Wrong (and probably non-existent) spectral class

[hharris]

I'm on Mac OS X using Celestia 1.4.1 with the original database.

The spectral class names for the stars are mostly correct, but there is one glaring error for a very important star. According to Celestia, the spectral class for Rigel Kentauras (a.k.a. Alpha Centuri) is "Bary." I believe it should be G2V.

Great program!

Henry

[Chuft-Captain]

Are you sure you've selected the star?

In the previous version (1.4.0) which I'm running, Rigel Kentaurus is not a star, it's the Barycenter that Rigel Kentaurus A and Rigel Kentaurus B revolve around.

[hharris]

Ahha! That explains a mystery. I've written a script that allows me to fly anywhere (using simulated sensors that track nearby objects) . I flew out to this "star" and found nothing but empty space.

Two questions about this:

(1) Should I use the spectral class to tell the difference between a star and a barycenter, or is there another, perhaps better, flag?

(2) What other types of non-physical "locations" are in the database?

Thanks!

Henry

[selden]

All of the Barycenters that come with Celestia are defined in STC catalog files that are in the /data/ directory. Other locations without associated objects can be defined as Nebulae or OpenClusters in DSC catalog files, but none of those types are included with Celestia yet. (They are used in various Addons.) Also, Class "invisible" can be used to define orbiting locations in SSC catalogs. In addition, models with fully transparent materials can be created, making them invisible, too.

Oh, and of course Locations can be defined in SSC files.

[hharris]

Interesting. Are Barycenters used by the program somehow, or is this just data of technical interest?

I noticed that the data I really wanted was not there, namely the mass of these objects, so I had to write code that estimates the mass of planets, moons and stars from the supplied radius using published mass-radius studies .

Henry

[selden]

Celestia does not use mass values. Although they can be specified, they're ignored. Celestia does not calculate gravitational effects, it uses Keplerian ellipses (EllipticalOrbits), ephemerides (CustomOrbits) or xyz trajectories (SampledOrbits).

Barycenters are the common foci for the orbits of multi-body systems in Celestia, just as they are in real life.

You probably should browse Celestia's catalog files for more information. Orbital parameters are a more reliable mass indication than a body's radius.

[hharris]

Thanks! Does that mean that Celestia binary system stars (for example ) are moving around each other? Wow. If that's true, I see why Barycenters would be in the database.

As for calcuating mass from orbital parameters. .. Mmmmm. I guess you could calculate the mass of the star from the orbits of the planets, but most stars in the database don't have planets. You could get an idea of the mass of the planets from the mass of the moons making the assumption that planet mass was much larger than the moon.

In any case, unless I'm missing your point entirely, I don't think you could write code that would be very general that way. But you can write a fairly general routine that calculates mass from radius for stars , planets and moons that comes pretty close. Look at a plot of radius versus mass for stars, for example; it's almost a straight line with small deviations when the star leaves the main sequence. You can't use the same code for all types of bodies of course, but it's fairly easy to segment it into classes. For example gas giants, large terrestial and small terrestrial planets are 3 convenient classes to model with simple math.

Henry

[selden]

Thanks! Does that mean that Celestia binary system stars (for example ) are moving around each other? Wow. If that's true, I see why Barycenters would be in the database.

Celestia does include many orbiting binaries.

My major quibble with radial mass approximations is for planetary and brown-dwarf objects with masses between 1x and ~80x Jupiter's mass. My understanding is that the radius decreases with increasing mass in that region, so there's significant ambiguity. That range seems to include many of the exo-planets that have been discovered so far.

[hharris]

Good point. My current code does not include stellar objects that small, which is good from my point of view. I enjoy researching the latest astrophysical findings and try to make predictive models that reflect current understanding. I expect my script will be continually updated as we find out more. Fun, fun ... and more fun!

Henry