by Chris Peterson » Sun Oct 23, 2011 6:39 pm
Charlie Mercer wrote:Thanks very much Ann. That is exactly what has me stumped, " light will be spread out over a very large cone", so if it is in fact diluted and has fewer photons how is it possible to see individual stars in a distant galaxy say 150 million light years away. The pix we see from the Hubble are sometimes listed as farther away than even this.? Still stumped.
Instead of a cone, think of a sphere, since that's much more typical of the way light spreads- especially from a star. Every time you double the radius of a sphere (which is like getting twice as far from the star at its center), the surface area increases by a factor of four. In other words, for any given patch, you'll have only one-fourth as many photons hitting it. This is the basis of the inverse square law: get ten times further away, and you'll only have 1/100 the number of photons on a given size patch (like the pupil of your eye).
We can see a galaxy 150 million light years away because, even though our telescope aperture only captures a tiny, tiny fraction of the total photons that were originally produced, that original number was so huge that we still have thousands or millions left to be collected by the camera. The photon count may be diluted, but it isn't diluted to zero (and instrumentally, we can detect single photons, although it typically requires at least a few dozen in each pixel to get a statistically meaningful signal).
[quote="Charlie Mercer"]Thanks very much Ann. That is exactly what has me stumped, " light will be spread out over a very large cone", so if it is in fact diluted and has fewer photons how is it possible to see individual stars in a distant galaxy say 150 million light years away. The pix we see from the Hubble are sometimes listed as farther away than even this.? Still stumped.[/quote]
Instead of a cone, think of a sphere, since that's much more typical of the way light spreads- especially from a star. Every time you double the radius of a sphere (which is like getting twice as far from the star at its center), the surface area increases by a factor of four. In other words, for any given patch, you'll have only one-fourth as many photons hitting it. This is the basis of the inverse square law: get ten times further away, and you'll only have 1/100 the number of photons on a given size patch (like the pupil of your eye).
We can see a galaxy 150 million light years away because, even though our telescope aperture only captures a tiny, tiny fraction of the total photons that were originally produced, that original number was so huge that we still have thousands or millions left to be collected by the camera. The photon count may be diluted, but it isn't diluted to zero (and instrumentally, we can detect single photons, although it typically requires at least a few dozen in each pixel to get a statistically meaningful signal).