If stars are balls of gas, then why do we draw them as pointed or spiky objects? Everyday Einstein dives into the science.
I recently found myself explaining to my five year old and a group of her friends how the stars in the sky are actually massive balls of (mostly) hydrogen and helium gas powered by nuclear fusion. (I’m such a fun mom; kids love me.) Some of them zoned out when I started talking about pressure equilibrium but one girl asked me: if stars are balls, then why do they have points?
The star symbol is ubiquitous: it’s used by many cultures and in a variety of contexts. And most often, the stars in the symbols have points. Five-pointed stars were drawn on Egyptian jars dating back to 3100 BCE and on tablets and vases in Mesopotamia around the same time. They were often seen in letters between the followers of Pythagorus (aka Pythagoreans) as a symbol of their group. Six- and seven-pointed stars have also been used throughout history as symbols of religion (like the Star of David) or symbols of patriotism (like the seal of the Cherokee Nation or the Great Seal of the United States).
Now, stars can be a symbol of fame (as in the Hollywood Walk of Fame), experience (five-star generals), or quality (five-star hotels). They top our Christmas trees and appear on flags or in company logos. But why do we draw stars as pointy objects so consistently if they are really round, symmetric orbs in the sky?
Stars Appear Pointy Thanks to Diffraction
Since a star is a massive ball of gas, much larger than our planet, we might expect stars to look like flat disks on the sky from our vantage point here on Earth. And this is, in fact, how we see the Sun. (Although a side note: please don’t look directly at the Sun unless you are wearing special eye protection.)
However, space is vast, and so all other stars are too far away for our eyes to resolve them. In other words, we see stars more like points of light than larger disks. But these points usually have some structure to them. Next time you’re out looking at the night sky, take a good look at the stars above and you’ll notice they look more like points of light with small spikes coming off of them. You can see these spikes best if you look just to the side of a star (putting the star in your peripheral vision) or if you squint. You can also cheat and get a good look at these spikes by squinting at a street lamp or even a light in your house.
The reason for these spikes is the phenomenon known as diffraction. To understand diffraction, we first have to remember that light sometimes likes to behave like a particle (called a photon) and other times it behaves like a wave. This split personality is known as the wave-particle duality of light. Diffraction is one of the clearest examples of light behaving like a wave. When a beam of light has to travel through a narrow opening or around a sharp edge, that light diffracts, or spreads out into a pattern called, you guessed it, a diffraction pattern.