[vsnet-chat 4837] Re: Question about W Sgr

[Fraser Farrell <fraser@trilobytes.com.au>]

> I Have noted that the variable star W Sgr (m1=12.1-14.8 P) located at
> position
> 17 57 3.2 -19 20 13 GSC Catalog is more brigth.
> I Have valued m1=10.15+-0.25 using Astroart and GSC Catalog.

Toni,

Did you mean VV Sgr ("vee vee Sgr")? This is a Mira variable at the 
position you quoted. Being a Red Star it is quite normal for it to appear 
brighter to the eye or to CCD than the -photographic- magnitudes listed 
in catalogues.

Photographic or "P" magnitudes were derived from blue-sensitive camera 
plates; which don't show Red Stars too well. And the redder the star is, 
the worse this problem gets. A couple of extreme examples which come to 
mind are R Scl and V Pav – both of which can be followed easily with 7x50 
binoculars, but you would never think this possible from their official P 
magnitudes!

The magnitudes in the GSC Catalog are, at best, only an approximation to 
the truth. They have been compiled from a wide variety of photographic 
sources with different spectral sensitivities. Use the Tycho catalogue's 
V magnitudes, or Brian Skiff's photometry compilation, or a good sequence 
from the AAVSO/BAA/RASNZ/VSOLJ charts to define the real magnitudes of 
stars you're examining.

If you're doing CCD imaging with an intention of getting magnitudes, then 
you should be using an infrared cutoff filter on your camera. Or 
preferably some filters which only let through genuine B, V or R -band 
light. Otherwise you're going to get embarrassed by the sometimes vast 
differences between "unfiltered CCD magnitude" and the magnitudes listed 
in catalogues.

Incidentally, the real W Sgr is a Cepheid variable of visual magnitude 
4.3 to 5.1 with a 7.6 day period. Good for school science projects 
because the kids can look at it once a night and get a lightcurve fairly 
quickly. It's also known as gamma-1 Sgr (and many other names), and is 
located at 18h 05m -29d 35' (2000). Just off the spout of the Sagittarius 
Teapot and not far from our latest nova V4740 Sgr. The trivia nuts will 
also tell you that it's the closest (in RA/Dec) naked-eye star to our 
Galaxy's centre.


cheers,
Fraser Farrell

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