As we mentioned before, all those shiny stars in the sky are almost spherical. And how come we see photos with stars having a cross or pointy bits and brighter stars have more noticeable crossed spikes.

This phenomenon is called diffraction spike and is caused by obstacles as starlight enters telescopes. These obstacles are the supports for secondary mirror in reflecting telescopes that is located upfront. Take Newtonian reflecting telescope and Hubble Space Telescope (Ritchey-Chrétien telescope) as an example, starlights are partially obstructed and diffracted by those cross-like supports before reaching the primary mirror, then that's why the images we observe have the spikes. In other words, the design of the reflecting telescope causes these optical add-ons.

You may find stars without diffraction spikes in some pictures, can you tell what type of telescope snapped these images? The three major types of telescopes are reflective, refractive and a combination of the two.

Telescope Credit: National Geographic RF360MM telescope, Vixen Optics' R200SS 200mm f/4 Reflector Telescope, Meade® LX850 Telescope.