Now apply to our gradient a filter from Filter > Distort > Glass with the following settings: texture is Thin lens, Distortion 5, softness 3, scale 100%.

 

Then Filter > Distort > Whirl, angle 260.

 

Paint our galaxy in the Hue/Saturation dialog from menu Image > Adjust, in the pop-up window set checkmark in Coloration and move sliders in the scales, changing Hue and Saturation.

 

That was a galaxy, top view. But you can easily obtain a sideview one by transforming the previous picture with Scale and Rotation from menu Edit > Transform.