Forum Index Register Members List Events Mark Forums Read Help

 International Skeptics Forum Luminosity and Flux

 User Name Remember Me? Password

 Welcome to the International Skeptics Forum, where we discuss skepticism, critical thinking, the paranormal and science in a friendly but lively way. You are currently viewing the forum as a guest, which means you are missing out on discussing matters that are of interest to you. Please consider registering so you can gain full use of the forum features and interact with other Members. Registration is simple, fast and free! Click here to register today.
 13th October 2021, 02:28 PM #1 Mike Helland Master Poster   Join Date: Nov 2020 Posts: 2,352 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?
 13th October 2021, 02:53 PM #2 Kid Eager Philosopher     Join Date: Nov 2010 Posts: 7,296 Originally Posted by Mike Helland 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? __________________ What do Narwhals, Magnets and Apollo 13 have in common? Think about it....
 13th October 2021, 05:05 PM #3 Mike Helland Master Poster   Join Date: Nov 2020 Posts: 2,352 Originally Posted by Kid Eager 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?
 14th October 2021, 06:16 AM #4 bobdroege7 Illuminator     Join Date: May 2004 Posts: 3,429 Real men measure things in rods. __________________ Un-american Jack-booted thug Graduate of a liberal arts college! Faster play faster faster play faster
 14th October 2021, 07:04 AM #5 Crossbow Seeking Honesty and Sanity     Join Date: Oct 2001 Location: Charleston, WV Posts: 13,799 Originally Posted by Mike Helland 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? __________________ "But Presidents are not kings, and Plaintiff is not President." - Judge Chutkan On 15 FEB 2019 'BobTheCoward' said: "I constantly assert I am a fool." A man's best friend is his dogma.
 14th October 2021, 09:31 AM #6 wea Critical Thinker     Join Date: Mar 2015 Location: EU, IT Posts: 432 Originally Posted by Mike Helland Flux = Luminosity / 4pi*r2 ... Any problem with that? Isn't it flux density? Last edited by wea; 14th October 2021 at 09:36 AM.
 14th October 2021, 12:58 PM #7 MRC_Hans Penultimate Amazing     Join Date: Aug 2002 Posts: 23,895 Originally Posted by Mike Helland 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. What is your point? Hans __________________ Experience is an excellent teacher, but she sends large bills.
 14th October 2021, 01:03 PM #8 Crossbow Seeking Honesty and Sanity     Join Date: Oct 2001 Location: Charleston, WV Posts: 13,799 Originally Posted by wea Isn't it flux density? I think that Flux Density would work out to: Flux = Luminosity /((4/3)*(pi)*(r3)) __________________ "But Presidents are not kings, and Plaintiff is not President." - Judge Chutkan On 15 FEB 2019 'BobTheCoward' said: "I constantly assert I am a fool." A man's best friend is his dogma.
 14th October 2021, 02:52 PM #9 marting Illuminator     Join Date: Sep 2003 Posts: 3,045 Originally Posted by Mike Helland 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. __________________ Flying's easy. Walking on water, now that's cool.
 15th October 2021, 02:30 AM #10 wea Critical Thinker     Join Date: Mar 2015 Location: EU, IT Posts: 432 Originally Posted by wea Isn't it flux density? Originally Posted by Crossbow 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

International Skeptics Forum

 Bookmarks Digg del.icio.us Google Reddit