When is a double star not a double star?

[David JM]

Nortons lists doubles that are nearly half an arc minute apart as well as closer doubles.

So, how far apart are an optical pair of stars before they stop being a double?

[DarkerSky]

That's an interesting thought. I've no idea on the answer. A Harvard paper on doubles some while ago quoted studies on optical doubles at up to 250". But what the limit is...?

[sologuitarist61]

I would have thought that a double star is more to do with two stars orbiting each other than how far apart they are.

[JamesF]

I guess people pick the maximum resolution of some reasonable size telescope as their cut-off point and that different list-compilers may choose different limits. Perhaps the upper limit is set by the optical telescope with the greatest resolution built thus far?

James

[JamesF]

I would have thought that a double star is more to do with two stars orbiting each other than how far apart they are.

Not always. Optical doubles can be totally unrelated stars, but on almost the same line of sight.

James

[barkis]

Gravitationally bound systems will be relatively close, as they orbit around a common centre of gravity. These systems can consist of several stars orbiting in a complex, but perfectly understandable way, although I'm no expert by a long way.

Some of the components of such a system, can only be identified spectroscopically, because they are much too close to separate via an optical telescope.

Optical double stars, are merely line of siight as you know, and with no physical attraction involved at all.

Their separation distance that would define them as optical doubles, as far as I know, has not been established.

Usually an optical double appear very close, but vary vastly in magnitude, which then becomes a test for the telescope that is being used to observe them.

Again, these optically close stars can consist of a number of them, again at different magnitudes, and temperatures (colours).

Ron.

[DarkerSky]

I would have thought that a double star is more to do with two stars orbiting each other than how far apart they are.

It is true. A 'true' double star is a binary star - i.e. stars that orbit one another around a common centre of gravity. However, double star catalogues also list what is known as optical doubles. These are stars that appear close together in the sky from our viewpoint on Earth, but which in reality are a huge distant apart from each other in space and are not gravitationally bound. In other words, optical doubles are essentially an illusion.

[sologuitarist61]

Not always. Optical doubles can be totally unrelated stars, but on almost the same line of sight.

James

Yep, point taken :o