Astronomy - Ch. 17: The Nature of Stars (10 of 37) Relationship Between Intensity and Luminosity

[@MichelvanBiezen]

Turjja, That is a good question, especially because those terms are sometimes used interchangeably which is not correct Flux is more of a mathematical term, it is the dot product between two vectors which give you the total energy (flow) through an area per unit time. (Thus when you put the area at an angle you have to multiply the energy by the cosine of angle between the direction of the energy and the normal to the area.) In astronomy, flux (or radiative flux) is the TOTAL amount of energy that flows per unit area per second. Intensity (or light intensity) is the total amount of light that passes through per unit are per second.

[@turjjadatta]

Is Flux and Intensity the same thing?

[@khushitrivedi4597]

Fabulous explanations sir!!😊

[@valentinabaccelliere3970]

Hello Professor I'm confused, my text book of astronomy (Fundamental Astronomy from Karttunen, H) says: " The flux density gives the power of radiation per unit area; hence its dimension is W*m^−2* Hz^−1 or W*m^−2, depending on whether we are talking about the flux density at a certain frequency or about the total flux density" and also says that sometimes the flux density is also called Intensity. Also says that " The flux emitted by a star into asolidangle ω is L=ωr^2*F , where F is the flux density observed at a distance r. Total flux is the flux passing through a closed surface encompassing the source. Astronomers usually call the total flux Luminosity" I am confused because I read and everywhere says different definitions of flux, intensity, specific intensity, astronomical flux (?) ecc. What should I belive?

[@MichelvanBiezen]

This is a confusing subject, especially since the definitions are poorly explained even in text books. There are some subtle differences that make it complicated.LUMINOUS FLUX: is the power radiated in the energy band that the human eye is sensitive to: (330 nm - 780 nm). RADIANT FLUX: is the power radiated in all the energy bands (IR + LIGHT + UV)Then units of flux is Lumens but can be converted to Watts. (This can be complicated because they use a weighted average using the spectral power distribution accounting for different sensitivities of the human eye at different wavelengths. FLUX DENSITY is indeed FLUX/AREA which can be calculated with m^2 and sterradiansUnless you are taking astrophysics or an advanced astronomy class I wouldn't go beyond these basic definitions.

[@valentinabaccelliere3970]

Ok thank you so much!! You helped me a lot with your videos

[@imalightbulb]

is light flux same with intensity?

[@MichelvanBiezen]

Luminous flux is defines as a quantity of light over a solid angle. Since the angle doesn't change with distance, the flux is not dependent on distance. But intensity is the amount of light produces divided by area, which increases with distance, so intensity decreases with increasing distance.

[@imalightbulb]

@@MichelvanBiezen Thanks! Your video helped me a lot on the competition I'm participating! (IACC)

[@imalightbulb]

@@MichelvanBiezen Flux is measured in Watts per Square meter, and is a measure of the net radiant energy passing through a given area, independent of the direction of that energy. Intensity, on the other hand, is measured in Watts per Square meter per Steradian. That means it's a measure of the amount of radiant energy passing through a given area, in the direction of a given solid angle. That means there's a lot more information contained in Intensity, since it can also be a function of direction (theta). Is this statement correct? Found it on Reddit

[@Rajeshgodsown]

Sir, When I hit NEXT button after this video, I am getting video from another channel. Ofcourse it is a good video, but if it is without your knowledge please act against it. Thank you for making such videos and showing the beauty of the STARS.

[@MichelvanBiezen]

We have no control over that. KZbin uses different algorithms to control which videos you get to chose from.

[@Walter-uy4or]

Or Eλ dλ = Iλ dλdt dA cos θ d = Iλ dλdt dA cos θ sin θ dθ dφ. Not that I could explain that