I think it's easier to explain the question with some images.
Here you can see a six-rayed one:
I cannot post more than 2 links, so:
www.i.stack.imgur.com/23YLa.jpg
www.i.stack.imgur.com/4WqEU.jpg
Why does it spread in such rays only? How does it happen?
I guess many of you have seen this before. Maybe the answer is quite simple but I don't know it and it did not seem to me it has been asked already.
This is just an effect of the optics used to take the images. The size of the aperture causes the light to be bend: Wikipedia: Airy Disk. Now, as the aperture of most lenses is not circular, the edges cause some visible rays in the image, as is depicted here. The left column shows an assumed shape of the aperture and the right column the resulting image of a point-like source of light.
Especially in telescopes using mirrors, this effect is also caused by the mechanics holding the mirrors in place: Wikipedia: Diffration Spikes
In photography this effect is often used for artistic reasons. If you own a good camera (DSLR or similar), try taking pictures of the same small light source with different aperture settings: The smaller the aperture, the more visible this effect is.
