[Aavso-photometry] Basis of magnitude system

[arne]

Peter Nelson wrote:
> A random question:  All magnitudes are relative and there must be a defined
> standard(s).  I am aware of the North Polar Sequence, and Morgans UBV
> standards.  How were these standards devised?  Was there an AO star which
> was defined as magnitude x and B-V=0 from which all other standards were
> carefully measured?   
> 
As Brad mentions, the system is defined by a handful of A0 stars.
These give zero color throughout the passbands, so are not a very
good representation of flux (obviously, the star's energy distribution
follows a blackbody and so must decrease towards the red, for example,
yet its magnitude remains the same).  The V magnitude is theoretically
defined by those A0 stars, though in practice it is defined by the
published Landolt standards (which are based on the Johnson standards,
which are based on the ...)
Arne