## Re: How do I calculate a star's luminosity/radius?

Originally Posted by

**Bipo10** Hello there!

I'm new to this forum and astrophysics in general. I'm also in early high school, so please be patient if I can't grasp the math at first.

Anyway, I've been looking for a way to calculate a star's radius without depending on luminosity, and/or a way to calculate luminosity without depending on the radius of the star. I have found this equation(1) : R/Rs = (Ts/T)^2(L/Ls)^1/2. The problem, though, is that although the numbers are quite accurate for stars such as

Sirius the calculations start getting off for stars such as Vega or

Polaris. Is this equation correct? Or am I lacking some variable I don't know?

The equation is correct as it stands.

Another problem i've encountered is with (2) L = 4

**pi**R^2

**sigma**T^4. No matter what numbers I plug into it, it never gives the correct luminosity or radius. What am I missing here? I'm pretty sure I did my calculations correct, but the results never match... Also, what are the all units I have to use in it? All numbers in solar units? Or in kilometers and watts? (and by extention, the luminosity I get will be in which unit?)

Units are likely your problem not the equations. The equation (2) requires input in SI units. https://en.wikipedia.org/wiki/Intern...ystem_of_Units

There's this equation too (3): L = 4

**pi**d^2B. Again, no explanations on which units I should use and/or if it works only for a set of stars, etc.

Where does this come from? Define "B"?

*To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.*

: Refractors:

*To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.*

ST80 (mods for white light solar), SV ED80 f7 **Newtonians:** Z12 f5 **Catadioptrics:** VMC110L, Intes MK66 **EPs:** KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans **Binocular** Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. **Mounts:** Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG