This shot reached #119 on Explore-interesting. Thanks!
Here's a "bare-camera" shot of comet Holmes with additional processing for dynamic range and color. This is just a little different than a naked-eye image, but it brings out the colors in the stars, which I love to do.
The colors here are the colors of the stars, but I have increased the saturation so they are more obvious.
The illusion of the various sizes of the stars comes from the relative brightnesses they have, which vary literally millions of times from one to another. In order to adequately expose the dimmer stars, the brighter stars get significantly overexposed, and quickly "burn out" the pixels near them, as the wavering atmosphere moves their apparent position around over the 8 second exposure time.
Comet Holmes was not visible to the naked eye, its brightness noted as magnitude 17, until October 24th, when it suddenly brightened, eventually reaching magnitude 2.5, an increase of brightness of almost a million times. The only phenomenon that can explain this is a sudden release of gas and dust which reflects a considerable amount of sunlight, literally a sudden explosion of material thrown out from the comet. This material has not had time to get strung out away from the comet, so there is no obvious "tail" (at least, not yet) and Holmes appears as moderately spherical. Holmes has been observed at nearly comparable brightness (magnitude 4) at least one other time, in 1892. Holmes takes about 6.9 years to make a complete orbit about the sun, and gets about as far away from the sun as Jupiter at the furthest extent of its orbit.
Comet Holmes, being much larger in the image (by virtue of being much nearer) than any of the stars, is more accurately reproduced than any of the stars because it covers many, many pixels instead of just one, and because although as a complete object, it is quite bright, any one pixel isn't all that bright so they don't tend to get overexposed.
Ultimate focus quality can be judged by how well the dimmest stars approximate one pixel. They are so dim that only the pixels they illuminate most often detect significant amounts of light; that's why the pixels around them, where no doubt they landed several times, don't show much or any light. The limiting factor there is the low pass filter ahead of the camera's sensor and the quality of the lens itself. This lens is very sharp, and as you can see, there are many stars that only occupy a few pixels.
If the focus were poor, the dim stars would land on more than one pixel and the light would be evenly divided between those pixels. That effect is minimal in this clip.
Another visible artifact I get questions about sometimes is the apparent uneven shapes of the burned out regions caused by the stars. This is caused by atmospheric lensing. The atmosphere has the ability to bend light, depending on density, temperature, etc. As the atmosphere is not of uniform density or temperature, and as it is "churning" internally as the air masses move about, the bending of the star's light that happens varies over time. The longer the exposure, the more random bending gets into an image, and consequently, the areas that burn out around any one bright star are not uniform in shape.
I try to shoot straight up, which minimizes the thickness of the atmosphere between the camera and the subject and I used that approach here, but as you see, it doesn't eliminate the effect. This is one of the reasons that the great optical observatories are located at high elevations; so that there is the least possible amount of atmosphere between them and the things they are trying to observe. These images were taken at an altitude of 2330 feet above sea level, which isn't all that high, though it is a lot better than trying to shoot an image like this at sea level!
There are a number of approaches that can be taken to minimize these effects, but few are practical with casual photos taken with an unaided camera like the shot you see here.
Canon EF 50mm f/1.8 II lens, Canon EOS 40D camera.
f/1.8, ISO 3200, 8 seconds, manual focus, 10 second stand-off shutter fire, placement on a paved road with very few light sources nearby. No moon, moderately dry air.