Stefan-boltzmann solved for temperature?

[~jet]

My physics is extremely rusty (I was bored to death back in school and had no practical application of the stuff to make it interesting and/or truly sink it.) Now, I have an application, and while I have a long way to go, it is pleasurable to be comprehending things as I go.

To a point, and here is where I ask for some help and confirmations. Namely, I need to find the effective (surface?) temperature of a star (presumably main sequence and not extraordinary in mass) based on its mass.

First, smash my face in if I miscombobulated the stefan-boltzmann law:

L = 4\piR^{2}\sigmaT^{4}_{eff}

... to look for T_{eff}: (and turning 4\piR^{2} into A for now)

T_{eff} = \sqrt[4]{L/A\sigma}

No?

Walp, I've already plugged this into my code, and I'm getting absolutely absurd results for a near-solar-mass star: 4.23581197119e+23 (x10^{23} for the noncoders out there) when I'm expecting to get roughly 5700-5800K. So I am forced to assume either that I flubbed my reconfiguring of S-BL or I am horribly misreading the S-B constant:

\sigma = 5.670373(21)×10^{−8}Jm^{−2}s^{−1}K^{−4}.

That's quite a unit... I'm almost certain I'm not reading it correctly. Tips?

 

[George Jones]

My physics is extremely rusty (I was bored to death back in school and had no practical application of the stuff to make it interesting and/or truly sink it.) Now, I have an application, and while I have a long way to go, it is pleasurable to be comprehending things as I go.

To a point, and here is where I ask for some help and confirmations. Namely, I need to find the effective (surface?) temperature of a star (presumably main sequence and not extraordinary in mass) based on its mass.

First, smash my face in if I miscombobulated the stefan-boltzmann law:

L = 4\piR^{2}\sigmaT^{4}_{eff}

... to look for T_{eff}: (and turning 4\piR^{2} into A for now)

T_{eff} = \sqrt[4]{L/A\sigma}

Yes, if you mean \sqrt[4]{L/\left(A\sigma\right)} or \sqrt[4]{L/A/\sigma}, and I get the correct result using this.

Walp, I've already plugged this into my code, and I'm getting absolutely absurd results for a near-solar-mass star: 4.23581197119e+23 (x10^{23} for the noncoders out there) when I'm expecting to get roughly 5700-5800K. So I am forced to assume either that I flubbed my reconfiguring of S-BL or I am horribly misreading the S-B constant:

\sigma = 5.670373(21)×10^{−8}Jm^{−2}s^{−1}K^{−4}.

That's quite a unit... I'm almost certain I'm not reading it correctly. Tips?

For troubleshooting, it would help if you posted \sqrt[4]{L/A\sigma} exactly as you coded it, including the numerical values that you used.

 

[~jet]

Yes, if you mean \sqrt[4]{L/\left(A\sigma\right)} or \sqrt[4]{L/A/\sigma}, and I get the correct result using this.

Yip yip, that's what I mean; good to hear.

For troubleshooting, it would help if you posted \sqrt[4]{L/A\sigma} exactly as you coded it, including the numerical values that you used.

the code is simple python, accepting solar masses as an argument:

star.mass = 1.98892e+30 (kg)
star.luminosity = star.mass ** 3.5 (1.1095850642735086e+106)
star.radius = (star.mass ** 0.8) * solar_radius (~695,500,000m)
star.surface_area = (4 * pi * (self.radius ** 2)) (~6.078608e+18 m²
star.effective_temperature = (self.luminosity / (self.surface_area * stefan_boltzmann_constant )) ** 0.25 (~4.2358e+23 wawahoozit units)

The code seems fine to me (assuming I've taken accurate information from the solar-comparative equations peppering the web.) I think I am mangling the units. Probably starting with luminosity... looking at it in erg/s (on the order of 10³³) I think I see a place to start

 

[George Jones]

star.luminosity = star.mass ** 3.5 (1.1095850642735086e+106)

This can't be correct. I think it should be something like
\frac{L}{L_{Sun}} = \left( \frac{M}{M_{Sun}} \right)^{3.5}.

 

[~jet]

pulled it from http://en.wikipedia.org/wiki/Mass–luminosity_relation ... I may be misusing the proportionality bit. Have we really not worked out how to model these based purely on mass yet? (i.e., independent of having to use the sun as a comparator.)

 

[~jet]

In any event, I shall plug away it at again; converting my radius to cm, I was able to get closER (as (erg/s)/(cm2erg/scm2K4) canceled down a lot easier)... at my next opportunity, I see what happens when I try that as you show it instead.

 

[~jet]

This can't be correct. I think it should be something like
\frac{L}{L_{Sun}} = \left( \frac{M}{M_{Sun}} \right)^{3.5}.

Kay, so still doing something wrong... I'm getting 32486K for a sunmass star when I use what wikipedia claims. Am I misconstruing your equation when I flip it to

L = \left(\left( \frac{M}{M_{Sun}} \right)^{3.5}\right)L_{Sun}.

Further, why do you say 3.5 and wikipedia claim 4?

 

[~jet]

Perhaps I'm also asking the wrong question; I may still be blending my units poorly.

T = \sqrt[4]{\frac{L}{A\sigma}} and I try to read this in units:

\frac{erg/s}{\frac{cm^{2} erg}{s cm^{2} K^{4}}}

If that isn't butchered, it does have a pleasing cascade of cancellations that lead me to

\frac{1}{K^{4}}

That's either wrong or I don't know how to ~read~ that properly. Beware, I'm many years out of practice and didn't have anything by way of application at the time to let it truly sink in.

 

[~jet]

Solved it; I was unwittingly trying to combine erg/s with watts