Why do stars look pointy to humans?
Overview of the diffraction of starlight.
Encyclopædia Britannica, Inc.
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Encyclopædia Britannica, Inc.Watch as a shadow slowly crosses the Earth.
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Contunico © ZDF Studios GmbH, MainzLearn about Eurasian beavers.
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Encyclopædia Britannica, Inc.Explanation of the periodic table.
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Contunico © ZDF Studios GmbH, MainzOverview of Planck's constant, including its discovery by Max Planck.
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Contunico © ZDF Studios GmbH, MainzLearn about Thomas Young's double-slit experiment that challenged Isaac Newton's theory of light.
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© MinutePhysics (A Britannica Publishing Partner)Infrared energy from stars causes space and the night sky to appear dark.
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© MinutePhysics (A Britannica Publishing Partner)Serge Haroche won the 2012 Nobel Prize for Physics for his experiments that studied the quantum mechanical behaviour of light.
Transcript
Imagine looking at stars in the night sky.
How would you describe their shape?
Pointy?
Sure. Now look at this image of stars from the Hubble Space Telescope.
They’re pointy, too.
But wait...stars are spheres!
So why do our eyes--and some telescopes--see these points of light? Starlight travels through space as a wave.
And, like any wave, it experiences diffraction when it hits an obstacle.
It bends, or diffracts, around the point of contact.
The shape of the obstacle determines the diffraction pattern of the wave. So what exactly is the obstacle that causes starlight diffraction in the Hubble images?
It’s the support bars for the secondary mirror! When light diffracts around Hubble’s crossed-shaped struts, the result is a diffraction pattern of crisscrossed dashes.
These are so common in telescope images that astronomers even have a nickname for them: diffraction spikes! That makes sense...for telescope images.
But why do people also see diffraction spikes?
Because the human eye lens contains tiny structural imperfections called suture lines.
When light passes through our lenses, it gets diffracted around the suture lines!
Every eye, and suture line, is unique, and so is the diffraction pattern for each one. Look closely again at one of the stars.
Closer.
Do you see any colors in the diffraction spikes?
Indeed!
Since diffraction spreads out the light, it also spreads out its wavelengths, too.
The colors confirm this: red, the longest wavelength, is at the end of the spike, while blue, the shortest, is at the center.
How would you describe their shape?
Pointy?
Sure. Now look at this image of stars from the Hubble Space Telescope.
They’re pointy, too.
But wait...stars are spheres!
So why do our eyes--and some telescopes--see these points of light? Starlight travels through space as a wave.
And, like any wave, it experiences diffraction when it hits an obstacle.
It bends, or diffracts, around the point of contact.
The shape of the obstacle determines the diffraction pattern of the wave. So what exactly is the obstacle that causes starlight diffraction in the Hubble images?
It’s the support bars for the secondary mirror! When light diffracts around Hubble’s crossed-shaped struts, the result is a diffraction pattern of crisscrossed dashes.
These are so common in telescope images that astronomers even have a nickname for them: diffraction spikes! That makes sense...for telescope images.
But why do people also see diffraction spikes?
Because the human eye lens contains tiny structural imperfections called suture lines.
When light passes through our lenses, it gets diffracted around the suture lines!
Every eye, and suture line, is unique, and so is the diffraction pattern for each one. Look closely again at one of the stars.
Closer.
Do you see any colors in the diffraction spikes?
Indeed!
Since diffraction spreads out the light, it also spreads out its wavelengths, too.
The colors confirm this: red, the longest wavelength, is at the end of the spike, while blue, the shortest, is at the center.