Originally posted by Stone G. Stars are point-like sources of light, but photograps of these are not 'points'. They are images of diffraction patterns, consisting of concentric bright and dark circles whith a diameter that depends upon the absolute aperture (size of entrance pupil) and focal length of your optics. The larger the f-number, the larger the image of the diffraction pattern.
The bright circles decrease rapidly in brightness with the distance from the center, and we often only capture the innermost central part, but he brighter the star, the more of this diffraction pattern you may/will actually capture.
Beside the diffraction paterns, the atmosphere is not a steady medium. Air is circulating in many directions, depending of altitude, and the light pass is nothing like a perfectly straight line. Also, there is dust, ice cristals, and these gases, with different refraction index, which make the light to bend and scatter even more. Anyone can see the star twinkling with his eyes, and this is a result of all these phenomena.
In a long exposure, this twinkle are translated in movements of the stars images in any direction, all recorded by the camera, and finally forming a round ball of light, bigger or smaller, depending on the luminosity of the star. You can clearly see that the bigger star twinkle much more than the small ones. That's because the scattered light from the small stars is to little for the eye, or the sensor, to give a circle of the same size.