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Why Do We Draw Stars with Five Points?

If stars are balls of gas, then why do we draw them as pointed or spiky objects? Everyday Einstein dives into the science. 

By
Sabrina Stierwalt, PhD,
Episode #313
NASA Hubble Space Telescope Image

In an experiment studied by high school physics students everywhere, Young’s Double Slit Experiment, experimenters shine a beam of light on a clear board after the light is sent through one and then two narrow slits. While a particle of light would simply pass through the slit or miss and be reflected back, a wave of light passes partially through the slit while also bending around the slit’s edges forming a distinct pattern on the board in the background.

So what does this have to do with the spikes we see around stars? Our best explanation for the spikiness of stars comes not from our understanding of outer space, but from the anatomy of the human eye. The light coming from the distant point in the sky must pass through the lens of our eye which itself is a small hole. When you squint, you make an even more exaggerated slit for the light to pass through. The lenses of our eyes also have fine imperfections known as suture lines that create small slits for the light to pass through and thus must result in their own diffraction pattern, the spikes. Since everyone’s eye is unique (including even your left versus your right eye), this would mean that no two people see a star the same way or with the same number or shape of points.

Even telescopes see what we call diffraction spikes around stars because there is usually not a clear path for starlight to reach the telescope lens or mirror. Something has to hold that lens or mirror in place. In the case of the Hubble Space Telescope, four metal bars or struts secure the mirror and so the brightest stars in Hubble images show four bright spikes from where light has had to pass around those struts. The upcoming James Webb Space Telescope will have three. 

Why Do Stars Appear to Twinkle?

Note that while the reason we see stars as pointy or spiked objects may be more Earth bound, the reason stars appear to twinkle still has to do with the atmosphere above. As a star’s light passes through a turbulent atmosphere, that light deviates from a direct path between us, the viewers, and the star, in a process called refraction. The star can thus appear momentarily dimmed and this low level flashing on and off appears as twinkling.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image Credit: NASA/ESA/H. Richer

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