How does Celestia calculate star distances from parallaxes?

[Paul]

I always used the simplistic equation:

d(ly) = 3264 / plx(mas)

But using the Hipparcos parallaxes directly from the VizieR search engine yields different star positions (distances) than those appearing in Celestia.
Chris, I was wondering:

* Is there a more accurate/sophisticated formula that you use, and if so, what is it? I'm beginning to wonder whether it's some trigonometric method.
* Do you use a different set of parallax data from a slightly different source that has been "massaged"?

[chris]

* Is there a more accurate/sophisticated formula that you use, and if so, what is it? I'm beginning to wonder whether it's some trigonometric method.
* Do you use a different set of parallax data from a slightly different source that has been "massaged"?

I use the same method as you, with a couple of modifications . . . For one, the distance for stars is clamped to a maximum value when the parallax is very small. Second, for components of multiple star systems have their parallaxes artificially constrained to the same values. Otherwise, measurement errors make almost all multiple star systems spread out over several light years along a line from the Sun.

--Chris

[Paul]

Second, for components of multiple star systems have their parallaxes artificially constrained to the same values. Otherwise, measurement errors make almost all multiple star systems spread out over several light years along a line from the Sun.

Ah, this is what I thought. So do you do this for all stars that are named as multiple, or do you do it according to a catalog of physical multiples?
If so, what's the catalog?

Thanks,
Paul

[chris]

Ah, this is what I thought. So do you do this for all stars that are named as multiple, or do you do it according to a catalog of physical multiples?
If so, what's the catalog?

The HIPPARCOS catalog contains information about which stars are members of multiple star systems, so I didn't have to use another database. In the future, I may try and pull in stars from other catalogs--it'd be nice to get some of the famous spectroscopic binaries.

--Chris

[Matt McIrvin]

This explains something about Celestia that had puzzled me. I had often read in the past that Mizar and Alcor are only an optical binary, that is, they're not actually a gravitationally bound pair. Yet Celestia lists them as the same distance from Earth, which would tend to imply that they are a real binary.

Well, it turns out that this is a matter of some contention, but this interesting page, at least, claims that in this case the Hipparcos error bars are small enough to say with confidence that they're a few light years apart, and probably not a real double:

http://leo.astronomy.cz/mizar/article.htm

As it happens, Mizar itself is a complicated multiple system, and Alcor is apparently a pulsating variable that was once mistaken for a spectroscopic double. This page talks about some of Mizar's components being resolved telescopically by Galileo!

But Alcor is probably not part of that system. On the other hand, they are both part of the Ursa Major moving cluster (Collinder 285), a nearby loose cluster that consists of most of the stars in the Big Dipper, plus several other bright and famous stars scattered all over the sky that share the same motion and probable origin. In a sense we are inside Collinder 285, though not of it-- just passing through...