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-   -   Luminosity and Flux (http://www.internationalskeptics.com/forums/showthread.php?t=354797)

 Mike Helland 13th October 2021 02:28 PM

Luminosity and Flux

https://lonewolfonline.net/luminosity-flux-stars/

Flux = Luminosity / 4pi*r2

That's the luminosity, divided by the area of a sphere, as defined by the sphere's radius.

That means, assuming a star's luminosity is fixed, the amount of flux changes with the distance to the star.

Alternatively, we could say:

Flux = Luminosity / 4pi*(ct)2

Where r is replaced by ct, which is the speed of light times the travel time to the star at the speed of light, which gives us the distance r.

Any problem with that?

 Kid Eager 13th October 2021 02:53 PM

Quote:
 Originally Posted by Mike Helland (Post 13628246) https://lonewolfonline.net/luminosity-flux-stars/ Flux = Luminosity / 4pi*r2 That's the luminosity, divided by the area of a sphere, as defined by the sphere's radius. That means, assuming a star's luminosity is fixed, the amount of flux changes with the distance to the star. Alternatively, we could say: Flux = Luminosity / 4pi*(ct)2 Where r is replaced by ct, which is the speed of light times the travel time to the star at the speed of light, which gives us the distance r. Any problem with that?
Yes - *why* substitute, when the unit of measurement of r for star flux is a light year?

 Mike Helland 13th October 2021 05:05 PM

Quote:
 Originally Posted by Kid Eager (Post 13628269) Yes - *why* substitute, when the unit of measurement of r for star flux is a light year?
Wouldn't you would need to convert that to meters to get back SI units?

 bobdroege7 14th October 2021 06:16 AM

Real men measure things in rods.

 Crossbow 14th October 2021 07:04 AM

Quote:
 Originally Posted by Mike Helland (Post 13628246) https://lonewolfonline.net/luminosity-flux-stars/ Flux = Luminosity / 4pi*r2 That's the luminosity, divided by the area of a sphere, as defined by the sphere's radius. That means, assuming a star's luminosity is fixed, the amount of flux changes with the distance to the star. Alternatively, we could say: Flux = Luminosity / 4pi*(ct)2 Where r is replaced by ct, which is the speed of light times the travel time to the star at the speed of light, which gives us the distance r. Any problem with that?
I am not entirely sure ...

If one is assuming that the Luminosity in question is radiated from a point source (where all of the radiation is distributed uniformly in all directions), and

If one is assuming that the all of the Luminosity is being measured in terms of some perfectly spherical surface at some uniform distance from the source of the Luminosity in question, and

If the speed of the Luminosity in question is equal to the speed of light,

Then 'Yes', your equation is valid.

Does this help?

 wea 14th October 2021 09:31 AM

Quote:
 Originally Posted by Mike Helland (Post 13628246) Flux = Luminosity / 4pi*r2 ... Any problem with that?
Isn't it flux density?

 MRC_Hans 14th October 2021 12:58 PM

Quote:
 Originally Posted by Mike Helland (Post 13628246) https://lonewolfonline.net/luminosity-flux-stars/ Flux = Luminosity / 4pi*r2 That's the luminosity, divided by the area of a sphere, as defined by the sphere's radius. That means, assuming a star's luminosity is fixed, the amount of flux changes with the distance to the star. Alternatively, we could say: Flux = Luminosity / 4pi*(ct)2 Where r is replaced by ct, which is the speed of light times the travel time to the star at the speed of light, which gives us the distance r. Any problem with that?
The square rule applies to the apparent luminosity of stars.

Hans

 Crossbow 14th October 2021 01:03 PM

Quote:
 Originally Posted by wea (Post 13629093) Isn't it flux density?
I think that Flux Density would work out to:

Flux = Luminosity /((4/3)*(pi)*(r3))

 marting 14th October 2021 02:52 PM

Quote:
 Originally Posted by Mike Helland (Post 13628246) https://lonewolfonline.net/luminosity-flux-stars/ Flux = Luminosity / 4pi*r2 That's the luminosity, divided by the area of a sphere, as defined by the sphere's radius. That means, assuming a star's luminosity is fixed, the amount of flux changes with the distance to the star. Alternatively, we could say: Flux = Luminosity / 4pi*(ct)2 Where r is replaced by ct, which is the speed of light times the travel time to the star at the speed of light, which gives us the distance r. Any problem with that?
The linked article incorrectly states flux as W^-2. It's W m^-2. Clearly knows it but just made an error in the doc.

 wea 15th October 2021 02:30 AM

Quote:
 Originally Posted by wea (Post 13629093) Isn't it flux density?
Quote:
 Originally Posted by Crossbow (Post 13629379) I think that Flux Density would work out to: Flux = Luminosity /((4/3)*(pi)*(r3))
ok, I had to look it up

http://star-www.rl.ac.uk/docs/sc6.htx/sc6se5.html

"different authors define the terms flux density, flux and intensity differently...sometimes...interchangeably"

Still waiting to see what OP thinks the problem is

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