Celestia/Tutorials/Globulars

This tutorial will describe how to add Globular into Celestia. Globular are a bit hard objects to add to Celestia, because most of them can be simulated as bits of STC code.

To add a Globular to Celestia, you need a .stc file. This can be created by taking any plain text file (.txt) and renaming the file extension to .stc. This file can be named anything as long as it has the .stc suffix. Then, it should be placed into the "extras" directory (i.e. a folder), or any folder within the "extras" directory.

Now, after you open the .stc file, you need to define a Globular by writing some code in it with your favorite text editor. Information for many Globular can be found in various places, outside of the milky way core

Basic definition

The basic definition looks like this:

Galaxy Name
Globular "Globular Name"        {
RA <number>	                 Dec <number>
Distance <number>
Radius <number>

With additional (optional) parameters, it looks like this:

SMC                      Globular "Globular Name"        {                               RA <number>                    Dec <number>               Distance <number>           Radius <number>         CoreRadius <number>
KingConcentration <number>
AppMag <number>               Axis  [ <Number> <Number> <number>]                   Angle <number>                  }

We'll go through the parameters one by one. Let's use the Globular NGC 1466 as an example.

Also, at any point you can add a comment to your .stc code. A comment is started by a # sign and lasts until the next line break. It's a good idea to add comments explaining if you calculated parameters yourself, or if they are guesses.

List of parameters

Name

Galaxy Name                          
"Name"

Here, Name is just the name (or names) of the Globular. If a Globular has multiple names, separate them with colons (:). like this:

SMC                           
"NGC 1466:PGC 2802621"

And it other names. so it would look like this:

SMC                           
"NGC 1466:PGC 2802621:ESO 54SC16"

RA and Dec

	RA <number>
	Dec <number>

These are the right ascension and declination of the Globular, i.e. the coordinates of the Globular on the sky. Note that in an STC file, RA is in degrees, unlike a DSC file where it's in hours. Usually the RA will be in hours/minutes/seconds format, and the Dec will be in degrees/arcminutes/arcseconds format. To convert to Celestia's decimal format, use a tool like the RA DEC flexible converter. You can also tell SIMBAD to output decimal coordinates by going to the Output options page and selecting "decimal" from the drop-down menu next to "Coordinates".

Distance

	Distance <number>

The distance to the Globular in light-years.

CoreRadius

	CoreRadius "Number"

the size of the Core of the Globular the core can be 0.2 (NGC 1466 Size)

AppMag or AbsMag

	AppMag <number>

or

	AbsMag <number>

This is the apparent magnitude of the Globular (how bright it appears from Earth), or the absolute magnitude (how bright it would appear from a distance of 10 parsecs), without extinction (dimming, caused by dust that is blocking light). The AppMag in Celestia corresponds to the V-band magnitude in sources such as SIMBAD.

---

The above parameters are all that's required to define a Globular. Here's the basic definition for NGC 1466:

SMC                           
"NGC 1466:PGC 2802621"
{
	RA 0.4478528
	Dec -71.5225806
	Distance 2.12e+05
	Radius 175
   CoreRadius 0.2
   KingConcentration 1.25
   Axis [-0.7429  -0.2364  -0.6263]                            
	AppMag 11.705
}

Now for the optional parameters...

---

Axis

	Axis [ <Number> <number> <number>

axis where the Globular rotation

KingConcentration

 	KingConcentration <number>

Where it rotation

Radius

	Radius <number>

Most globular clusters have a half-light radius of less than ten parsecs (pc), although some globular clusters have very large radii, like NGC 2419

Angle

	Angle <number>

It Angle where it on our view

Example code

For the sake of reference, here is what the final .stc code might look like for NGC 1466:

SMC                           
"NGC 1466:PGC 2802621"
{
	RA 0.4478528
	Dec -71.5225806
	Distance 2.12e+05
	Radius 175
   CoreRadius 0.2
   KingConcentration 1.25
   Axis [-0.7429  -0.2364  -0.6263]                            
	AppMag 11.705     
}