I How is light modeled in astrophysics?

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Light from distant stars can be modeled as parallel rays due to the vast distances it travels, with amplitude decreasing proportionally to 1/d, where d is the distance. This approach suggests that a Gaussian beam model with a constant beam radius could be appropriate. Starlight is emitted in all directions, but by the time it reaches Earth, it can be treated as a collection of parallel rays with uniform intensity. The discussion emphasizes the importance of understanding light propagation and its modeling in astrophysics. Overall, the treatment of starlight as parallel rays aids in simplifying complex astrophysical calculations.
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After propagating a long distance, how light is modeled?
Suppose a light beam is sent from a distant star. When the light beam hit a telescope orbiting around the Earth. How is the light beam modeled? I have searched for many keywords without success.

In my opinion, since the beam has gone so far, it can be viewed as parallel rays, with a decaying amplitude proportional to ##1/d## where d is the propagation distance. Therefore, could I model it as a Gaussian beam with a constant beam radius?
 
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Starlight is not beamed. It is sent out in all directions. Arriving at the Earth starlight is essentially a bunch of parallel light rays of equal intensity.
 

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