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Why do bright objects feature a stellate around them (like in the lights on the bridges below)?

If this were due to scattering of light, I'd have expected it to be like a luminous sphere around rather than a stellate.

Image source: 1, 2

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    $\begingroup$ See Mike Stone's answer. I must add that the quality of the stars created by bright lights is usually a factor in the quality of a camera objective. The nicer, sharper they are, usually the higher the grading of the objective, which was designed to take that into account. Each blade of the aperture will diffract light. The images you posted, specially the first, are taken with a quite small aperture to emphasize the diffraction spikes effect. The number of arms is equal to the number of blades if its an even number, and 2x the number of blades for uneven numbers. $\endgroup$
    – José Andrade
    Commented Nov 21, 2022 at 8:43

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If you are refering to the six pointed "stars" around the bright lights, they are diffraction spikes due to the shape of the iris in the camera.

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Looks like this effect (the names are "terrestrial scintillation" or "atmospheric boil" is caused by refractive index variations on the optical path between the source of light and the observer. See, e.g., National Bureau of Standards Technical Note 225 issued April 5, 1965, p. 48.

The main result of the studies is that terrestrial scintillation is primarily a function of vertical temperature gradients. Maxima were observed at midday when they are due to solar heating and at night due to radiation cooling of the ground.

I would think the stellate shapes are due to random changes in the conditions on the optical path during the time exposure of the camera. Probably, the characteristic time of the changes are comparable to exposure time of the camera, so they are not averaged in the image. I would guess the shapes might be closer to round ones for longer exposure.

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  • $\begingroup$ I believe OP was asking about the diffraction spikes that create the star shaped diffraction around a bright object. This happens in photography and although is an optics phenomenon, its not mediated by the environment. $\endgroup$
    – José Andrade
    Commented Nov 21, 2022 at 8:38

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