[vsnet-chat 6] alpha Ceti and visual color response

[bas@lowell.Lowell.Edu (Brian Skiff)]

     In re brighter Mira comp stars:  the moderately-red alpha Ceti should
appear _fainter_ than its V mag. under any but bright light levels.  The peak
visual response shifts toward the blue as you become dark-adapted, and thus
red stars will appear fainter than otherwise.  The peak wavelength shifts from
about 5500A for daylight response down to about 5100A when fully dark-adapted. 
The effect on perceived magnitude is like a color term in ordinary
photoelectric or CCD photometry, and is _roughly_:

m(vis) = V + 0.2(B-V).

There have been several articles about this in the Journal of the AAVSO and
also the Journal of the BAA, and probably other places, too.  There are
certainly variations in color sensitivity among individuals, so the coefficient
in the equation above can vary as well.
     The effect is easy to see in photometric sequences such as the Landolt
stars in Selected Area 110, which lies in the dark rift of the Aquila Milky
Way.  In small fields here are stars of ordinary color beside very red ones
that lie behind the dark cloud.  Since the V magnitudes and B-V colors are
precisely known, you can get a feeling for what happens by examining some of
these stars.  Compare for example the stars near the asterism including the
standards SA 110-499 to -507.
     The published photometry for alpha Ceti suggests it is quite stable in
brightness (range of only a few percent), despite its spectral type (M2III)
and the fact that it's been designated as a variable in the GCVS.  Of course,
what we need is some reliable photometry done with a very small telescope that
can look at such bright stars!

\Brian Skiff  (bas@lowell.edu)

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