t00fri wrote:After such comments it is time to make clear that in case of the missing Castor system (and many further binary and multiple stars), it's not at all a matter of "data fixing" in Celestia!
As a basic strategy of Celestia development we have always decided to implement officially only those objects where roughly complete and reliable astronomical data are available. The more complex a multiple star system is, the smaller the chances that all needed orbital parameters have been measured. That's why Celestia offers only relatively few binary systems via my visualbins.stc and spectbins.stc files and Grant's nearstars.stc. Not because we are too lazy, for example, to include incompletely known multiple star systems!
Castor is one such example.
While a sixfold system like Castor is certainly a great potential target for visualization, there are still a fairly substantial number of orbital parameters missing, such as orbital inclinations and semi-major axes. Fortunately, the 6 star Castor system may be regrouped into 3 binary pairs, since we know that the partners of the visual Castor.A, CastorB and CastorC stars are all spectroscopic in nature and correspondingly very close to their much wider separated parent stars!
However, spectroscopic pairs have also certain disadvantages, namely that it is impossible to determine separately orbital inclinations and the semi-major axes with purely spectroscopical techniques (unless we are dealing with an eclipsing spectroscopic binary)!
Therefore, in such challenging and interesting cases like Castor, we are very happy about the initiative of users preparing corresponding add-ons that are resting on incomplete scientific data and a number of unavoidable assumptions. This kind of approach is fully satisfactory to many users.
CC's add-on is certainly a nice piece of work along these lines.
Fridger
Fridger makes some good points here about the challenges of measurement.
Anyone who has ever taken an undergraduate course in Physics, Astronomy, or Astrophysics will know well the difficulties and issues associated with the various observation methods.
For those who have not, here are some references and pictures which may help to understand some of the methodologies and associated issues:
On Binaries:
http://en.wikipedia.org/wiki/Binary_star#Methods_of_observationhttp://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.htmlOn Tools of the trade:
http://hubblesite.org/newscenter/archive/releases/1990/05/image/a/http://imgsrc.hubblesite.org/hu/db/images/hs-1990-05-a-full_jpg.jpghttp://www.rocketmime.com/astronomy/Telescope/ResolvingPower.htmlIt's important to understand that "Scientifically Accurate" is not the same as "100% accurate".
Good science requires you to follow a valid methodology, and to document the methods used as well as the errors associated with any measurements (which are always present).
At the distances involved, the errors and uncertainties can be large.
- See how accurately you can measure the diameter of an orange at even 100 meters distance! (Oh, and by the way you are not allowed to measure out the 100 meters with a tape measure, but must find a way to estimate the distance without changing your position.)
Celestia makes no claims to be 100% accurate. By definition, there are errors associated with all the included content, because it has been acquired by scientific method. It's just that some content inherently comes with larger errors or uncertainties than others.
Most binary systems. including Castor, would fall into this latter category. This does not make it in-valid.
FWIW, I consider the Castor addon to be a visualization of the system based on what is currently known scientifically (along with it's measurement uncertainties.).
This is somewhat better than an "artistic realization" which makes no attempt at all to incorporate known scientific quantities or qualities.
The webpage from which most of the "facts" used for this addon are sourced, goes to considerable lengths to quote it's own sources, and to qualify any uncertainties and/or assumptions they have made. This is indicative of a scientific approach.
That said, there are enough uncertainties and aspects that just can't be or haven't been determined for this system, that we cannot say with any great confidence that this is "how the system is."
For example, this system has only been observed as a multiple since the mid 17th century, so we've only had the opportunity to observe and measure at most about 2% of YY Gem's estimated 14,000 - 26,000 year orbit.
For this reason, we can't even know yet that YY Gem is gravitationally bound in a closed elliptical orbit with the other 2 pairs. It's orbit may in fact be hyperbolic.
Also, given the number of existing unknowns and assumptions, if I remember correctly, I made no efforts to ensure the correct inclination of this system relative to Earth was represented in the addon.
Then a right-click on Castor, for example, will correctly reveal that the system consists of 6 stars in form of 3 spectroscopic pairs, with only partially determined orbits.
The webpage on which the design of the addon is based can be accessed via the right-click -> Info function on each of the 6 stars in the addon.
CC
