Stc Part Addons Tutorial

## Introduction

An STC file, or star catalog, specifies the position and other parameters of a star or barycenter.

This is a brief description of the STC parameters recognized by celestia V1.4.1 and newer versions of the program. STC functionality has undergone a substantial improvement when compared to the previous versions of celestia, V1.3.2 and earlier.

## List of parameters

* Barycenter
* Star
	+ OrbitBarycenter

Or

* + RA
	+ Dec
	+ Distance

* + AppMag(Apparent magnitude) or AbsMag(Absolute magnitude)
	+ SpectralType
	+ Radius (optional; the default value is calculated from the Star's SpectralType and magnitude.)
	+ SemiAxes or Mesh (optional)
	+ Texture (optional)
	+ EllipticalOrbit (optional)
		- Orbital elements (not listed here: see SSC documentation)
	+ Rotational elements (not listed here: see SSC documentation; optional)

## Description

### Type and Identifier

* **Barycenter** or **Star**
* *<number>*
* "*<name>* : *<name>*"

An STC catalog file can be used to describe either **Stars** or **Barycenters** around which **Stars, Barycenters** and SSC objects can orbit.

A **Barycenter** defines only a position in space. You can't see it. It's the center of mass of a collection of bodies. A **Barycenter** can orbit around another **Barycenter** or around a **Star**.

A **Star** is a glowing object described by a position plus visual parameters like **SpectralType**, **AppMag** or **AbsMag** and optional **Mesh**, **Radius**, **SemiAxes** and **Texture** declarations. A **Star** can orbit around another **Star** or around a **Barycenter**.

There are three optional fields used to introduce an STC object. They specify the type of object it is and provide unique identifiers for it. In their required order of appearance, they are

* (Optional) type of object: either **Barycenter** or **Star**. If omitted, **Star** is assumed.
* (Optional) Hipparcos catalog number: if this Hipparcos catalog number is the same as the Hipparcos catalog number of a previously existing Star or Barycenter, that object is replaced by this one. Barycenters and Stars can replace each other or objects of the same type.
* (Optional) quoted name: can be a list separated by colons (:). This will replace the names of a previously defined object with the same catalog number. If no name is specified, the original names will be retained.

Of the three, at least a HIP number or a Star name is required.

Examples:

* Type of object, Hipparcos catalog number, and two names

```
           Barycenter 100000 "My Barycenter:My System" { ... description ... }

```

* Name of object (type of object is assumed to be Star)

```
           "My Star" { ... description ... }

```

All of the other parameters used to describe a particular STC object are enclosed within curly-brackets: { }

#### Bayer Star Names

When a Bayer star name is specified, the 3-character abbreviation for the Greek letter as used in the Hipparcos catalog should be specified. These abbreviations are

```
   alpha = ALF   beta= BET   gamma= GAM   delta= DEL   epsilon= EPS
   zeta  = ZET   eta = ETA   theta= THE   iota = IOT   kappa  = KAP
   lambda= LAM   mu  = MU.   nu   = NU.   xi   = KSI   omicron= OMI
   pi    = PI.   rho = RHO   sigma= SIG   tau  = TAU   upsilon= UPS
   phi   = PHI   chi = CHI   psi  = PSI   omega= OMD

```

e.g.

```
Star "ALF Cen" { ...description... }

```

This table is taken from *HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index* Kostjuk N.D., Institute of Astronomy of Russian Academy of Sciences (2002), which is available at http://cdsarc.u-strasbg.fr/viz-bin/Cat?IV/27

#### Constellation Names

Similarly, when specifying a constellation name as part of a star's name, use its official 3 letter abbreviation. The International Astronomical Union provides a list of constellation abbreviations.

### Position

* **RA**
* **Dec**
* **Distance**

or

* **OrbitBarycenter**
* SSC-style **EllipticalOrbit** parameters (optional)

The nominal location of a Star or Barycenter can be specified using either RA, Dec and Distance or by using the keyword OrbitBarycenter to specify the name (and thus the position) of a previously defined **Barycenter** or **Star**.

For example:

```
     RA 299.0    # in fractional degrees, 0-360
     Dec -10.5   # in fractional degrees, -90 to + 90
     Distance 30 # in light years

```

or

```
     OrbitBarycenter "My Star"

```

Note that in an STC catalog, **RA** must be specified in degrees. This is unlike a DSC catalog, where **RA** must be specified in hours.

* A **Star** or **Barycenter** can orbit around its own nominal position, around another **Star** or around a **Barycenter**.

The orbit of a **Star** or **Barycenter** is specified using the same **EllipticalOrbit** declaration as is used in SSC files. (A list of parameters is omitted: see the chapters on SSC files and Trajectories. Celestia v1.5.0 Frame declarations are not yet supported in STC catalogs.)
A **Star's** or **Barycenter's** elliptical orbit is defined relative to the object's nominal location, which is specified using either an **RA**, **Dec**, **Distance** triplet or by **OrbitBarycenter**.
Specifying **RA,Dec,Distance** is slightly more efficient than **OrbitBarycenter**, but only at load time, when Celestia has to lookup the position of the Barycenter. Despite this, OrbitBarycenter should be used for multi-star systems so that Celestia can determine that two stars orbiting the same point are actually associated.

### Appearance

* **SpectralType** *"string"*
* **AbsMag** *f* or **AppMag** *f*

* **Radius** *f* (optional)
* **SemiAxes** [*f* *f* *f*] (optional)
* **Mesh** <*filename*> (optional)
* **Texture** <*filename*> (optional)

* SSC-style orientation parameters (optional)

* By default, the appearance of a **Star** is controlled by its **SpectralType** (coloration) and magnitude (brightness: either **AbsMag** or **AppMag**). Celestia's spectral type field can include any of the standard astronomical spectral type and luminosity codes. See below for a description of the magnitude standard used by Celestia.

