Celestia Forums

In Celestia, it shows up as .08 rad, when in real life, it's .145 rad. I have tried to change it in the nearstars.stc, but it doesm't wanna change it..

here is relevant info on Proxima Cen
---------------
Component Proxima:
Proper names: Proxima Centauri, V645 Centauri, Alpha Centauri Proxima, Alpha Proxima
Catalog numbers:
Gliese (Gl) 551, Luyten Half-Second (LHS) 49, Hipparcos Input Catalog (HIC) 70890
Right Ascension and Declination: 14h29m42.91s, -62?°40'47.2" (epoch 2000.0)
Distance from Sol: 4.223 light-years (1.295 parsecs)
Standard error in distance: 0.31236%
Source for distance: Hipparcos
Celestial (X,Y,Z) coordinates in ly: -1.539, -1.178, -3.752
Galactic (X,Y,Z) coordinates in ly: 2.946, -3.021, -0.169
Proper motion: 3.809 arcsec/yr (281.7?° from north)
Radial Velocity: -16 km/sec
Source for proper motion and radial velocity: Gliese
Galactic (U,V,W) velocity components in km/s: -24.68, -2.473, 13.67
Spectral class: M5e
Luminosity Class: V
Apparent visual magnitude: +11.05 (increasing to +10.12)
Absolute visual magnitude: +15.49 (increasing to +14.56)
Visual luminosity: 0.0000555 x Sol (increasing to 0.000131 x Sol)
Variable type: UV Ceti flare star (eruptive variable)
Color indices: B-V= +1.83, U-B= +1.43, R-I= +1.66
Mass: 0.123 x Sol
Source for mass: European Southern Observatory
Diameter: 0.145 x Sol
Source for diameter: European Southern Observatory
Comfort Zone (visual): 0.00745 A.U.s
Orbital period in CZ: 16.0689 hours
Tidal index in CZ: 297597
Angular size of star in sky in CZ: 10.397477 degrees

doctrellor,

Celestia calculates the radius from other information: the magnitude and stellar type. It isn't going to be precise, but I'm surprised it's off so much.

Where did you get the information you provided?

Celestia uses data only from formal astronomical databases, not information that happens to wander in off the street

According to nearstars.stc, the data it uses is from the RECONS Web site at
http://joy.chara.gsu.edu/RECONS/TOP100.htm
but which recently changed its name to
http://www.chara.gsu.edu/RECONS/TOP100.htm

[quote="selden"]doctrellor,

Celestia uses data only from formal astronomical databases, not information that happens to wander in off the street ;)

[/quote]

I guess professional astronomers in Europe are just presenting info "off the street"

from my post above..lol

Mass: 0.123 x Sol
Source for mass: European Southern Observatory
Diameter: 0.145 x Sol
Source for diameter: European Southern Observatory

I'll get the site for ya in a sec

http://www.stellar-database.com/

on the search for star by name...

Put in Proxima Cen then click Submit Query, and it spits out the info I posted above..:)

as far as Proxima Cen having .145 rad, try

http://en.wikipedia.org/wiki/Proxima_Centauri
http://www.solstation.com/stars/alp-cent3.htm
http://www.daviddarling.info/encycloped ... maCen.html

and a few other reputable sites has the same info

That's what the text says, but did you look up those values yourself or did you cut-and-paste it all from a Web site?

A quick search of the ESO Web site didn't turn up a page with that particular information.

Again, what site(s) did the information come from?

FWIW, the RECONS site shows a mass estimate of 0.11x the Sun's, which is consistant with the value you show, but one still must be skeptical when claims are made without proper references.

The inclusion of a "Comfort Zone" suggests that the page was cut-and-pasted from a site like Solstation. While most of their information is reasonably good, they do have their own biases. When providing data, it's usually best to go to the original source. There are too many chances for typos to be introduced when getting the information from secondary sources like Solstation. Sites like Wikipedia one has to be especially careful of. Their pages too frequently get modified by well-meaning but misinformed individuals or even defaced by not-so-well-meaning folks.

p.s. Ok, it wasn't solstation
But it isn't a primary source, either. It's one individual's personal labor-of-love.

selden wrote:That's what the text says, but did you look up those values yourself or did you cut-and-paste it all from a Web site?

A quick search of the ESO Web site didn't turn up a page with that particular information.

Again, what site(s) did the information come from?

FWIW, the RECONS site shows a mass estimate of 0.11x the Sun's, which is consistant with the value you show, but one still must be skeptical when claims are made without proper references.

Actually, the radii calculated by Celestia for low mass stars are probably wrong if it's using the same radius formulae as for larger stars. I'm using a a formula of radius = SQRT(L)/((T^2)*SQRT(4.pi.sigma)) where radius is in metres, L is in watts, and T is in Kelvin - though conveniently I can't remember how I derived that, but it seems to make sense. That formula breaks down for the lowest mass stars though - they come out as being smaller than Jupiter (sometimes about the size of Earth!) if you use the unmodified formula . This formula works fine for more massive stars, but below about 0.3 solar masses it produces inaccurate results. I think the radius-mass relationship must change at lower masses, because evidently this formula doesn't work for them.

I've got an empirical fudge formula of radius = (0.85*Ms)+0.05, where Ms is the mass of the star in solar masses and the radius is in solar radii. That kinda fits observed values for solar metallicity stars.

It's easy enough to modify the radius of Proxima if it's important to you.
Place a little ssc file in the extras folder, that looks like this:This is just a cut-and-paste from the nearstars.stc entry with an extra line specifying the Radius. It'll override the Proxima definition and force its radius to whatever you want (in this case I've used 0.145 solar radii).

I don't do this generally because:
1) Radii are not well confirmed for most stars
2) There are a lot of stars to fix by hand
3) I don't know what else in Celestia gets broken when I force a radius away from the calculated value

Celestia underestimates the radii of cool stars generally, because they deviate significantly from the perfectly spherical, uniformly glowing black bodies of the sort required by the formula Celestia uses (which is as Malenfant describes). They none of them have spectra that look very much like the ideal black body curve for their temperature, they have big star spots, and (in the cool supergiants) huge limb darkening and great big lumps sticking out of them.

Grant

cool, thx guys..:)