Why are the 'Fixed Stars' so FIXED?

"Henri Wilson" HW@.... wrote in message
...
On Sun, 1 Apr 2007 15:04:46 +0100, "George Dishman"

wrote:


"Henri Wilson" HW@.... wrote in message
news
On 29 Mar 2007 10:25:26 -0700, "George Dishman"

wrote:


For years you have been saying that Cepheids were plain
constant-luminosity stars and the variation was due to
c+v effects because they are in binary systems that have
not been recognised as such.

No I changed that opinion some time ago George. I accepted that the
presence of
harmonics in the brightness curves was pretty hard to explain on purely
'orbit'
grounds.
So it is quite likely that two factors are contributing to the
brightness
curves of these stars. Their orbit motion and the huff-puffing of their
surfaces.

Given that you now accept the huff-puff nature, you
need to reconsider your justification for saying
that Cepheids that are currently thought of as
isolated might actually be part of a binary.

Every one I read about seems to have a companion star.

Put "solitary cepheid" into Google and you get a number
of hits. At least one was a survey listing both categories
with similar numbers of entries. I looked it up earlier
at work and don't have the reference here and it was in
postscript but I'm sure you can find a readable version
with a little hunting.

If you are now switching to say they are single stars, why
on Earth would your software be modelling binary systems
and restricting the solutions to Keplerian orbits when the
motion of the surface is due to internal pressure?

I think it is my turn to say you are getting very confused
Henry.

It is a fact that most 'cepheids' appear to have a companion...

It is a fact that something around half of _all_ stars
are in binary systems so there is no reason why Cepheids
should be an exception.

...all stars are obiting some kind of mass centre.

They all orbit the galaxy, so what. The orbital period
needs to be a few years or less for any significant
effects to show up.

which means they
are in some kind of orbit.
I reckon the movement of their surfaces would feature similar radial
velocities
to those of an orbit. It is distinctly possible that the huffing is
linked
to
the orbit period. It is also possible that the stars are in tidal lock
..

It is certainly possible, especially for close binaries,
but less likely for those with greater separations.

and
distorted into some kind of dumbell shape,

No, each would be more like an egg shape. Look up "Roche Lobe".

Yes, egg shaped...that would cause a brightness variation at double the
orbit
frequency.

leading to a brightness variation as
they orbit....but that wouldn't account for the short periods of many of
them.

It wouldn't account for any where the period of the Cepheid
differs from the orbital period, nor does it account for those
that are not in binary systems.

That is true. That's why I accept the possibility.
However it doesn't make any difference to the fact that the brightness
variation of huff-puff stars conforms with BaTh.

First you need to model them correctly. Your new program
should do that if you match the red velocity curve to the
published data. The grre curve then gives the luminosity
variation due to c+v and any extra is intrinsic. So Henry,
revisit your matches and tell me how much is c+v and how
much is intrinsic for some examples 1.5 magnitude variation

In every paper I have read about cepheids, the authors admit the have no
theory
to link the surface movement to the brightness curve.

I won't comment on that without doing some study for myself.

George