Accurate star data

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Averus
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Accurate star data

Post #1by Averus » 25.09.2012, 23:49

PlutonianEmpire wrote:1. Unfortunately, it appears as though the lack of core stars is indeed part of the original distribution.

Oh, okay, thanks for the info.

PlutonianEmpire wrote:2. The 2 million stars database is a bit outdated. It was last updated for 1.4.0 in 2009. Even if it's only been 3 years, I'm sure the current data available for a lot of the stars is a lot better than when these .dat's were last compiled.

Not really that much of a problem, the .dat files merge rather nicely, so any few you'd care to use simultaneously could be glued together.

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Re: 1 Mega-stars STC file for M31

Post #2by PlutonianEmpire » 26.09.2012, 00:15

Averus wrote:
PlutonianEmpire wrote:1. Unfortunately, it appears as though the lack of core stars is indeed part of the original distribution.

Oh, okay, thanks for the info.

PlutonianEmpire wrote:2. The 2 million stars database is a bit outdated. It was last updated for 1.4.0 in 2009. Even if it's only been 3 years, I'm sure the current data available for a lot of the stars is a lot better than when these .dat's were last compiled.

Not really that much of a problem, the .dat files merge rather nicely, so any few you'd care to use simultaneously could be glued together.
Actually, I was thinking more along the lines of having the stars use public data available as of the last 6 months. AFAIK, the data for the stars in all these .dat's (the default one for 1.6.1, and 2.1A and C) are all 3 years old. I'd prefer something with more current info.
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Re: 1 Mega-stars STC file for M31

Post #3by omega13a » 26.09.2012, 08:13

bdfd wrote:Hi,
False plutonian :!:
The file "2 million stars database" is a solid foundation :)
That is why there are several files in the data directory like :
nearstars.stc, revised.stc, extrasolar.*, spectbins.stc, visualbins.stc, ...
They update the latest data. :)
qED.

They don't fix all of the data. The last time I checked, Celestia still had Castor as a single star when its really a hexanary star. You have to download an add-on from Motherload to correct that and even then, the add-on has its mistakes.
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t00fri
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Re: 1 Mega-stars STC file for M31

Post #4by t00fri » 26.09.2012, 14:06

omega13a wrote:
bdfd wrote:Hi,
False plutonian :!:
The file "2 million stars database" is a solid foundation :)
That is why there are several files in the data directory like :
nearstars.stc, revised.stc, extrasolar.*, spectbins.stc, visualbins.stc, ...
They update the latest data. :)
qED.

They don't fix all of the data. The last time I checked, Celestia still had Castor as a single star when its really a hexanary star. You have to download an add-on from Motherload to correct that and even then, the add-on has its mistakes.

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
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Re: 1 Mega-stars STC file for M31

Post #5by Hungry4info » 27.09.2012, 02:02

t00fri wrote: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)!

I'm not advocating any changes, as I really do like things they way it is in this respect, but extrasolar planets appear to be exempt from being excluded from Celestia due to their inclination degeneracies. Was there a conscious decision some time in the past to permit extrasolar planets more leniency?
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Re: 1 Mega-stars STC file for M31

Post #6by t00fri » 27.09.2012, 09:56

Hungry4info wrote:
t00fri wrote: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)!

I'm not advocating any changes, as I really do like things they way it is in this respect, but extrasolar planets appear to be exempt from being excluded from Celestia due to their inclination degeneracies. Was there a conscious decision some time in the past to permit extrasolar planets more leniency?

Not a conscious decision AFAIK. Since extrasolars represent a pioneering field of high general interest and with lots of research activity, the initiative of the devs who are taking care of regular updates was never opposed. Personally, I think this is the correct attitude concerning this fast moving field. Yet I would prefer a well visible warning about the inherent experimental limitations...

In contrast to extrasolar planets where inclination degeneracies always will remain, a number of binary systems do exist where the orbits are fully known. Precisely these we have included into Celestia, at least as concerns my work in form of the visualbins.stc and spectbins.stc data.

Since many thousands of multiple star systems are known with highly incomplete orbit parameter sets, a clearcut distinction of the few fully known orbits seems a good policy. However, a very sensible improvement would be to implement the existing info from multiple star catalogs by listing multiple star properties in the info panels of all Celestia stars. This is a relatively easy task and is on my todo list for celestia.Sci.

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.

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Re: 1 Mega-stars STC file for M31

Post #7by Chuft-Captain » 27.09.2012, 18:21

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_observation
http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html
On 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.jpg
http://www.rocketmime.com/astronomy/Telescope/ResolvingPower.html

It'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
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