Re: Detecting variables

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

  Robert Bradbury wrote:

> The question would be whether there was an
> even distribution over variables going bright
> vs. variables going dark. 

  Hmmm.  Most variables in catalogs have been observed
to go through at least one complete cycle -- bright to dark
and back to bright again.  I believe that Robert is 
suggesting here that he'd like to concentrate on the number of 
stars which have simply gotten darker, but never returned to
their original brightness.

  That's not easy to determine with confidence.  We have
decades of information on stars in the past, so that an 
object which suddenly appears ("goes bright") can be 
searched for on old photographic plates, etc.  We can
make a quantitative statement on how long it has been
"dark" before this recent brightening.

  However, the only way to be sure that a source has "gone dark"
for good is to notice its fading and then .... keep checking.
If you wait for 10 years, and don't see it re-appear, all
you can say is "it might have period longer than 10 years".

  In other words, the degree to which you can claim 
"goes bright only" is not symmetric with the degree to which
you can claim "goes dark only".  Well, not unless you are
very, very patient.

                                    Michael