[vsnet-chat 2511] Re: [AAVSO-DIS] B-V to V-R

[Lew Cook <lcoo@yahoo.com>]

Since I posed the question, I'll take a stab at the answer as best as
I can. I'm sure any misstatements I make will be caught and
corrected.
=====
It means there is a relatively simple relationship between the colors
measures of stars measured thru B (blue) and V (visual - green)
filters versus those measured between V  and R (red) filters in
photometry systems.

Arlo Landolt did a phenomenal amount of work in setting up a standard
star system. Using his standard star measurements, a relationship
between the colors is found to exist. 

The relationship is linear. The difference in brightness measured in
V and R is about half (0.508) of that measured between B and V
filters. There is a small correction factor (0.04). 

((Stars of spectral class A 0 - like Vega, a hot blue star - should
have a color measurement (color index) of Zero - by definition. the
factor of 0.04 makes a correction which could be due to a variety of
factors, including interstellar reddening by dust and the definition
of the photometric systems. 

Yellow stars like the sun have a B-V color index of around 0.6. the
R-I color index would be around 0.34 magnitudes.

Very red stars have color indices (B-V) of 1.5 or greater. Generally
speaking the B-V index can be used as a measure of the temperature of
the star.

Hope that Helps!
Lew
--- Beka Lou Taylor <bekaloutaylor@email.com> wrote:
> Can someone tell me what this means?
> email:<bekaloutaylor@email.com>
> 
> At 12:11 PM 1/6/00 -0700, Brian Skiff wrote:
> >     A simple regression against colors of Landolt stars yields:
> >
> >(V-R) = 0.508(B-V) + 0.040
> >
> >or:  R = V - 0.508(B-V) - 0.040
> >
> >Note that the sign difference on the zero-point is opposite what
> Arne posted,
> >which was my arithmetic error, not his.  This relation works quite
> well
> >for stars bluer than B-V = 1.2.  More complex functional forms
> were derived
> >by John Caldwell and published in the South African Astronomical
> OBservatory
> >Circulars a few years ago.  These cover all the UBVRIJHK, uvby,
> and DDO
> >colors with extended range in color space.
> >
> >\Brian
> 


=====
Regards, Lew 
http://vsnet.geocities.com/lcoo/
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