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.