When shooting pinpoint light sources like these Christmas tree lights and stopping the lens aperture down to a high f/number like f/11 or f/16, the lens usually creates these nifty star points. The number of points on the star is actually determined by the geometry of the the lens aperture, specifically by the number of aperture blades. Lenses with odd numbers of blades produce twice the number of star points as aperture blades. For example, in this photo, the lens has 9 aperture blades and so the lights all have 18 pointed stars. If the lens had 7 aperture blades, it would produce 14 point stars.
Lenses with even numbers of aperture blades, (like most Canon or Nikon lenses) will only produce an equal amount of star points as aperture blades. An 8 bladed aperture will produce only 8 star points which looks a bit more boring than a 14 point star. This is one of my favorite aspects of most Sigma lenses: they all tend to have either 7 or 9 aperture blades which produces some of the best star highlights when stopping down at night. This is a rarely considered trait when choosing a camera lens but is something that I have always appreciated in Sigma lenses. I hope to eventually get a Sigma 35mm f/1.4 Art (Or maybe replace my Rokinon 24mm f/1.4 with a new Sigma 16-20mm f/2.0 if they release one as rumored) and I think that the 9 bladed aperture is one of the things I look forward to using the most. Merry Christmas.
Canon EOS 6D
Sigma 50mm f/1.4 @ f/16