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rocketoctopus
Joined: 11 Nov 2010 Posts: 1 Location: London

Posted: Thu Nov 11, 2010 2:59 pm Post subject: Help with stellar magnitudes and distances 


Hi there, i was reading your bit about stellar distances and magnitudes on the internet as I am studing astronomy and am stuck on a particular question and your formulas didnt help me....the question is this....
The stars Vega and Phad are both of spectral type A0V. Vega lies at a distance of 7.8 parsecs and has an apparent visual magnitude of 0.03. Phad appears 9 times fainter than Vega to an obsever on the Earth.
(i) calculate the parallax of vega. (I calculated this to be 0.13)
(ii) Calculate the absolute visual magnitude of Vega. (using M = m  5logd +5 I calculated it to be 0.57
(iii) Calculate the apparent visual magnitude of Phad. (using m1  m2 = 2.5log(b1/b2) I calculated this to be 2.42
(iv) Estimate the distance of Phad from the Sun assuming negligible interstellar extinction.State any assumptions you make?
This question I have tried for so long to calculate and cannot do it using any formulas how do you work this out.
Kind regards
Brad 

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hepcj Site Admin
Joined: 23 Jun 2007 Posts: 125

Posted: Mon Nov 15, 2010 1:38 am Post subject: 


Hi Brad,
Parts (i)  (iii) seem reasonable.
Part (iv) also had me stumped for a while. I was going through different ways of trying to find the distance modulus but I think the key point is that you know the relative brightness of Vega and Phad is 1:1/9.
Because of the inverse square law, Phad will be 3 times as far a Vega. You know the distance of Vega, so a rough estimate ofthe distance to Phad will be
3 x 7.8 = 23.4 pc
It is a pretty crude approximation and it relies on both stars being the same size and luminosity. The real value is 25.648 pc. 

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