there is probably no substantial difference between a main 
sequence (luminosity V) star and a supergiant (luminosity I). In this 
part of the HR diagram, main-sequence stars ARE supergiants. 
    On the usual HR diagram the luminosity curves often are cut off in 
the left center, causing no end of question and confusion for the home 
astronomer. They are just crowded together and have far less meaning 
than on the middle and right side of the chart. 
Example stars 
 -----------
    In turns out that many of the MK standard stars are dim ones or 
ambiguous components of binaries. In the stead, here I selected 
examples from the star catalog in the Observer's Handbook. In fact, 
this is the very one on the NYSkies website which I uploaded a month 
or so ago. Being in digital form I immported it into Excel and 
manipulated it as a spreadsheet.
    This list has stars brighter than +3-1/2 magnitude, so all the 
examples are visible to bare eye from New York, except for latitude 
cutoff. In my selection i allowed southern examples for travels beyond 
New York's latitude. 
    I left out binary members unless both components are of the same 
class or the comes was several magnitudes dimmer than the main star. 
Else you see a blend of two spectra. I also passed over variable stars 
that shift spectral class. 
    Where there was a choice of stars I picked the brighter one. Stars 
brighter than +2.0 magnitude are asterisked. Many classes have no 
suitable examples in the Observer's Handbook catalog. I omitted these 
classes here to compact the table. 
    ------+---------+---------+---------+---------+---------+ 
     \lum I  I      |  II     |  III    |  IV     |  V      | 
       \  | super-  | bright  | giant   | subgiant| main se-| 
    sp cl\| giant   | giant   |         |         | quence  | 
    ------+---------+---------+---------+---------+---------+ 
     O 5  | zet Pup |         |         |         |         |   
     O 9  |         |         | iot Ori |         |         |   
     O 9.5|         |         |         |         | zet Oph | 
    ------+---------+---------+---------+---------+---------+ 
     B 0  | eps Ori*|         |         | gam Cas | tau Sco |     
     B 0.3|         |         |         | del Sco |         |   
     B 0.5| kap Ori |         | bet Cru*|         | bet Sco |     
     B 1  | zet Per |         | bet Cen*|         | alp Vir*| 
     B 1.5|         |         | alp Lup | lam Sco*|         |  
     B 2  |         | eps CMa*| gam Ori*| del Cen | alp Ara |     
     B 2.5|         |         |         | zet Cen | alp Pav*|  
     B 3  | omi2CMa |         |         | sig Sgr | alp Eri*|  
     B 4  |         |         |         |         |   p Car |  
     B 5  | eta CMa |         | del Per |         |         |  
     B 7  |         |         | bet Tau*| alp Col | alp Gru*|  
     B 8  | bet Ori |         | gam Crv |         | bet CMi |  
     B 8.5|         |         | zet Peg |         |         | 
     B 9  |         | gam Lyr |         | alp And | lam Aql |  
     B 8.5|         | lam Cen | del Cyg | del Crv |         | 
    ------+---------+---------+---------+---------+---------+ 
     A 0  | eta Leo | eps Sgr*|  mu Ser | eps UMa*| alp Lyr*|  
     A 1  |         |         | bet Car*| bet Aur*| del Vel*| 
     A 2  | alp Cyg*|         |         | zet Sgr | iot Cen |  
     A 3  |         |         | gam UMi | bet Eri | alp PsA*|  
     A 4  |         |         |         | del Leo | bet Ari | 
     A 5  |         |         |         | del Cas | alp Oph |   
     A 6  |         | ups Car |         | bet Pav | alp Pic | 
     A 7  | iot Car |         | the2Tau | gam Boo [ alp Aql*| 
     A 9  |         | alp Car*|         |         |         |   
    ------+---------+---------+---------+---------+---------+ 
     F 0  | alp Lep |         | zet Leo | bet TrA |         |   
     F 1  |         |         | the Sco*|         |         | 
     F 2  | iot1Sco |         | bet Cas | del Aql | eta Sco |   
     F 5  | alp Per*|         |         |  xi Gem |         |   
     F 6  |         |         |         | the UMa |  pi2Ori |   
     F 8  | del CMa*|         |         |         |         |   
    ------+---------+---------+---------+---------+---------+ 
     G 0  | bet Aqr |         |         | eta Boo | eta Cas |   
     G 1  |         | eps Leo |         | bet Hyi |         |   
