Determining the radius of a star

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Discussion Overview

The discussion revolves around determining the radius of a star, specifically the sun, and modeling its density. Participants explore various methods and equations related to stellar modeling, including the geometry involved in calculating volume and density, as well as the use of established equations in astrophysics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant proposes a formula for density based on volume and mass, using a cylindrical model, and seeks feedback on its correctness.
  • Another participant suggests that the problem is already solved and references the Lane-Emden equation, questioning the appropriateness of using a cylinder for a spherical object like the sun.
  • A third participant acknowledges a mistake in the cylindrical model and shares their recent experience with stellar modeling, expressing interest in further resources for learning.
  • Another participant explains that calculating the volume of a distant star is feasible with known distance and surface brightness, mentioning the use of binary systems for mass estimates.
  • One participant reflects on the original poster's intent to derive equations independently and expresses uncertainty about the correctness of the initial equation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correctness of the initial density equation. There are multiple competing views regarding the modeling approach and the methods for determining the radius of the sun.

Contextual Notes

There are unresolved assumptions regarding the appropriateness of the cylindrical model for a spherical star, as well as the specific methods for calculating mass and volume in stellar contexts.

travwg33
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I'm working on trying to determine a formula that can model the density of the sun.

density:
\rho = m/v

the volume of a three dimensional figure is b*h
therefore: v of cylinder = Area of a circle * h (delta r = h)
v=\piR2\Deltar

therefore: \rho = (\Deltam)/(\piR2\Deltar)

If this equation is wrong please comment.
Well in order for me to model this I need to figure out the radius of the sun; I know I can easily search for the value on the internet, but how is it actually determined?
 
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travwg33 said:
I'm working on trying to determine a formula that can model the density of the sun.

This is a solved problem: google Lane-Emden equation for one of the early models.

travwg33 said:
If this equation is wrong please comment.

Why are you modeling cylinders if the sun is a sphere?

travwg33 said:
Well in order for me to model this I need to figure out the radius of the sun; I know I can easily search for the value on the internet, but how is it actually determined?

We know the distance to the sun and the sun's angular size. Geometry tells us the rest.
 
You are right, I have no idea why I said cylinder, but thanks for the other suggestions.

Just last week I went to an astronomy camp and during one of the lecture, the professor introduced the class to rudimentary stellar modeling, namely modelling our own sun using excel. During the lesson we derived the equation for hydrostatic equilibrium. During the prosess we made an educated guess as to the equation for density in order to save time which we just modeled as a simple line with negative slope.
After doing so we modeled various parts of the star. This sparked my interest in stellar modelling. Are there any decent websites that can help me learn more about this?
 
Calculating the volume of a distant star is not terribly difficult if you know the distance [not difficult]. The spectrum gives you the average surface brightness and its magnitude tells you how much surface is required. Once you have the mass [a bit more difficult], problem solved. There are more than enough binary systems to give us good mass estimates.
 
i think what Travwg33 was asking, and what everyone seems to be missing, is his equation correct. he was deviving his thoughts, not just asking FOR the answer. i think he wants to learn how to devive equations on his own, a noble persuit. if that is your question... i have no frakking clue. if the others answered your question however, feel free to tell me to shut it. i probably should anyway. =3
 

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