Oval Aureoles & M42 Orion

Scattering from far and near. The brighter stars have oval aureoles. Scattering in our atmosphere created them. The dust clouds in 1300 LY distant M42 shine by scattering the light of the nebula's embedded stars. The red glow is the light of hydrogen atoms excited by the stars' UV radiation.

Imaged from central Stockholm by Peter RosÚn. 4 exposures of 30 sec, 800 ISO, Canon EOS5D MkII through Williams Optics FLT 110 f/6.5 Apo refractor.   

M42 was 25° high.

Images ©Peter RosÚn.
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Diffraction by ice crystals in the air probably creates the elongated ovals.

The crystals are sufficiently large to be aerodynamically oriented as they drift downwards relative to local air currents. To starlight from a relatively low star the crystals - hexagonal plates or columns - appear on average to be horizontally elongated. The resulting scattering and diffraction creates a vertically elongated aureole.

We need near simultaneous images of star aureoles near the horizon and zenith to check this hypothesis!

The field's brightest star is mag 2.75 iota Orionis. To its lower right the star pair has magnitudes of 4.75 and 5.65. This negative view shows ovals on stars down to 7th magnitude.