New double star and binary star site

http://www.martin-nicholson.info/twins/twins.htm

How do you know it is a true double, and not just two stars in our line of
sight but light years apart?

Steve

Steven Gill wrote:

How do you know it is a true double, and not just two stars in our line of
sight but light years apart?

We can observe their motion around each other.

"Steven Gill" wrote in message
. uk...
How do you know it is a true double, and not just two stars in our line of
sight but light years apart?

Steve


1. Watch the pair for years or decades and see if there is orbital motion.
Works best on very close pairs.

2. Check for common proper motion of wider pairs, since they won't show much
orbital motion on timescales of a few centuries. If you can use
spectroscopy to check for common radial velocity that can also help, but
this is difficult for very faint stars.

3. Use statistical methods to assign a probability of a pair of given
magnitude and separation being a physical pair. The famous double-star
observer Robert G. Aitken offered the formula

log (separation in arcsec) = 2.8 - 0.2 m

where m is the magnitude of the double star (I think this is for combined
magnitude of two nearly equal components). A wide pair of fourth or fifth
magnitude stars might well be physical (e.g., epsilon Lyrae) by this
criterion, but the same separation for 11th magnitude stars would not be.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

Question to all re Dark Matter, etc.
Does Einstein maintain (in his 'M'' equation) that there is no such
thing as ''empty'' space in the cosmos? Vacuum ??
tks
ClaudiaMarie

Question to all;
In some galaxies are some stars only gas & therefore able to pass
through each other ( without collisions or exerting dominate magnetism
one over another) ??
tks,
ClaudiaMarie