I just want to add stars

[Sadachbia]

Long-time Celestia user here, first time posting on the forums. I'm not 100% sure this is the correct subforum for my request, so if not, please let me know.

Anyway, after Googling and browsing the User Guide and general dinking around with the Celestia program itself, I seem unable to find a good, clear, concise how-to for making my own addon containing my fictional stars/or adding fictional data to existing stars. The ability to add planets to stars would be nice too, but at the moment, I would be happy for just the ability to insert my own objects into space. (I'm not sure how the XYZ placement is determined. Distance from other known objects? Free navigation in all 3 dimensions? This would probably be explained in a how-to guide, I suppose...)

Eventually, I think it would be cool to be able to zoom out from the Milky Way and have small pins flagging the locations of stars of note. If any of you have played the video game Mass Effect, the galactic map when you choose a travel destination looks kind of like what I want to do. I dunno if this is within the capabilities of simple addon-programming, however. ^_^;; (I'm not a web-developer or computer code-writer. I have no experience with programming languages other than some really basic HTML. I'm willing to learn a little if the need arises, though.)

[selden]

Welcome to the Celestia forum!

Celestia uses the standard astronomical coordinates Right Ascension, Declination and distance to specify where stars are located, not xyz coordinates.

Please take a look at
http://www.lepp.cornell.edu/~seb/celest ... intro.html
It briefly explains how the various types of catalogs work and then shows how to create a STarCatalog to define a star and a SolarSystemCatalog to define a planet orbiting around it. Please let me know how it can be improved.

One can define Nebula objects in a DeepSpaceCatalog which might be close enough to what you want for "pins" visible from far away.

[Sadachbia]

Whoa, that's weird, my Celestia folder looks nothing like the sample one shown in that how-to. o_O

I'm running Celestia on a Mac, but I don't think that's the issue. It's telling me to add a folder (directory) in the "Data" directory, and I don't have a Data folder. I don't seem to have catalog files in there either. The only things in my Celestia folder are generic documents (no file extension, but they open in a text editor) named AUTHORS, ChangeLog, COPYING, and README, plus an RTF document "README for Mac OS X", and a "License" folder that contains several "COPYING_" documents with suffixes denoting different languages (en, ja, ru, etc.).

The version I have is 1.6.0, but maybe I missed downloading something. I'll go see if I can download another version.

UPDATE: Hmm, no, the version 1.6.1 I downloaded is the same. No Data folder to be found. Maybe I need to have an existing addon installed for it to have such a folder and catalog files...?

'nother update: For some reason, I have two copies of Celestia in my Applications folder? Apparently one of them has the stuff I need and the other one doesn't. Heh. Well, time to try these instructions out. xD

[Sadachbia]

Wow, it turns out it is VERY easy to add new stars (and planets). I had no idea it would be so simple, so thank you very much for the link

Just one more thing I'm wondering, if I want to place stars in other galaxies, is there a more precise way than just setting a distance and then a RA/decl. that falls somewhere between the edges of the galaxy as it would appear from Earth? (A relatively "close" and "large" galaxy like Andromeda would take up multiple arcminutes and degrees of declination, but some of the more distant ones appear too small from Earth to allow as much of a range between edges.)

[selden]

RA and Dec are the only coordinates available for positioning Stars and DSC objects. Celestia uses double-precision arithmetic, so it has the equivalent of about 16 decimal digits available. As a result, if you look closely, you'll notice Stars are placed on a grid in distant galaxies, although the effect isn't obvious in nearby ones.

Celestia used to be limited to placing Stars within 16K LY of the Sun, which was quite a bit more restrictive!

[John Van Vliet]

--- edit ---

[Sadachbia]

If I want to add additional names to known stars, would I do that in Celestia's stars.dat file? (I would make a backup first, obviously, which I will be doing anyway since I downloaded the additional stars addon John suggested :3)

[selden]

If you expect to be wanting to share what you do with other people, you should define additional stars in STC catalogs, similar to what is done in Celestia for binary stars. See, for example, the catalogs data/spectbins.stc and visualbins.stc. Your own STC catalogs should be put in Celestia's "extras" folder, though, not in the data directory.

[Sadachbia]

If you expect to be wanting to share what you do with other people, you should define additional stars in STC catalogs, similar to what is done in Celestia for binary stars. See, for example, the catalogs data/spectbins.stc and visualbins.stc. Your own STC catalogs should be put in Celestia's "extras" folder, though, not in the data directory.

