When stars are portrayed in media, they are often shown with long spikes emanating from them. Perhaps the most common example is that of the “star of Bethlehem” which, according to the story, led the wise men to baby Jesus. Of course when we look at stars in the night sky, we don’t see any such spikes. Stars twinkle due to atmospheric disturbances, but that’s about it. In photographs, however, bright stars often have such long spikes. So what causes them? It all has to do with an interesting bit of optics.
In astronomy, they are known as diffraction spikes, and they appear with certain types of telescopes. Optical telescopes broadly fall into two types: lens-based and mirror-based. Lens-based telescopes were the first to be developed, and are basically a long tube with two or more lenses. Starlight is refracted by the lenses to produce a magnified image. Since the light goes straight through the telescope unimpeded, you don’t get any spikes on stars. The big disadvantage of lens telescopes is that they tend to get be fairly long for large magnifications, and large lenses are difficult to make well.
But the main reason we see diffraction spikes so often is that astrophotographers often use them to artistic effect. They transform a bright point of light into a wondrous stellar image.
Share This Article