Celestia Forums

One of the things that I'd like to be able to do is display the orbits of stars within the galaxy.
In particular, I'd like to view flows of stars like those described at http://www.eso.org/outreach/press-rel/p ... 08-04.html


Has anyone considered trying to implement an Addon to show this?

Celestia can almost do this now: you can declare Star orbits and make them relative to a Barycenter that's at the center of the galaxy. The distance units are in AU, though, which is somewhat awkward. Also, the orbits of stars associated with the disk have an up-and-down component as they go above and below the disk. This happens several times during a single orbit around the galaxy's center, unlike a Keplerian orbit, which would go up and down only once. Orbiting a Barycenter around the galaxy and having a Star orbit in a smaller loop around that almost does the right thing.

I guess you could model it that way. Barycenter at the center of the galaxy and then a barycenter for the star to orbit on with a really high eccentricity to account for up/down. Not very realistic though. You would have backwards motion since you cannot set the eccentricity to model a line, hmm, something to try tonight. I would much rather see the ability to add radial and translational motion to stars, at least the major constellation stars, since Sol is the center of the universe (0,0,0) in Celestia (off by a smidge astronomically speaking.)

Buggs,

I think you're using the wrong terminology.

The orbit of the secondary barycenter would have a large inclination (90 degrees) relative to the primary orbit around the galactic center, not necessarily a large eccentricity (high ellipticity).

Also, it wouldn't necessarily cause the star to go backward in its orbit around the galactic center. That depends on the ratio of speeds of the two "orbits". (e.g. relative to the Sun, the Moon never goes backward. Its orbit actually is everywhere concave toward the Sun although it seems to orbit around the Earth about 12 times in a year.)

I found an article which summarizes the Sun's galactic orbit relatively clearly:
http://www.astro.ncu.edu.tw/contents/fa ... Galaxy.pdf

Its vertical epicycle has a period of something like 63MegaYears compared to a galactic orbital period of ~240MegaYears -- certainly not one that would produce backward motion!

Gotcha, I had to envision a circular orbit wheeling around the GC to get it straight in my mind. Kind of like watching a sin curve map out along a horizontal axis WHILE seeing it's x,y position on a unit circle... I think two dimensionally too much.

Well, I spent a little time on this. I am sure my orbital calcs are all wrong and forced. However, I got the barycenter to orbit the GC in the galactic plane. I tried having the sun orbit the EC barycenter (of course it disappeared, 16000 ly thing?). Instead of all of this, wouldn't it be easier to have the 0,0,0 point in Celestia orbit the galactic center in the code?

buggs_moran wrote:Well, I spent a little time on this. I am sure my orbital calcs are all wrong and forced. However, I got the barycenter to orbit the GC in the galactic plane. I tried having the sun orbit the EC barycenter (of course it disappeared, 16000 ly thing?). Instead of all of this, wouldn't it be easier to have the 0,0,0 point in Celestia orbit the galactic center in the code?

Code: Select all

Barycenter "GC"
{
        RA           266.25 # 17.7500
        Dec         -28.9300
        Distance   2.772e+04
}

Barycenter "EC"
{
      OrbitBarycenter "GC"

   EllipticalOrbit {
      Period             240e9
      SemiMajorAxis      1760000000.0
      Eccentricity           0.0
      Inclination      61.64
      AscendingNode      269.7   
      ArgOfPericenter     83.71
      MeanAnomaly        90.0
   }


I think Buggs are on to something here, and it's definietly more logical to have the 0,0,0 coordinates in the center of our galaxy.

Bye, Anders

You wouldn't necessarily want to move 0,0,0 to the center of the galaxy. Redefining all of the objects in Celestia from Heliocentric to a new Galactocentric origin would be daunting to say the least. RA, Dec and Distance might be simple mathematically, but getting orientations of objects like binary stars would probably be evil, I don't know for certain, maybe Grant can chime in here. I was thinking more along the lines of a new coordinate system. You can already define a barycenter coordinate system and stars around it, a star coordinate system and planets around it, and a planet coordinate system withe moons around it. Having a galactic coordinate definition would allow us to define our system in relation to it. What we have to do now is define the center of the galaxy from the sun, then define the orbit of the sun around the galaxy center, seems rather Ptolemaic. Not to mention the fact that the star disappears due to the OpenGL constraints. A long time ago Chris mentioned that one of the long term goals to overcome this would be to have multiple definable frames of reference given RA, Dec and Distance from Sol. I don't know if he ever got anywhere with that.

This is an older post on coordinate systems http://www.celestiaproject.net/forum/viewtopic.php?t=8662

For whatever reason, if you specify unrealistic AbsMag values, like -20, the stars remain visible at very large distances (outside the galaxy). They disappear at intermediate distances (within the galaxy) and then reappear when you get close.

A related problem: in order for the epicycles to display the vertical motions properly, they'd have to precess, staying perpendicular to the direction toward the galactic center. Celestia does not (yet?) support precession of orbits, only of planetary rotations.

Oh, and a minor point: the period is 240e6 (MegaYears), not 240e9 (GigaYears).

selden wrote:Oh, and a minor point: the period is 240e6 (MegaYears), not 240e9 (GigaYears).

THAT"S IT, I am packing my bags and going home. No more teaching math for me, nosiree Bob. Duh *smacks self upside the head*. Check your numbers they always said, and did I, No! I was "LOST" bound (TV show), I swear your honor, I didn't even see the 9, silly zeros. It seems like each one makes ten times the problem.