I took several photos of the interior of a music store, and only one of the pictures came out with a star effect on some lights. I was not using a star filter, and have no idea how to reproduce this if I wanted to.
The settings on this shot were f22, speed 1/4, ISO 1600, on a Canon 20D digital SLR.
Other exposures, all at 1600, were f4.5, 1/80; f4.5 1/160; and f5.0, 1/80. None of those had this effect.
What caused this?
It’s light diffraction distortion. At very small apertures (and f22 is very small) you get this distortion. I believe it’s because the light actually hits the shutter blades at sharp angles and distorts around the edges of the blades. It won’t show up at larger apertures.
You didn’t happen to poke the lens with your thumb after eating a big greasy chicken sandwich, did you?
I only ask because fingerprints on the optics can cause a star effect.
A more usual method of getting a star effect on the cheap is to smear a little vaseline onto a spare UV filter.
f22 caused it, as Hodge pointed out. What you’re seeing is the points of light squeezed along the blades of the tiny aperture.
f4.5 and f5 are larger openings, so you won’t get this effect.
As noted above, diffraction effects because of the small aperture. I can also tell that you used a lens with 6 blades on the aperture diaphragm - possibly the 20D kit lens? Can I also ask why you decided to shoot at that aperture, rather than opening up the lens for a faster shutter speed? Merely curious.
You can see the same effect here, even though this was shot only at f/8:
Look at the star effect on the lights under the bridge; f/22 would have increased this, while also reducing available light.
This photo has a clear star effect, which is not surprising, since I was pointing the camera almost straight at the nearest star, It used at 10-20mm zoom lens at 10mm (very wide angle), and was shot at f16.
Right, diffraction from the edges of an aperture or shutter iris. There are multiple blades in a circular array that are hinged to travel together. In almost all cameras this is what sets the aperture diameter, though there are a few cameras in which a mechanism like this acts as the shutter, opening and closing to control the exposure time. In 35 mm SLRs the multi blade aperture diaphragm often creates a star pattern, particularly if the edges of the blades are made relatively straight and you use a small aperture which makes diffraction stronger and makes the part of the blade actually touching the light path more nearly a straight edge because it’s a smaller segment of the curve.
You can also use photoshop to add effects post-exposure 
The lens that came with the kit is a Canon 18-55mm zoom and that’s what I used. I took multiple shots to experiment with depth of field, and wanted to include one stopped all the way down for maximum depth of field. (BTW I am starting to notice that this lens has some chromatic aberration and I will probably open another thread to ask about that.)
That didn’t sound quite right to me. I checked out your link, and sure enough, the Vaseline was used as it has been for decades, as a diffusion filter. The diffusion causes a glow, and this type of filter is often used for glamour photography to hides flaws such as wrinkles in the models’ skin. If you revisit the link you will see that the first photo used the diffusion effect. The second photo, which has the star effect, was the result of “Off with the smeared UV filter, on with an 8-point diffraction, or star filter…” Yes, the first photo does have a slightly noticeable star effect but I’m guessing that it’s due the factors that other posters have mentioned; it was shot at f11.
You can get a star effect with vaseline. You just have to smear it on so it has ridges, like a fingerprint. I agree that in this case it was probably refraction off your aperature that caused the effect.
