Why does the expansion of dying stars cause their emitted light to be redder?
The correct answer and explanation is:
Correct Answer:
The expansion of dying stars causes their emitted light to be redder due to cooling of the star’s surface, which shifts its peak radiation to longer wavelengths—i.e., the red end of the spectrum.
Explanation (300 words):
As a star nears the end of its life, it undergoes significant changes in its structure and temperature. One common phase in stellar evolution is when a star expands into a red giant or red supergiant. This happens when the star exhausts the hydrogen in its core, which reduces the outward pressure from nuclear fusion that balances gravity. The core contracts under gravity, heating up, while the outer layers expand greatly.
During this expansion, the energy from the star’s core is spread over a much larger surface area. Although the total energy output may stay the same or even increase, the surface temperature decreases because the same energy is being radiated over a bigger surface. Temperature is a key factor in determining the color of the light a star emits.
According to Wien’s Law, the peak wavelength of emitted radiation is inversely proportional to the surface temperature of the star. This means that as a star’s surface cools, the peak wavelength shifts to longer wavelengths, which are redder in the visible light spectrum.
This is why dying stars that have expanded appear redder. They are cooler on the outside compared to when they were younger and more compact, leading to light that is more in the red and infrared part of the spectrum.
Additionally, the term “redshift” might sometimes be mentioned in this context, but in stellar aging, it is primarily thermal redshift due to cooling, not the Doppler redshift caused by motion.
Thus, the expansion of dying stars leads to cooler surfaces, and cooler stars emit redder light, making this a clear physical consequence of stellar evolution.