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All of the relevant technical details about this data can be found in the Astrophysical Journal paper by Silva and Cornell (1992)
This document contains links to PostScript Figures. Direct retrieval of the data can be achieved here:
A9 - O5 main sequence stars These are hot
stars, with surface temperatures greater than 10,000 K. As
a consequence their spectra are rising into the blue. You should
also note the steady increase of the strength of the hydrogen
absorption lines.
K5 - F7 main sequence stars Now
we see a progression from stars with temperatures around 7000 (F7)
to those with temperatures of around 4000 (K5). The strength of
the hydrogen lines decreases as collisional excitation in these
cooler atmospheres is insufficient to populate excited stats in
hydrogen. Metallic lines now become much stronger. In particular,
Calcium II H and K (wavelength around 3900 Angstroms), Magnesium I
(5150 angstroms - see especially K stars) and Sodium D around 5800
angstroms. As you get to lower temperature proportinately more flux
is emitted at longer (redder wavelengths).
G8 -- A7 main sequence stars .
Here we see a narrower range of temperatures represented with again
the general trend of increasing metallic line strength and decreasing
hydrogen line strength. A collection of absorption features,
predominately due to Iron, produces a depression, known as the G-band
around 4100 angstroms which can be clearly seen in the G-star
spectra (try using the zoom option in ghostview for this).
Blue Giants Here are examples
of massive stars as they appear when they begin to evolve off the
main sequence and head to the red giant branch (movie of this not
yet available). Not the extremely well-defined hydrogen absorption
lines. As the star expands its surface gravity decreases, relative
to what it was on the main-sequence, and this improves the sharpness
of the asbsorption lines.
Red Giants Here are example
spectra of cool, red giant stellar atmospheres. They look
similar to their main sequence counterparts of the same spectral
type but again the features are sharper, most notably, the
calcium triple at around 8500 angstroms, in which all 3 lines of
calcium are much more prominent in Giants than main sequence stars.
Very Cool Giants Here are example
spectra of very cool (T < 3500 K) red giant stellar atmospheres.
The degree of excitation is so low here that molecules are able to
form in these atmospheres. The complex rotational and vibrational
energy states of a molecule give rise to depp absorption troughs
(called bandheads) that occupy 1-300 angstroms. Missing data in
these spectra is due to calibration problems associated with
sky subtraction.
Extreme Molecular Band Heads
These are the coolest known giants (and likely to be very rich
in heavy elements). The bandhead structure should be obvious,
particularly longward of 7000 angstroms.
They look
similar to their main sequence counterparts of the same spectral
type but again the features are sharper, most notably, the
calcium triple at around 8500 angstroms, in which all 3 lines of
calcium are apparent.
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