Do you mean that I need to make new objects (with my fictional names) and put them at the same spatial location as existing stars, instead of adding fictional names to known stars? E.g., in my universe, the star Epsilon Eridani is also called Arcadia; so, if I want to type in "Arcadia" and go to Epsilon Eridani, or have that show up as one of the names if I type in "EPS eri", do I change something in stars.dat, starnames.dat, or make an entirely new star that just overlaps EPS Eri's real-universe location?

I don't have any plans right now to make my addons sharable with other people; they're mostly for my reference, and I don't think anyone would even be that interested, since it's a fictional universe I'm still in the process of writing (i.e., it's not published and nobody but me has ever heard of it). This doesn't mean I won't EVER distribute the addon, though.

[selden]

Unfortunately, renaming a star does require overwriting its definition in Celestia. This is done by specifying the star's Hipparcos ID number in the STC file. The procedure for doing this is described in Celestia's WikiBook at http://en.wikibooks.org/wiki/Celestia/S ... ming_Stars

[Sadachbia]

Ahhh, I see! *nodding* I guess that means I can just use the same .stc file to both place new fictional star systems and append fictional names to known stars.

Now, as for the placement, you said that it's via the RA and declination (in other words, where they would appear as viewed from Earth). Is there a way to display this in Celestia, or a map somewhere that grids out the celestial hemispheres with RA and declination? (Like how maps of Earth are gridded with latitude and longitude.) I don't want to place stars blindly, check where they show up in Celestia, and then re-open the STC file to tweak the numbers; and putting them near known stars would mean I would have to look up the RA/declination of each individual star and then alter the numbers so mine would just be "next to" the known star in Earth's sky.

After some thought, I suppose it might not be feasible to have a 2-D representation of even one celestial hemisphere that isn't really distorted. Perhaps I'll make notes of the RA/declensions of constellation borders and then place objects by instead of trying to plot out the entire sky. Hehe!

[Sadachbia]

Okay, now that I've got the basics down, I can get into the more specific questions. :3

So, I know how to make a star, and I know how to make a barycenter. I'm trying to make two stars orbiting that barycenter (using the format on the Wikibooks page), but there must be an error in my code, because I can select and "visit" the barycenter in Celestia but not the stars orbiting it. One is a yellow-white bright giant, F9 I-b, and the other one is a brown dwarf, which I have in my notes as T1 VI (I'm not sure if "T" is the actual proper spectral class for a brown dwarf, or if I assigned that myself. I'm pretty sure using "VI" to designate a white or brown dwarf is my own invention, so maybe I shouldn't include that in Celestia if I don't want to confuse it >.>)

Here is the copy-pasta'd code from my custom STC file (this binary star system is located in the Andromeda Galaxy, hence the very large distance. Also, _ is used to represent tab space, since the forum coding can't recognise that):

Code: Select all

#######################
500002 "Eazurcha A" {
   RA  39.50
   Dec 40.0001
   Distance 2389445.12
   SpectralType "F9I-b"
   AbsMag
}
#######################
500002 # Eazurcha A
{
   OrbitBarycenter "Eazurcha"
   SpectralType "F9 Ib"
   AppMag
   EllipticalOrbit
   {
      Period 201.129
      SemiMajorAxis 8.5
      Eccentricity 0.1
      Inclination 0.1
      AscendingNode 67.708
      ArgOfPericenter 3.774
      MeanAnomaly 200.119
   }
}

500003 # Eazurcha B
{
   OrbitBarycenter "Eazurcha"
   SpectralType "T1 VI"
   AppMag
   EllipticalOrbit
   {
      Period 201.129
      SemiMajorAxis 100.2
      Eccentricity 0.75
      Inclination 0.1
      AscendingNode 67.708
      ArgOfPericenter 183.774
      MeanAnomaly 200.119
   }
}
#######################



Some of the numbers I altered from the example on the Wikibooks page, which uses the Alpha Centauri system as the template; others I just left as-is because I don't know exactly what they refer to and hope that they aren't required to be different >_> Obviously a massive evolved star and its non-fusing companion aren't going to be orbiting a barycenter with distances as similar as those for ALF Cen A and B, since the latter have similar masses (or, more similar, anyway), but AscendingNode? Argument of Pericenter? The most astronomy I've taken was up to ASTR 122 at the university, so I don't even know what that's referring to >_< I also left "AbsMag" blank because I'm not sure how to calculate the absolute magnitude of these stars, and was hoping Celestia could calculate it automatically... (I didn't want to use apparent magnitude because that would be a really really big number.) If I can't find a mathematical function or a chart online to do it, I might just look at the absolute magnitudes of known stars of the same spectral class and just invent a number that falls within their range.

