IS Forum
Forum Index Register Members List Events Mark Forums Read Help

Go Back   International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology
 


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.
Reply
Old 13th October 2021, 02:28 PM   #1
Mike Helland
Master Poster
 
Join Date: Nov 2020
Posts: 2,257
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?
Mike Helland is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 13th October 2021, 02:53 PM   #2
Kid Eager
Philosopher
 
Kid Eager's Avatar
 
Join Date: Nov 2010
Posts: 7,288
Originally Posted by Mike Helland View Post
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....
Kid Eager is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 13th October 2021, 05:05 PM   #3
Mike Helland
Master Poster
 
Join Date: Nov 2020
Posts: 2,257
Originally Posted by Kid Eager View Post
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?
Mike Helland is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 06:16 AM   #4
bobdroege7
Illuminator
 
bobdroege7's Avatar
 
Join Date: May 2004
Posts: 3,370
Real men measure things in rods.
__________________
Un-american Jack-booted thug

Graduate of a liberal arts college!

Faster play faster faster play faster
bobdroege7 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 07:04 AM   #5
Crossbow
Seeking Honesty and Sanity
 
Crossbow's Avatar
 
Join Date: Oct 2001
Location: Charleston, WV
Posts: 13,721
Originally Posted by Mike Helland View Post
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?
__________________
I can barely believe that I made it through the Trump presidency.

On 15 FEB 2019 'BobTheCoward' said: "I constantly assert I am a fool."

A man's best friend is his dogma.
Crossbow is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 09:31 AM   #6
wea
Critical Thinker
 
wea's Avatar
 
Join Date: Mar 2015
Location: EU, IT
Posts: 427
Originally Posted by Mike Helland View Post

Flux = Luminosity / 4pi*r2

...

Any problem with that?
Isn't it flux density?

Last edited by wea; 14th October 2021 at 09:36 AM.
wea is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 12:58 PM   #7
MRC_Hans
Penultimate Amazing
 
MRC_Hans's Avatar
 
Join Date: Aug 2002
Posts: 23,789
Originally Posted by Mike Helland View Post
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.
MRC_Hans is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 01:03 PM   #8
Crossbow
Seeking Honesty and Sanity
 
Crossbow's Avatar
 
Join Date: Oct 2001
Location: Charleston, WV
Posts: 13,721
Originally Posted by wea View Post
Isn't it flux density?
I think that Flux Density would work out to:

Flux = Luminosity /((4/3)*(pi)*(r3))
__________________
I can barely believe that I made it through the Trump presidency.

On 15 FEB 2019 'BobTheCoward' said: "I constantly assert I am a fool."

A man's best friend is his dogma.
Crossbow is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 14th October 2021, 02:52 PM   #9
marting
Master Poster
 
marting's Avatar
 
Join Date: Sep 2003
Posts: 2,805
Originally Posted by Mike Helland View Post
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.
marting is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 15th October 2021, 02:30 AM   #10
wea
Critical Thinker
 
wea's Avatar
 
Join Date: Mar 2015
Location: EU, IT
Posts: 427
Originally Posted by wea View Post
Isn't it flux density?
Originally Posted by Crossbow View Post
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
wea is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Reply

International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -7. The time now is 09:17 PM.
Powered by vBulletin. Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.

This forum began as part of the James Randi Education Foundation (JREF). However, the forum now exists as
an independent entity with no affiliation with or endorsement by the JREF, including the section in reference to "JREF" topics.

Disclaimer: Messages posted in the Forum are solely the opinion of their authors.