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Radius of a star 
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#1
May2506, 10:53 AM

P: 230

How do I go about working out the radius of a star when I have the bolometric flux, the wavelength of the peak flux of its spectrum, and its parallax?
I've also calculated the temperature of its photosphere (Wein's law right?). Or is it more to do with the parallax? 


#2
May2506, 01:58 PM

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#3
May2506, 02:03 PM

P: 230

I haven't touched on luminosity for this question, as it asks for that in the next part (and I know how to get that).
As I understand it, radius of the star is d*tanP where d is the distance to the star, and P is the parallax. Is [tex]10^11[/tex] the correct order of magnitude for a star do you think? Its bigger than the sun, but its not overly huge for a star is it? 


#4
May2506, 10:35 PM

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Radius of a star
you have its parallax? so it must be fairly close ... right?
that means it should appear fairly bright ... right? Temperature determines how much light is emitted by each sq. meter of surface  not how much light comes off the entire surface. Do you know how these are related? 


#5
May2606, 01:00 PM

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Please don't doublepost, it's very annoying to those trying to help you.



#6
May2606, 01:02 PM

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#7
May2606, 01:08 PM

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[tex]d=\frac{1}{\theta}[/tex] where [itex]\theta[/itex] is the parallax angle in arcseconds. 


#8
May2606, 01:13 PM

P: 230

Is it in metres if you use radians and parsecs if you use it in degress, arcmin and arcsec then?
I've just realised how its done though  distance can be used with the bolometric flux to find the bolometric luminosity. This is turn can be used with [tex] P = \sigma AeT^4[/tex] to find the raduis of the star right? I'm a little confused how the question was intended to be worked out though  2 marks for this, and a further two in the next part for writing down something you've already calculated in order to do this question (luminosity). 


#9
May2606, 01:20 PM

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[tex]d=\frac{d_{earth}}{\theta}[/tex] where [itex]d_{earth}[/itex] is the distance from earth to sun. The expression I gave you is only valid in units in which the angle is in arcseconds and the distance is in parsecs. Use the above expression for meters and radians. 


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