Due to the wave nature of light (light travels as both a wave and a particle), the wave will bend around edges (much in the way that water going through a gap in a wall will bend around the wall). The light bends in the direction orthogonal to the edge. The effect is called diffraction and the "sunbeams" are called diffraction spikes.

The effect is most noticeable when edges are straight ... aperture blades that are flat such that the aperture is a polygon. If the aperture blades are rounded, the effect isn't noticed (or at least not as much) because the curve causes the light to spread out along the curve ... rather than all light bending in the same direction and concentrating the effect. It is possible (or it was ... I used to use one) to get a filter that has very fine wires embedded in the glass. These were sometimes called starlight filtered because they turn every point of light into a "star" with diffraction spikes. There are variations on these that used etched grooves ... but these create spikes with a prismatic effect.

Some reflector telescopes (such as Newtonian reflectors -- but there are others that do this) have a secondary mirror near the front of the telescope supported by a "spider" ... usually four thin vanes that support the secondary mirror, but occasionally three. These telescopes also create diffraction spikes. There are variants that used curved vanes specifically to spread out the diffraction effect so that the spike isn't noticeable.

Every edge causes the light to bend in both directions. When you have an even number of aperture blades, the diffraction spikes from the blades on opposite sides of the aperture are overlaid on each other and they double up the effect -- creating fewer spikes but the spikes are larger and brighter. A lens with an odd number of blades will also create spikes in both directions (hence a 9-blade aperture gets 18 spikes) but since the spikes aren't paired up with a blade on the opposite side of the lens, the effect isn't intensified. This results in more spikes ... but not as large or bright.