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The Difference Between Brightness and Flux Density

-- chiu | August 19, 2024

Question:

Is the distance information used when calculating flux density (intensity) of an object?

If my understanding is correct, telescopes (or radio arrays) observe flux, while people present flux density (intensity) in literature. While flux depends on the distance of the object, intensity should be distance-independent. Does that suggest that we need to use the information of distance when calculating the flux density of an object? If so, how can one calculate the uncertainty of the final intensity measurement due to the uncertainty of distance?

-- chiu

Answer:

You are correct in that brightness (you used the term “intensity”) is independent of distance, but flux density is distance-dependent. The chapter on radiation fundamentals in Chapter 2 of Essential Radio Astronomy provides an excellent discussion of these concepts.

-- Jeff Mangum

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