Register to reply 
HR diagram: magnitude  luminosity relationby stargazer3
Tags: hr diagram 
Share this thread: 
#1
Mar1513, 08:58 AM

P: 44

Hey PF,
I'm just being stupid today, because I can't figure out the way HR diagram is constructed, hope you guys can help. So suppose that you complete an observation in several different filters, and you need to work out the luminosity and temperature for each object in your field of view. Now for the temperature you could just plot an instrumental magnitude against filter wavelength and fit a blackbody profile to it, the peak of the curve showing the effective temperature of the star. But what about the luminosity? Is it reasonable to take an area of one pixel and, knowing average photon count rate, integrate it over 4π steradians? Then, after multiplying by a typical photon energy (assuming the filters are quite narrow), I'd get a luminosity from a particular source? Something seems wrong here, but I can't figure out what. Is it because the atmospheric absorption is not considered? If so, how to do it properly? 


#2
Mar2513, 06:16 AM

P: 112

Luckily for us there is a relation between the apparent magnitude m & luminosity L for a star.
L=4πmd^{2} And the apparent magnitude is easily measured with or without a CCD/DSLR etc. This is commonly done by people who observer variable stars. 


#3
Mar2913, 03:09 PM

Emeritus
Sci Advisor
PF Gold
P: 5,196

L = (4πd^{2}) F It's true that a CCD measures flux, but it's not true that flux and apparent magnitude are the same thing. They are related though: m = 2.5*log(F/F_{0}). where F_{0} is an arbitrary reference flux (the flux of a source that you define to have magnitude 0 in whatever magnitude scale you're using). 


#4
Mar2913, 03:35 PM

P: 112

HR diagram: magnitude  luminosity relation
I also see on Wikipedia the relationship you mentioned http://en.m.wikipedia.org/wiki/Luminosity#section_1 At luminosity formulas. Thanks! :) Regards, Robin. 


Register to reply 
Related Discussions  
MassLuminosityTime relation (Astronomy)  Advanced Physics Homework  2  
Luminosity to magnitude  Advanced Physics Homework  3  
Radial velocity/magnitude/luminosity  Introductory Physics Homework  1  
Massluminosity relation problem  Astronomy & Astrophysics  19  
Detailed MassLuminosity Relation for Main Sequence Stars  Astronomy & Astrophysics  12 