Update: After some dinking around with other stars, I think the biggest problem is the lack of a magnitude for these two fictional stars. It looks like if you don't specify a magnitude (either absolute or apparent), Celestia just doesn't bother placing an object.

[fungun]

Here is an example of one of mine.

Code: Select all

Barycenter 80763 "Antares:HIP 80763"

   {
   RA      247.351917
   Dec        -26.432003
   Distance        604
   }


Star "Antares A"


   {
   OrbitBarycenter    "Antares"
   SpectralType    "M1I"
   AbsMag       -5.14
   SemiAxes       [1 0.75 0.75]
   EllipticalOrbit
      {
                      Period      878
                      SemiMajorAxis         537
                      Eccentricity          0
      Inclination   82.980
      AscendingNode      67.708
      ArgOfPericenter    3.774
      MeanAnomaly        200.119
      }
   RotationPeriod    24
   }


Star "Antares B"

   {
   OrbitBarycenter    "Antares"
   SpectralType    "B4V"
   AbsMag       -0.84
   SemiAxes       [1 0.75 0.75]
   EllipticalOrbit
      {
                      Period      878
                      SemiMajorAxis         537
                      Eccentricity          0
      Inclination   82.980
      AscendingNode   67.708
      ArgOfPericenter 183.774
      MeanAnomaly     200.119
      }
   RotationPeriod    24
   }

Tim

[selden]

A brief description of the various orbital parameters is at http://www.lepp.cornell.edu/~seb/celest ... eters.html

[Sadachbia]

Ah, okay, thank you Selden... *looks* OH GOD MATH. Is now a bad time to point out I tried taking calculus 3 times at the university and never passed? ._.

Don't worry, reading it anyway xD I'm sure I'll figure it out.

[ThinkerX]

For what little it might be worth, I made a data base, now an 'addon' containing over 30,000 more or less sunlike stars not in the primary version of Celestia.

[Sadachbia]

After a whole bunch of tweaking (and making up absolute magnitudes that seemed to fit with the binary stars' stellar classifications), I ALMOST have it to work...the only problem now is getting the brown dwarf component to show up. I can select it and go there, it's just invisible. At first I thought perhaps I had made its orbit around the system barycentre too small and it was inside the radius of the larger star, but I've kept increasing its semimajor axis (and finally the larger companion's as well) to about 8000 (though I don't know what unit that's in. AU? Kilometres?) and I still don't see it.

Still fiddling with the declination so that the binary star system is actually INSIDE the Andromeda Galaxy, instead of slightly above or below. Argh.

ETA: I'm thinking it might help to plug in numbers to an equation as posted on http://www.lepp.cornell.edu/~seb/celest ... eters.html, instead of just making them up... but I don't know the mass of the stars, and that seems to be kind of an important thing to know. Googling has shown me that the luminosity of a star is related to its mass by the equation L = M^3.5, and I know the luminosities of the objects whose mass I want to calculate (Celestia kindly auto-calculates that), but I have no recollection of what operation inverses raising something to an exponent. Natural log isn't the answer; taking ln() of both stars' luminosities gave me a mass of 6.301 MSol for the F9 Ib star (luminosity 545x Sol, obviously a larger star, so a higher mass makes sense) but a mass of -10.289 for the brown dwarf (T1) companion (whose luminosity is .0000340x Sol). I think I may have remembered at that point that you're not supposed to take logs of numbers less than 1, but like everything else, I'm nowhere near sure.

[selden]

Unfortunately, Celestia does not draw stars which are "too dim".
Hopefully this will be fixed in a future version.

For the time being, though, you'll have to increase the absolute magnitude until they're drawn. Sorry: it isn't obvious to me what values are needed -- it seems to depend on how far away they are from the viewpoint.

[Sadachbia]

The bigger problem I'm having at the moment is that my brown dwarf is waaayyy too big. Celestia tells me that its radius is 8.1 Rsol, which is nowhere near right. I tried changing its class from T1 to T6 (since I guess I've only ever seen brown dwarfs of class T6 or cooler in catalogues) and that made it BIGGER. Now it's 290x the Sun's radius. What's going on here? D:

[selden]

Celestia's method for calculating a star's radius doesn't work for Brown Dwarfs, so you should specify an appropriate Radius value in the STC file.

Code: Select all

Star "my_bd"
{
   Texture "browndwarf.*"
   RA 162
   Dec -39
   Distance 13.167
   SpectralType "M8.5V"
   AppMag 17.29
   Radius 100000
}