Re: NSV 10892

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Tom wrote:

> The spectrum seems to show this star brighter in V than Ic.  Yet the Ic is 
> brighter than V?

  Ah, the wonders of the astronomical magnitude system :-)

  To a very good approximation, astronomers have decided to use
stars like Vega -- with a temperature of about 10,000 K, much hotter 
than our Sun -- as the "fiducial" stars for optical photometry.
The magnitude system is arranged so that a star like Vega has
colors of zero.  That is,

             U - B  =  0
             B - V  =  0
             V - R  =  0
             R - I  =  0 

implying, of course,

             V - I  =  0

  In other words, if a star like Vega has apparent magnitude V = 8.2,
then it will also have magnitude I = 8.2.  

  Yes, we decided to do it this way, even though 

      - the _peak flux_ (amount of energy per cm^2 per sec per Hz)
                 is not the same at the centers of the two different
                 passbands.  Hot stars like Vega have a much, much
                 larger flux in the blue portion of the optical
                 than the red

      - the _integrated flux_ (amount of energy per cm^2 per sec across
                 the entire passband) is not the same in the
                 two different passbands

      - the _number of photons_ collected per cm^2 per sec is not 
                 the same in the two different passbands

  Okay.  Now, it turns out that stars as hot as Vega are rare.  The 
great majority of stars are much cooler.  That means that their
ratio of blue light to red light is _smaller_ than Vega's -- they
look redder.  NSV 10892 is one such star.  It has relatively more
red light ... but _still_ emits more flux in the V-band than
in the I-band, as Tom noted.  

  What does this mean for its magnitudes?

          Vega           has color  V - I = 0    

          NSV 10892      is redder
                         produces relatively more red light
                         hence, I-band mag becomes relatively smaller
                                (small mag means brighter)
                         hence, V - I > 0
                         
              
  The important point is that there is NO SIMPLE RELATIONSHIP between
the magnitudes in different passbands and the relative amounts of 
energy emitted/collected in those passbands.  Alas.  Radio and
X-ray astronomers are smarter: they measure fluxes and describe
their sources in terms of fluxes.  A radio galaxy with a flux
of 10 milliJanskies at 6 cm and 20 milliJanskies at 3 cm really
is putting out more flux at 3 cm.  But the traditions of history --
and the collected scientific measurements of several centuries --
keep us optical astronomers bound to magnitudes.

                                                Michael Richmond