As everybody knows, Celestia does not take visual extinction (Av) in consideration in its calculations. This has some consequences, and one of them is the underestimation of the AbsMag and, hence, the calculation of the stars' radius when using the black body aproximation.

I'd like to proppose, for future consideration, the creation of a new, optional variable "Extinction" in the stc codes. When ommited, all the Celestia's calculations concerning the magnitudes of the star would continue as they are today. But if one declares it's value, the Av would then enter in the calculations, thus resulting in more realistic values for the AbsMag and the estimated radius of the star.

Take, for example, the star Schulte 16:

Code: Select all

`1010293161 "Schulte 16:Cyg OB2 16:LS III +41 33" {`

InfoURL "http://simbad.u-strasbg.fr/simbad/sim-id?Ident=schulte+16"

RA 308.1607096

Dec 41.4204975

Distance 4940 #Gaia DR2+Bailer-Jones

SpectralType "O7.5V"

AppMag 10.84

}

Without Av, Celestia calculates AbsMag=-0.06 and Radius=1.1Rsun (~782000Km). These data do not match those of a tipical O7.5V star. It's an unrealistic star and it would be too faint for its class when observed from stars close to it.

According to Chentsov

*et al*. (https://arxiv.org/abs/1306.1087), the Av towards this star is 4.5mag. So, if we could declare this Av in the stc code, like this:

Code: Select all

`1010293161 "Schulte 16:Cyg OB2 16:LS III +41 33" {`

InfoURL "http://simbad.u-strasbg.fr/simbad/sim-id?Ident=schulte+16"

RA 308.1607096

Dec 41.4204975

Distance 4940 #Gaia DR2+Bailer-Jones

SpectralType "O7.5V"

AppMag 10.84

Extinction 4.5

}

We would get Absmag=-4.56 and Radius=8.9Rsun (~6210000Km), much closer to the data of a typical O7.5V star.

Celestia would have to assume isotropic and homogeneous extinction though (still better than no extinction at all, IMHO). So, it would store the Av/pc for that star in order to calculate the AppMag for other distances.

**The math behind it:**

Let d be the distance of the star from Sol in parsecs. AppMag is its apparent magnitude.

Let Av be the total visual extincion in magnitudes declared in the code. It must be a positive value. The AbsMag of the star is given by:

AbsMag=AppMag-5*(log10(d)-1)-Av [eq.1]

Now, let Avpc=Av/d [eq.2]

This will be the extinction per parsec. We will consider the extinction homogeneous and isotropic, so the value given by [eq.2] will be used in any direction from the star.

So, for any given distance d' (in parsecs) from the star:

AppMag=AbsMag+5*(log10(d')-1)+Avpc*d' [eq.3]