     G 2  | alp Aqr |         |         |         |     Sun |   
     G 5  |         | bet Crv | omi UMa |  mu Her |         |   
     G 6  |  xi Pup |         |         |         |         |   
     G 7  |         |         | bet Her |         |         |   
     G 7.5|         |         | eta Her |         |         |   
     G 8  | eps Gem |         | eta Dra |         | tau Cet |   
     G 9  |         |         | eps Vir |         |         |   
     G 9.5|         |         | eps Oph |         |         |   
    ------+---------+---------+---------+---------+---------+ 
     K 0  |         |         | bet Gem*| eta Cep |         |   
     K 1  |         |         | lam Sgr |         |         |   
     K 1.5| zet Cep |         | alp Boo*|         |         |   
     K 2  | eps Peg |         | alp Ari |         |         |   
     K 2.5|         |         | del Sgr |         |         |   
     K 3  |  pi Pup | iot Aur | alp Tuc |         |         |   
     K 4  |         |         | bet UMi |         |         |   
     K 5  |         |         | alp Tau*|         |         |   
     K 7  | sig CMa |         | alp Lyn |         |         |   
    ------+---------+---------+---------+---------+---------+ 
     M 0  |         |         | bet And |         |         |   
     M 1  |         |         | del Oph |         |         |   
     M 1.5| alp Sco*|         |         |         |         |
     M 2  | alp Ori*|         | alp Cet |         |         |   
     M 2.5|         |         | sig Lib |         |         |   
     M 3  |         |         |  mu Gem |         |         |   
     M 3.5|         |         | gam Cru*|         |         |   
     M 4  |         | rho Per |         |         |         |   
     M 5  |         |         | bet Gru |         |         |   
    ------+---------+---------+---------+---------+---------+ 
Star colors 
 --------- 
    The classes are commonly assigned schematic colors: 
        --------
        O - blue 
        B - blue-white 
        A - white 
        F - yellow-white 
        G - yellow 
        K - orange 
        M - red 
        -------
    These are NOT the color the star 'should look like' by eye. Color 
perception varies widely among observers and with ambient conditions. 
The colors are used in printing charts and tinting projected images. 
Think of them as a code and not the actual hue the star shines with. 
    A misleading analogy is the color of a heated metal piece, like in 
an iron mill. It will NOT show these colors for a very simple reason. 
Metals vaporize at a couple thousand degrees Centigrade, at the lower 
range of star temperatures. An other reason is that a piece seen close 
up, as while working at a kiln, is dazzling white even at temperatures 
still within the middle of spectral class M. 
Nonstellar spectra
 ----------------
    Spectral classification applies only to real stars, not any other 
type of celestial body. The spectrum is formed by a blackbody emission 
from the star's photosphere and modulated by the atmosphere around the 
star. Altho spectra can be collected from any source emitting 
electromagnetic radiation -- not just visible radiation -- there are 
no 'spectral classes' for them. 
    A spectrum of reflected radiation, like off of a nebula or 
asteroid, is basicly that of the stellar illuminant and further 
modulated by absorption in the target's surface. The new spectral 
features are distinguished by being those appropriate for a low, 
substellar, temperature. 
Conclusion 
 -------- 
    A study of this table reveals some interesting features. There are 
no examples of M IV or M V stars. These are very weak stars mostly out 
of bare-eye reach, even taking the limit to 5th or 6th magnitude. Yet 
these stars are the most populous in the Sun's vicinity out to a 
hundred lightyears. 
    We can see the iuminosity I and II stars at huge distances, many 
hundreds to thousands of lightyears away, because they are so 
brilliant. Probably thankfully there are no such stars near to us, 
within a hundred lightyears.
    The Sun's region of the Milky Way galaxy is filled with stars of 
all luminosities in spectral classes B thru G, providing examples in 
most combinations of spectrum and luminosity. 
    Novices in astronomy can do well to recognize the letters for 
spectral class, ignoring for the while the number subclass.