* The size of a Star is usually calculated from its SpectralType and its Magnitude. Optionally one can force a Star to be a particular size by specifying its **Radius** in km.

* The default spherical shape of a Star can be modified by the optional declaration **SemiAxes**. **SemiAxes** specifies the relative sizes of a **Star's** radii in three dimensions:

```
     SemiAxes [1 0.85 1]

```

* The default spherical shape of a **Star** can be replaced by using the optional model declaration **Mesh**.

```
     Mesh "accretion_disk.cmod"

```

A **Mesh** may be a 3DS model, or one of Celestia's proprietary model types, either CMOD or CMS, which are described elsewhere.
* The default surface texture of a **Star** (normally determined from its **SpectralType**) can be replaced by using a **Texture** declaration

```
     Texture "image filename"

```

Note: as of Celestia v1.4.0, only one default texture is used for all stars: astar.jpg. Its coloration is determined by the **Star's** **SpectralType**.
* The orientation of a **Star** is controlled by the same optional rotational element declarations as are used in SSC catalogs, although v1.5.0's BodyFrames and related functions are not yet available. (A list of rotational parameters is available in the SSC documentation.)

* Other SSC parameters are not supported. In particular, Bumpmaps, Normalmaps, NightTexture and Atmosphere (including CloudTextures) are ignored.

Also, AltSurface and Location declarations are not (yet?) supported.
## Example STC definition

This example is taken from Grant Hutchison's star catalog file Celetia/data/nearstars.stc. It's one of the catalogs which are included with Celestia v1.4.0 and later.

```
Barycenter "Rigel Kentaurus:Toliman:ALF Cen:Gliese 559"
{
 RA 219.917516 # mass ratio 1.09:0.92
 Dec -60.837128 #
 Distance 4.365
}

71683 #  ALF Cen A
{
 OrbitBarycenter "ALF Cen"
 SpectralType "G2V"
 AppMag 0.01
 EllipticalOrbit
 {
  Period 79.914
  SemiMajorAxis 10.765 # mass ratio 1.09:0.92
  Eccentricity 0.5179
  Inclination 82.980
  AscendingNode 67.708
  ArgOfPericenter 3.774
  MeanAnomaly 200.119
 }
}

71681 # ALF cen B
{
 OrbitBarycenter "ALF Cen"
 SpectralType "K0V"
 AppMag 1.34
 EllipticalOrbit
 {
  Period 79.914
  SemiMajorAxis 12.755 # mass ratio 1.09:0.92
  Eccentricity 0.51479
  Inclination 82.980
  AscendingNode 67.708
  ArgOfPericenter 183.774
  MeanAnomaly 200.119
 }
}

```

## Renaming Stars

The following procedure can be used to change the names associated with a Star or Barycenter in Celestia.

* Create an STC catalog file using your favorite text editor.
* Specify the star's Hipparcos number.
* Enclose the replacement name(s) in quotes, separated by colons.
* Include the full location and stellar type information for the star.

For example, this STC file (betel.stc) renames Betelgeuse:

```
27989 "Betelgeuse:Al Mankib:Almerzamo nnagied:Gula:Besn:ALF Ori:58 Ori"
{
 RA 88.792958
 Dec 7.407064
 Distance 419.13
 AppMag 0.58
 SpectralType "M1Iab"
}

```

The alternative names above are from *Star Names, Their Lore and Meaning* by Richard Allen, Dover, 1963. The location and spectral information are from the Simbad catalog server.

## Visual magnitudes in Celestia

by Fridger Schrempp.

In view of various different standards used to characterize (apparent) magnitudes, it might be worth specifying what Celestia actually wants:

In Celestia, AppMag corresponds to the so-called V-magnitude in the Johnson UBV photometric system!

Confusion may arise, since Celestia uses data from the Hipparcos catalog and that has introduced its own visual filter profile. The Hipparcos "visual" magnitude is called Hp.

This profile is much wider in wavelength than the "standard" Johnson one.

Moreover, Tycho data again use different photometry, designated with V\_T and B\_T for the visual and blue magnitudes, respectively.

There exist detailed mutual transformation algorithms and/or lookup tables. The relations are quite non-trivial in general.

For interested people, here is a good summary of transformations etc.: http://www.tass-survey.org/tass/catalogs/tycho.html

## Hipparcos and Tycho Catalog Numbers

Celestia uses Hipparcos catalog numbers as the primary identifier for its Stars. Hipparcos catalog numbers, often identified in documents with the prefix HIP, range from 1 to 120404. In Celestia, Tycho catalog numbers start at 1 000 000 000.

Tycho catalog numbers can be used, but they have to be "encoded" into fake Hipparcos numbers. They all are encoded into a 10 digit number with a value greater than 1 000 000 000. To do this, the three numeric fields of a Tycho catalog number are reversed and concatenated into a single number:

```
     TYC aaaa-bbbbb-c becomes cbbbbbaaaa

```

E.g. TYC 654-32-1 becomes 1000320654. (Note, however, that this number does not correspond to an actual Tycho catalog entry. It is only used to show how the numeric fields are rearranged.)

Celestia rearranges these numbers to display the correct Tycho catalog number on the screen.

The following STC catalog entry defines the star TYC 654-32-1 named "TYC Star"

```
1000320654 "TYC Star:Test Star"
{
 RA 123.45
 Dec 45.67
 Distance 89
 SpectralType "G1V"
 AppMag 8.7
}

```

Note that Celestia uses 32-bit integers to represent star numbers, so cannot use star numbers above 4294967295 (which would correspond to TYC 7295-29496-4): this should not limit the implementation of real Tycho stars.