StarTalk Radio

Things You Thought You Knew – The Color of the Sun

December 30, 2025

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  • The sun's true color, as seen from space or through thin clouds on Earth, is white, not the yellow or red we perceive due to atmospheric scattering of blue light. 
  • The sound of thunder is a shockwave caused by rapidly heated air from a lightning bolt, and the characteristic 'crackle and pop' comes from constructive and destructive interference between shockwaves generated by the lightning bolt's non-straight path. 
  • Friction is essential for nearly all terrestrial movement and processes, including walking and driving, and the kinetic energy of high-speed atmospheric re-entry is dissipated primarily through friction and shockwaves converting energy into heat. 

Segments

Sponsor Messages and Intro
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(00:00:00)
  • Key Takeaway: The episode of StarTalk Radio, “Things You Thought You Knew – The Color of the Sun,” covers the sun’s color, weather acoustics, and friction.
  • Summary: The initial segment features advertisements for Smilo Cancer Hospital and Aldi on Uber Eats. The hosts then introduce the episode, confirming the topics to be covered are the color of the sun, weather acoustics, and friction.
Debunking Sun’s Yellow Color
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(00:01:41)
  • Key Takeaway: The sun is inherently white, and its perceived yellow or red color at sunrise/sunset is an optical illusion caused by Earth’s atmosphere scattering blue light.
  • Summary: When the sun is overhead, atmospheric particles preferentially scatter blue light, which is why the sky appears blue; the remaining light reaching the surface is slightly less blue, appearing white. When the sun is low on the horizon, the light travels through much more atmosphere, scattering away even more blue and violet light, resulting in the observed amber or red color.
Light Color and Photography
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(00:08:35)
  • Key Takeaway: White objects, like snow, appear white because they reflect an equal mixture of all colors of light, proving the illuminating light source (sunlight) is white.
  • Summary: Photographers historically distinguished between ‘daylight film’ (balanced for the sun’s full spectrum) and ‘indoor film’ (overly sensitive to blue to compensate for the weak blue light from incandescent bulbs). Emotionally, blue is associated with cold and red with hot, but in lighting technology, achieving a ‘cooler’ (bluer) scene often required a ‘hotter’ (higher temperature) light bulb.
Acoustics of Weather Phenomena
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(00:18:37)
  • Key Takeaway: The varied sound of thunder results from constructive and destructive interference of shockwaves generated by the kinks along the lightning bolt’s path.
  • Summary: Sound travels approximately one mile every six seconds in air, allowing distance to thunderstorms to be estimated by counting seconds between the flash and the thunder. High-frequency sounds from weather events dissipate faster, meaning distant storms sound lower in pitch than nearby strikes, which can sound like the sky is tearing apart.
Snow Sound Absorption and Crunch
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(00:27:08)
  • Key Takeaway: Fresh snow acts as a natural soundproofing material by absorbing sound waves that would normally reflect off rigid urban surfaces.
  • Summary: Snow muffles city noise because its soft, variegated surface is highly absorbent rather than reflective like concrete or glass, leading to the quietness associated with a ‘Silent Night.’ Snow crunches when stepped on only in very cold temperatures (low 20s Fahrenheit or lower) because the ice crystals are rigid and fracture instead of yielding and melting under pressure.
The Necessity of Friction
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(00:37:20)
  • Key Takeaway: Friction is a necessary force for nearly all terrestrial motion, including walking and driving, and its absence prevents movement against the Earth’s surface.
  • Summary: All transportation methods, except rockets, rely on friction between tires/shoes and the ground to propel forward, stop, or steer; without it, wheels would simply spin. If all humans ran due west, the momentum generated by their friction against the ground would cause a minuscule, yet real, acceleration in the Earth’s rotation.
Friction in Space Re-entry
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(00:48:25)
  • Key Takeaway: The intense heat experienced by astronauts during atmospheric re-entry is the conversion of their massive kinetic energy (17,000 mph) into heat via friction and shockwaves, not merely the act of falling.
  • Summary: Astronauts rely on atmospheric friction to slow down from orbital velocity, avoiding the need to carry massive amounts of fuel for deceleration. Heat shields function by ablating—burning off sacrificial layers—which carries the heat away from the capsule.