# Help to determine the luminosity of a star

1. Mar 25, 2009

### victoriafello

I need to work out the luminosity of 3 stars 85 PEG A / B / C using the following data and equation,

Absolute Mag of stars A/B/C/SUN
MA = 5.28
MB = 8.53
MC = 15
MS = 4.84
Apparent Mag of stars A/B/C
mA = 5.75
mB = 9.00
mC = 15.47
LS = Luminosity of Sun – 1 ?

Equation
MA-MS = -2.5 LOG ( LA/LS)

I have got a result of 0.667 for A and 0.033 for B, But when i have looked online at star catalogues the actual lumisosity is different, where am i going wrong ?

2. Mar 25, 2009

### George Jones

Staff Emeritus
What luminosities do the catalogues give for these stars? Even better, can you give links to these catalogues?

3. Mar 25, 2009

### victoriafello

I looked on a few sites at home which i cant remember now but one was on Wiki, i know thats not always the most reliable source so i wasnt sure if it was me or them that was incorrect

http://en.wikipedia.org/wiki/85_Pegasi

4. Mar 25, 2009

### George Jones

Staff Emeritus

I thinks that this uses bolometric, not visual, magnitudes to calculate luminosities.

5. Apr 8, 2009

### philippa65

I have been investigating the same question. I have used the same method as you but not sure if it is correct. Waiting for my tutor to get back to me whether they have told us to use the correct equation! Using the software Celestia A = 0.558 and B = 0.032 but i assume that is absolute luminosity. I will tell you if the tutor comes up with anything.

6. Apr 9, 2009

### philippa65

I have been lead to believe that my values (the same as yours for this question) have been calculated in the correct way. If you used the Celestia software, which is good fun and recomend but cant find the link at the mo, sorry, and use their figs for M then you get closer to their values for L. One bonus from going through countless pieces of paper and lots of pencil lead is that I am now quite au fait with solving log equations!
My tutor has been brillant on this matter and actually bothered to look at the question unlike the tutor on my other course when there were queries.
Good luck with the rest of the paper.