Cosmic Kaleidoscope: Exploring Diffraction Spikes in Cosmic Imagery
Why do stars look pointed in drawings and images despite being seething balls of plasma?
Article by Sammit Dhar, a member of the Astronomy Club, IIT BHU.
We all know stars are giant, angry balls of plasma. Yet when asked to draw a star, nobody would draw a circle. It’s always a pointed shape that we recognise as a star. But why exactly does this happen?
Most images of stars we see have beams of light coming out of them. In fact, any small source of bright light can have these beams. This phenomenon is also observed when you squint at a bright source of light. Of course, they are not real at all, just artefacts in our images and vision caused due to the wave nature of light.
Whenever there are small disturbances in the path of light, such as obstacles, openings, etc., light bends around them, giving us the well-known phenomenon of diffraction. Edges of objects and shapes, in general, which cause a sudden change in the path of light, can give rise to diffraction.
Diffraction is always happening around us, but the effects generally cancel out with an increase in distance from the source of the disturbance. On some occasions, where these effects interfere constructively to give rise to noticeable changes, we get these beams, formally called diffraction spikes.
Diffraction spikes are perpendicular to the edge of the object causing them. An edge at the aperture of a regular camera or the primary mirror of a telescope, therefore, yields two spikes 180 degrees apart. N-sided regular apertures yield 2N spikes when N is odd, and only N spikes when N is even. The spikes may be diffused in all directions for a circular aperture or even form a ring.
In addition to the aperture shape, the struts holding the secondary mirror also give rise to diffraction spikes in a reflecting telescope.
Notably, the Hubble Space Telescope has four struts 90 degrees apart, giving rise to 4 spikes the same angle apart. In pictures taken by the JWST, two dim spikes come from the spider vane supports that hold the secondary mirror. These spikes are on top of the spikes coming from the edges of the primary mirror. On closer inspection, we can see that the six bright spikes in images by JWST are also made of smaller spikes. These smaller spikes arise due to the individual edges of the primary mirror segments.
Remarkably, these spikes serve more than just an artistic purpose. Diffraction spikes also allow us to differentiate between stars and galaxies in an image since galaxies don’t have diffraction spikes. Even when we squint, diffraction from our eyelashes produces similar spikes, illustrating the pervasive nature of this phenomenon. The delicate intricacies of capturing celestial bodies through telescopic lenses highlight how even minor imperfections can yield profound effects, underscoring the delicate interplay between science and art in our exploration of the cosmos.
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