Orbital Separation

[Evil Dr Ganymede]

Another starlist-related question (possibly a silly one):

Some of the stars on the RECONS list are binaries, but the distances between the stars are only expressed as angular separations.

So how can one convert that angular separation into a separation measured in AU? I know how far the stars are from Sol, if that helps.

[Spaceman Spiff]

It's nothing special, but sadly I suspect you won't get what you really want from this. The angular separation only tells you the separation projected on the plane of the sky, not the true separation: we often don't know how far in front or behind one star is relative to the other, even which star is closer.

The angular separation does tell you the minimum distance between the stars. This is found by multipying the distance by that angle.

Example, a pair of stars with angular separation one arc second at 100 light years distance:

1" : 1/3600 ? pi/180° = 4.8481368?10^-6 radians.

100 light years : 100 * 299,792,458 m/s * 365.2422 * 24 ? 60 ? 60s = 9.4605284?10^17m.

Minimum star separation = 4.8481368?10^-6 ? 9.4605284?10^17m = 4.5865936?10^12m.

In A.U.s., this is 4.5865936?10^12m / 1.496?10^11m = 30.659 A.U.

Spiff.

[Evil Dr Ganymede]

It's nothing special, but sadly I suspect you won't get what you really want from this. The angular separation only tells you the separation projected on the plane of the sky, not the true separation: we often don't know how far in front or behind one star is relative to the other, even which star is closer.

Thanks, Spiff. In fact, now I'm thinking of this, ISTR that Grant (I think) gave me a catalogue of multiple stars that might have the orbital distances a while back, so I'll check that too.

I just wanted to get a vague idea of the size of the orbit is all, so I think your method should be fine for me if there's no other data available.