Differece between brightness and intensity?

Hi all i was going through my aqa physics A module 5 (astro option) specification and came across this:

15.3.1 Classification by luminosity: Relation between brightness and apparent magnitude

15.3.2 Apparent magnitude, m: Relation between intensity and apparent magnitude

Whats the difference between the two?? am i right in thinking that luminotsity is the power radiated by the star at the surface. And brightness and intensity is what we can see/recieve at earth?? So whats the difference between the 2.

Im thinking that maybe brightness is a scale i.e log(ratio of brightness)*-2.5=difference in brightness

and intensity is the amount of photons that we can record using photographic plates or CCD and obeys inverse square law.

Many thanks all... this is such a confusing area for me... :mad:

Reply 1

redbuthotter

and the formula b = L / （4 * pi * R^2)

what is b exactly??

Reply 2

F1 fanatic

redbuthotter

and the formula b = L / （4 * pi * R^2)

what is b exactly??

b is a flux, which is how you define brightness. The luminosity is the power that the star outputs, so that however far away you are that is always the same. It's a value that quantifies the energy per second that the star is giving out.

However, when you observe a star it appears a lot brighter if you are closer to it, because of the fact that the energy is spread over a larger surface area. You can think of the energy being equally distributed over the surface of a sphere with the star at the centre. The further away you are, the more the energy is spread out and the dimmer it appears.

Reply 3

redbuthotter

ok thanks, that i sort of get with the whole inverse square law and area thing, but what about intensity and brightness?? with respect to the specification.

Reply 4

Morbo

For the purposes of the specification, it looks like 'intensity' is the power per unit area received at earth. The apparent magnitude has a log-scale definition using the 'intensity', and in common language, this scale tells you how bright a star looks from Earth.

I think the specification classes 'brightness' as characteristic of the star i.e. a star that outputs more energy is brighter. This is related to the absolute magnitude of the star because the absolute magnitude is the apparent magnitude it would have if it were at a distance of 10pc.

Thus, the absolute magnitude brings all stars to 10pc and then gives them a magnitude at that distance. Stars with larger brightness will, therefore, have more absolute magnitude.

Reply 5

phys981

Curiously enough, I think this was a question on yesterday's (can we talk yet?) Edexcel Atrophysics.

I only got a quick glance though and won't see it properly until Monday.

I hope my students wrote something like what Worzo said - luminosity is the total emitted by the star and intensity is the part received at earth, the inverse square law bit.

Reply 6

redbuthotter

And is brightness just a classification? and the light flux equation, is that brightness or intensity, or neither? Just wanted some black and white clarity... tho i fink im begginging to understand it... thanks!

Reply 7

F1 fanatic

rsk

Curiously enough, I think this was a question on yesterday's (can we talk yet?) Edexcel Atrophysics.

I think we're ok, it's more than 12 hours after the exam, so should be fine.

redbuthotter

Brightness is an actual value, it's a physical quantity that you can measure (Wm^-2). It is the absolute magnitude which uses the brightness that is a classification. Sticking with Worzos definitions which seem to make sense then the brightness and intensity are both related to the flux equation, but it is the distance that appears in that equation that varies, the brightness using a distance of 10pc and the intensity using a distance from the star to the earth.