Celestia Forums

Hi everyone,

Here's something a bit different. Populating galaxies with stars is something that's been done before, but doing so with data from SIMBAD is something new, I believe.

This add-on adds about 370 stars in the dwarf galaxy Wolf-Lundmark-Melotte (WLM). These stars are real, and the data is from SIMBAD. Specifically, I've extracted names, coordinates, and spectral types (if known) from SIMBAD. For stars whose spectral types are not known, variable types have been used to fill in random spectral types; for example, Carbon stars would be given a spectral type of "C". Because the distance of these stars aren't known, I used special formulae to come up with random (but realistic) distances for each of the stars.

Note that in the default version of Celestia, WLM is rendered as a blob (Type "Irr") and not its more elongated shape. So, for the stars to actually fit in the galaxy, go to your galaxies.dsc and replace the definition of WLM with this:



Enjoy:

Looks quite good, thank you!

(Okay, the newer add-on was lost, so I'll have to reupload it.)

More stars!

Here's about 1600 stars in the Sculptor Dwarf Spheroidal Galaxy (Sculptor dSph). If you look at a picture of the galaxy you'll see it has very blurry edges, and this is reflected in the addon, where stars are even scattered outside the boundaries of the rendered galaxy.

Enjoy:

I finally got off my butt and made another one of these. Here's about 3300 stars in the Fornax Dwarf Spheroidal Galaxy (Fornax dSph). Like Sculptor dSph, the stars are quite scattered. Fornax dSph also happens to have five globular clusters, but I haven't added those yet, and I've kind of forgotten how to do those...

Enjoy:

Please do Messier 110, please?

Gurren Lagann, I'll try to do Messier 110, but note that sometimes the stars might not be able to be properly fitted into the galaxy. Also, these take a lot of time, which is why it took a few days to finish off the one I was working on. Which is this.

This adds about 670 stars in the Draco Dwarf. Like the previous two files, the stars are quite scattered, and that's a very common thing in dwarf galaxies.

Enjoy:

I've published two files. The first one about 140 stars in Messier 110 (M 110). The other adds about 250 stars in Messier 32 (M 32).

For both galaxies, SIMBAD's coverage of the stars is uneven, so the galaxies won't be fully populated, but that's fine.

I wonder if you can do M 31/Andromeda and M 33/Triangulum. Or even Smith's Cloud.

Gurren Lagann, M 31 and M 33 are in theory possible but because they're spiral galaxies, populating them is a bit different. It would be easy if they were face-on (inclination = 0°) or edge-on (inclination = 90°) but it's not the case for either of these galaxies.

As for Smith's Cloud, it's just a cloud of hydrogen gas without any stars (probably), and SIMBAD lists no children objects anyway. You could try making a nebula model, maybe?

Oh then. Good luck on populating M 31 and M 33!

I'm no model maker btw

EDIT: Jeez, 10790 children for M 31 and 4301 children for M 33

Does anybody know what the "Radius" parameter means when defining a globular cluster in Celestia? There are ways to define the size of a globular cluster, including the half-light radius (r_h) and the tidal radius (r_t). I assumed it meant the tidal radius, but I'm not so sure anymore.

The tidal radius, or Hill sphere, is the distance from the center of the globular cluster at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself. This is the distance at which the individual stars belonging to a cluster can be separated away by the galaxy.

Gurren Lagann, thanks, but I'm looking for the definition for Celestia code; i.e. which of the three radii it refers to.

Maybe the tidal radius? Because the half-light radius is either the CoreRadius or KingConcentration parameters.

This add-on adds 52 globular clusters around the Andromeda Galaxy (M 31).

Many of the parameters are approximations. Some random variation has been added to the "Distance" parameter, so the globular clusters don't appear in one plane. That means that the "CoreRadius" and "AbsMag" will also be slightly skewed. The data were cobbled from seven papers, and Mayall II has been custom-built from data from Ma et al. (2007).

Did you manage to determine which radius is used by Celestia?

I was about to suggest that if you compare the values of the parameters used in Celestia's globular cluster catalog file globulars.dsc with the published values for various globular clusters, that should tell you which is meant.

LukeCEL wrote:This add-on adds 52 globular clusters around the Andromeda Galaxy (M 31).

Many of the parameters are approximations. Some random variation has been added to the "Distance" parameter, so the globular clusters don't appear in one plane. That means that the "CoreRadius" and "AbsMag" will also be slightly skewed. The data were cobbled from seven papers, and Mayall II has been custom-built from data from Ma et al. (2007).

only the stars are left

selden wrote:Did you manage to determine which radius is used by Celestia?

Sadly I couldn't. Just to be sure, I looked at two globulars, M 71 and 47 Tucanae. 47 Tucanae's "Radius" is 106.7 light years, which at its distance would be 0.416° = 24.96′. This corresponds to the SEDS 2007 catalog, which gives a diameter of 50.0′. M 71's "Radius" is 13.66 light years, which at its distance would be 0.06° = 3.6′. It also corresponds to the SEDS 2007 catalog, which gives a diameter of 7.2′. So, I assumed that it was used for the "Radius" parameter, but when I looked to see its definition, it just said "Apparent dimension in arc minutes".

The "Radius" parameter might just be half the apparent diameter of the object, which is a shame because that's not really rigorously defined. (Also, I may have to re-do my entire file.)

Added after 11 minutes 26 seconds:
If you've downloaded my LMC and SMC add-on, it's fine, because I used the apparent dimensions for that file.

Another source of info about galactic clusters is the database available at https://heasarc.gsfc.nasa.gov/W3Browse/all/globclust.html
It includes both the "core radius" and the "half light radius" for more than 150 of them.

selden, thank you for that. It won't really be relevant to my add-on, which is about globular clusters around Andromeda, but it might be possible to extract a few more globular clusters to add to the original globulars.dsc file.

Added after 8 hours 54 minutes:
The new version contains only 24 globular clusters, and the new Radius values feel uncomfortably small. For example, Mayall II's radius is 0.0283′ which corresponds to 10.34 ly. That's less than its half-light radius, which conceptually makes no sense...